Tự động hóa - Chuẩn truyền tin hart trong đo lường và điều khiển tự động mạng công nghiệp

Nội dung text: Tự động hóa - Chuẩn truyền tin hart trong đo lường và điều khiển tự động mạng công nghiệp

1. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer TRƯỜNG ĐẠI HỌC BÁCH KHOA KHOA ĐIỆN BỘ MÔN : TỰ ĐỘNG HÓA CHUẨN TRUYỀN TIN HART TRONG ĐO LƯỜNG VÀ ĐIỀU KHIỂN TỰ ĐỘNG MẠNG CÔNG NGHIỆP Version 1.0 – Lưu hành nội bộ ĐÀ NẴNG 2007 1 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
2. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer GIỚI THIỆU CHUNG HART là một giao thức truyền thông được giới thiệu vào năm 1980, những ứng dụng của HART được phát triển bởi tổ chức HCF. HART cho phép thiết bị làm việc trong môi trường công nghiệp có nhiễu cao và tương thích với các chuẩn 4-20mA. Nó được kiến trúc dựa trên sự xếp chồng tín hiệu số trên nền tín hiệu tương tự 4 – 20mA, nghĩa là nó có dạng tín hiệu lai, cộng tín hiệu một chiều với tín hiệu đã được mã hóa. Do đó các thiết bị có thể nhanh chóng định dạng và xác định đúng thông số cần dùng khi có nhiều thiết bị nối vào chung mạng công nghiệp. Cũng như các chuẩn công nghiệp đã có trong lịch sử, để người sử dụng và các môi trường tiếp nhận không bị ảnh hưởng về tâm lí vật lí, HART cũng cho phép nối Master-Slave dạng PPI và MPI. Các liên kết PPI cho phép kéo dài đường truyền đến 3000m và MPI là 1500m, tối đa của MPI lến đến 15 thiết bị. Tuy nhiên HART có nhược điểm là tốc độ truyền thấp, hiện nay đến 4800 baud. Ngược lại, HART lại cho phép cả thiết bị tương tự và số có thể làm việc trên cùng một mạng. Sau đây sẽ trình bày cụ thể hơn những đặc điểm cơ bản về HART. Tài liệu sau đây vừa trình bày những kiến thức về HART, đồng thời cũng đưa ra những mạch điện cụ thể sử dụng cho các chuẩn đo lượng hiện đại hiện nay. Sinh viên có thể sử dụng các phần kiến thức đó để phục vụ cho quá trình làm bài tập, đồ án môn học, tốt nghiệp và các công tác khác sau này. 2 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
3. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer About HART Part 1 Part1: Preliminaries Introduction HART (Highway Addressable Remote Transducer) provides digital communication to microprocessor-based (smart) analog process control instruments. Originally intended to allow convenient calibration, range adjustment, damping adjustment, etc. of analog process transmitters; it was the first bi-directional digital communication scheme for process transmitters that didn't disturb the analog signal. The process could be left running during communication. HART has since been extended to process receivers, and is sometimes also used in data acquisition and control. HART Specifications continue to be updated to broaden the range of HART applications. And a recent HART development, the Device Description Language (DDL), provides a universal software interface to new and existing devices. HART was developed in the early 1980s by Rosemount Inc. [1.4]. Later, Rosemount made it an open standard. Since then it has been organized and promoted by the HART Communication Foundation [1.5], which boasts some 114 member companies. As the de-facto standard for data communication in smart analog field instruments, HART is found in applications ranging from oil pipelines to pulp and paper mills to public utilities. As of June 1998 an estimated 5 million nodes were installed [1.1]. Among the many HART products now available are Analog Process Transmitters Digital-only Process Transmitters Multi-variable Process Transmitters Process Receivers (Valves) Local (Field) Controllers HART-to-Analog Converters Modems, Interfaces, and Gateways HART-compatible Intrinsic Safety Barriers HART-compatible Isolators Calibrators Software Packages New HART products continue to be announced, despite encroachment by Foundation Fieldbus and other faster networks. Analog transmitters continue to flourish [1.2], which suggests that HART will, also. A recent study [1.3] predicts that, of all smart pressure transmitters sold in the next few years, sales of HART units will increase at 17.5% per year. Analog Services, Inc., a leader in HART development, is pleased to present this on-line book about HART. We have tried to present many topics that do not appear in the HART Standards or App Notes. This is still a work in progress. If there are other topics that you would like to see 3 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
4. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer covered or corrections to what we have presented, please send us an e-mail at stevea@analogservices.com. Overview: HART and The Conventional Process Loop HART is sometimes best understood by looking at how it evolved from a conventional process loop. Figure 1.1 is a simplified diagram of the familiar analog current loop. The process transmitter signals by varying the amount of current flowing through itself. The controller detects this current variation by measuring the voltage across the current sense resistor. The loop current varies from 4 to 20 mA at frequencies usually under 10 Hz. Figure 1.1 Conventional Process Loop Figure 1.2 is the same thing with HART added. Both ends of the loop now include a modem and a "receive amplifier." The receive amplifier has a relatively high input impedance so that it doesn't load the current loop. The process transmitter also has an AC-coupled current source, and the controller an AC-coupled voltage source. The switch in series with the voltage source (Xmit Volt Source) in the HART controller is normally open. In the HART Controller the added components can be connected either across the current loop conductors, as shown, or across the current sense resistor. From an AC standpoint, the result is the same, since the Pwr Supply is effectively a short circuit. Notice that all of the added components are AC-coupled, so that they do not affect the analog signal. The receive amplifier is often considered part of the modem and would usually not be shown separately. We did it this way to indicate how (across which nodes) the receive signal voltage is derived. In either the Controller or the Transmitter, the receive signal voltage is just the AC voltage across the current loop conductors. 4 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
5. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.2 Process Loop With HART Added To send a HART message, the process transmitter turns ON its AC-coupled current source. This superimposes a high-frequency carrier current of about 1 mA p-p onto the normal transmitter output current. The current sense resistor at the controller converts this variation into a voltage that appears across the two loop conductors. The voltage is sensed by the controller's receive amplifier and fed to the controller's demodulator (in block labeled "modem"). In practice the two current sources in the HART process transmitter are usually implemented as a single current regulator; and the analog and digital (HART) signals are combined ahead of the regulator. To send a HART message in the other direction (to the process transmitter), the HART Controller closes its transmit switch. This effectively connects the "Xmit Volt Source" across the current loop conductors, superimposing a voltage of about 500 mV p-p across the loop conductors. This is seen at the process transmitter terminals and is sent to its receive amplifier and demodulator. Figure 1.2 implies that a Master transmits as voltage source, while a Slave transmits as a current source. This is historically true. It is also historically true that the lowest impedance in the network the one that dominates the current-to-voltage conversion was the current sense resistor. Now, with some restrictions, either device can have either a low or high impedance. And the current sense resistor doesn't necessarily dominate. Regardless of which device is sending the HART message, the voltage across the loop conductors will look something like that of figure 1.3; with a tiny burst of carrier voltage superimposed on a relatively large DC voltage. The superimposed carrier voltage will have a range of values at the receiving device, depending on the size of the current sense resistor, the amount of capacitive loading, and losses caused by other loop elements. Of course the DC 5 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
6. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer voltage will also vary; depending on controller supply voltage, loop resistance, where in the loop the measurement is made, etc. Figure 1.3 HART Carrier Burst HART communication is FSK (frequency-shift-keying), with a frequency of 1200 Hz representing a binary one and a frequency of 2200 Hz representing a binary zero. These frequencies are well above the analog signaling frequency range of 0 to 10 Hz, so that the HART and analog signals are separated in frequency and ideally do not interfere with each other. The HART signal is typically isolated with a high-pass filter having a cut-off frequency in the range of 400 Hz to 800 Hz. The analog signal is similarly isolated with a low-pass filter. This is illustrated in figure 1.4. 6 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
7. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.4 Separation of Analog and HART (Digital) Signals The separation in frequency between HART and analog signaling means that they can coexist on the same current loop. This feature is essential for HART to augment traditional analog signaling. Further information on the frequencies involved in HART transmission is given in the section entitled HART Signal Power Spectral Density. For a description of FSK and other forms of data/digital communication, see [3.5]. For convenience, Figure 1.4 shows the Analog and HART Signals to be the same level. Generally, this isn't true. The Analog Signal can vary from 4 to 20 mA or 16 mA p-p (unusual, but possible), which is vastly larger than the HART Signal. This, in turn, can lead to some difficulties in separating them. HART is intended to retrofit to existing applications and wiring. This means that there must be 2-wire HART devices. It also means that devices must be capable of being intrinsically safe. These requirements imply relatively low power and the ability to transmit through intrinsic safety barriers. This is accomplished through a relatively low data rate, low signal amplitude, and superposition of the HART and analog signals. Power consumption is further reduced through the half-duplex nature of HART. That is, a device does not simultaneously transmit and receive. Therefore, some receive circuits can be shut down during transmit and vice-versa. Intrinsic Safety and retrofitting to existing applications and wiring also explain why HART was developed at all, despite other advanced communication systems and techniques that existed at the time. None of them would have met the low power requirements needed in a 2-wire 4-20 mA device. Further information on intrinsically safe HART devices is given in the section entitled HART and Intrinsic Safety . In HART literature the process transmitter is called a Field Instrument or HART Slave Device. (These terms will be used interchangeably throughout our presentation.) And the current loop is a network. The controller is a HART Master. A hand-held communicator can 7 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
8. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer also be placed across the network temporarily. It is used in place of, or in addition to, the fixed controller-based HART Master. When both types of Masters are present, the controller is the Primary Master and the hand-held unit is the Secondary Master. (Note: It becomes difficult to describe process devices in a data communication setting, because the terms transmitter and receiver have more than one meaning. For example, a process transmitter both receives and transmits data bits. We hope we've avoided confusion by providing sufficient context whenever these words are used.) HART now includes process receivers. These are also called Field Instruments or HART Slaves and are discussed in the section entitled Process Receiver. Overview: Signaling The HART signal path from the the processor in a sending device to the processor in a receiving device is shown in figure 1.5. Amplifiers, filters, etc. have been omitted for simplicity. At this level the diagram is the same, regardless of whether a Master or Slave is transmitting. Notice that, if the signal starts out as a current, the "Network" converts it to a voltage. But if it starts out a voltage it stays a voltage. Figure 1.5 HART Signal Path The transmitting device begins by turning ON its carrier and loading the first byte to be transmitted into its UART. It waits for the byte to be transmitted and then loads the next one. This is repeated until all the bytes of the message are exhausted. The transmitter then waits for the last byte to be serialized and finally turns off its carrier. With minor exceptions, the transmitting device does not allow a gap to occur in the serial stream. 8 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
9. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer The UART converts each transmitted byte into an 11 bit serial character, as in figure 1.6. The original byte becomes the part labeled "Data Byte (8 bits)". The start and stop bits are used for synchronization. The parity bit is part of the HART error detection. These 3 added bits contribute to "overhead" in HART communication. Figure 1.6 HART Character Structure The serial character stream is applied to the Modulator of the sending modem. The Modulator operates such that a logic 1 applied to the input produces a 1200 Hz periodic signal at the Modulator output. A logic 0 produces 2200 Hz. The type of modulation used is called Continuous Phase Frequency Shift Keying (CPFSK). "Continuous Phase" means that there is no discontinuity in the Modulator output when the frequency changes. A magnified view of what happens is illustrated in figure 1.7 for the stop bit to start bit transition. When the UART output (modulator input) switches from logic 1 to logic 0, the frequency changes from 1200 Hz to 2200 Hz with just a change in slope of the transmitted waveform. A moment's thought reveals that the phase doesn't change through this transition. Given the chosen shift frequencies and the bit rate, a transition can occur at any phase. 9 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
10. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.7 Illustration of Continuous Phase FSK A mathematical description of continuous phase FSK is given in the section entitled Equation Describes CPFSK. The form of modulation used in HART is the same as that used in the "forward channel" of Bell-202. However, there are enough differences between HART and Bell-202 that several modems have been designed specifically for HART. Further information on Bell-202 is given in the section entitled What's In a Bell-202 Standard? At the receiving end, the demodulator section of a modem converts FSK back into a serial bit stream at 1200 bps. Each 11-bit character is converted back into an 8-bit byte and parity is checked. The receiving processor reads the incoming UART bytes and checks parity for each one until there are no more or until parsing of the data stream indicates that this is the last byte of the message. The receiving processor accepts the incoming message only if it's amplitude is high enough to cause carrier detect to be asserted. In some cases the receiving processor will have to test an I/O line to make this determination. In others the carrier detect signal gates the receive data so that nothing (no transitions) reaches the receiving UART unless carrier detect is asserted. Overview: HART Process Transmitter Block Diagram A block diagram of a typical HART Process Transmitter is given in figure 1.8. 10 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
11. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.8 Typical HART Process Transmitter Block Diagram The "network interface" in this case is the current regulator. The current regulator implements the two current sources shown in the "process transmitter" of figure 1.2. The block labeled "modem", and possibly the block labeled "EEPROM", are about the only parts that would not otherwise be present in a conventional analog transmitter. The EEPROM is necessary in a HART transmitter to store fundamental HART parameters. The UART, used to convert between serial and parallel data, is often built into the micro-controller and does not have to be added as a separate item. The diagram illustrates part of the appeal of HART: its simplicity and the relative ease with which HART field instruments can be designed. HART is essentially an add-on to existing analog communication circuitry. The added hardware often consists of only one extra integrated circuit of any significance, plus a few passive components. In smart field instruments the ROM and EEPROM to hold HART software and HART parameters will usually already exist. Overview: Building Networks The type of network thus far described, with a single Field Instrument that does both HART and analog signaling, is probably the most common type of HART network and is called a point- to-point network. In some cases the point-to-point network might have a HART Field Instrument but no permanent HART Master. This might occur, for example, if the User intends primarily analog communication and Field Instrument parameters are set prior to installation. A HART User might also set up this type of network and then later communicate with the Field Instrument using a hand-held communicator (HART Secondary Master). This is a device that clips onto device terminals (or other points in the network) for temporary HART communication with the Field Instrument. A HART Field Instrument is sometimes configured so that it has no analog signal only HART. Several such Field Instruments can be connected together (electrically in parallel) on the same network, as in figure 1.9. 11 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
12. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.9 HART Network with Multi-dropped Field Instruments These Field Instruments are said to be multi-dropped. The Master is able to talk to and configure each one, in turn. When Field Instruments are multi-dropped there can't be any analog signaling. The term "current loop" ceases to have any meaning. Multi-dropped Field Instruments that are powered from the network draw a small, fixed current (usually 4 mA); so that the number of devices can be maximized. A Field Instrument that has been configured to draw a fixed analog current is said to be "parked." Parking is accomplished by setting the short-form address of the Field Instrument to some number other than 0. A hand-held communicator might also be connected to the network of figure 1.9. There are few restrictions on building networks. The topology may be loosely described as a bus, with drop attachments forming secondary busses as desired. This is illustrated in figure 1.10. The whole collection is considered a single network. Except for the intervening lengths of cable, all of the devices are electrically in parallel. The Hand-Held Communicator (HHC) may also be connected virtually anywhere. As a practical matter, however, most of the cable is inaccessible and the HHC has to be connected at the Field Instrument, in junction boxes, or in controllers or marshalling panels. 12 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
13. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.10 HART Network Showing Free Arrangement of Devices In intrinsically safe (IS) installations there will likely be an IS barrier separating the Control and Field areas. A Field Instrument may be added or removed or wiring changes made while the network is live (powered). This may interrupt an on-going transaction. Or , if the network is inadvertently short-circuited, this could reset all devices. The network will recover from the loss of a transaction by re-trying a previous communication. If Field Instruments are reset, they will eventually come back to the state they were in prior to the reset. No reprogramming of HART parameters is needed. The common arrangement of a home run cable, junction box, and branch cables to Field Instruments is acceptable. Different twisted pairs of the same cable can be used as separate HART networks powered from a single supply, as in figure 1.11. Notice that in this example the 2nd network has two multi-dropped Field Instruments, while each of the other two networks shown has only one. 13 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
14. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.11 Single Cable With Multiple HART Networks Circuit 1 in the diagram is connected to A/D converter 1 and Modem 1. Circuit 2 is connected to A/D converter 2, Modem 2. And so on. Or else a multiplexor may be used to switch a single A/D converter or single Modem sequentially from Circuit 1 through Circuit N. If a single Modem is used, it is either a conventional Modem that is switched in between HART transactions; or it could be a special sampled-data type of Modem that is able to operate on all networks simultaneously. HART networks use shielded twisted pair cable. Many different cables with different characteristics are used. Although twisted pair cable is used, the signaling is single-ended. (One side of each pair is at AC ground.) HART needs a minimum bandwidth (-3 dB) of about 2.5 kHz. This limits the total length of cable that can be used in a network. The cable capacitance (and capacitance of devices) forms a pole with a critical resistance called the network resistance. In most cases the network resistance is the same as the current sense resistance in figures 1.1 and 1.2. To insure a pole frequency of greater than 2.5 kHz, the RC time constant must be less than 65 microsecond. For a network resistance of 250 ohm, C is a maximum of 0.26 microfarad. Thus, the capacitance due to cable and other devices is limited to 0.26 microfarad. Further information on cable effects is given in the section entitled Cable Effects. Digital signaling brings with it a variety of other possible devices and modes of operation. For example, some Field Instruments are HART only and have no analog signaling. Others draw no power from the network. In still other cases the network may not be powered (no DC). There also exist other types of HART networks that depart from the conventional one described here. These are covered in the section entitled HART Gateways and Alternative Networks . Overview: Protocol 14 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
15. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Normally, one HART device talks while others listen. A Master typically sends a command and then expects a reply. A Slave waits for a command and then sends a reply. The command and associated reply are called a transaction. There are typically periods of silence (nobody talking) between transactions. The two bursts of carrier during a transaction are illustrated in figure 1.12. Figure 1.12 Carrier Bursts During HART Transaction There can be one or two Masters (called Primary and Secondary Masters) per network. There can be (from a protocol viewpoint) almost an unlimited number of Slaves. (To limit noise on a given network, the number of Slaves is limited to 15. If the network is part of a super network involving repeaters, then more Slaves are possible because the repeater re-constitutes the digital signal so that noise does not pass through it.) A Slave accesses the network as quickly as possible in response to a Master. Network access by Masters requires arbitration. Masters arbitrate by observing who sent the last transmission (a Slave or the other Master) and by using timers to delay their own transmissions. Thus, a Master allows time for the other Master to start a transmission. The timers constitute dead time when no device is communicating and therefore contribute to "overhead" in HART communication. Further information on Master arbitration is available in the section entitled Timing is Everything. A Slave (normally) has a unique address to distinguish it from other Slaves. This address is incorporated into the command message sent by a Master and is echoed back in the reply by the Slave. Addresses are either 4 bits or 38 bits and are called short and long or "short frame" and "long frame" addresses, respectively. A Slave can also be addressed through its tag (an identifier assigned by the user). HART Slave addressing and the reason for two different address sizes is discussed in more detail in the next section. Each command or reply is a message, varying in length from 10 or 12 bytes to typically 20 or 30 bytes. The message consists of the elements or fields listed in table 1.1, starting with the preamble and ending with the checksum. 15 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
16. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Part of Message Length in Bytes Purpose Synchronization & Preamble 5 to 20 Carrier Detect Synchronization & Start Delimiter 1 Shows Which Master Choose Slave, Indicate Address 1 or 5 Which Master, and Indicate Burst Mode Command 1 Tell Slave What to Do Indicates Number Bytes Number Data Bytes 1 Between Here and Checksum Slave Indicates Its 0 (if Master) Status Health and Whether it 2 (if Slave) did As Master Intended Argument Associated Data 0 to 253 with Command (Process Variable, For Example) Checksum 1 Error Control Table 1.1 Parts of HART Message The preamble is allowed to vary in length, depending on the Slave's requirements. A Master will use the longest possible preamble when talking to a Slave for the first time. Once the Master reads the Slave's preamble length requirement (a stored HART parameter), it will subsequently use this new length when talking to that Slave. Different Slaves can have different preamble length requirements, so that a Master might need to maintain a table of these values. A longer preamble means slower communication. Slave devices are now routinely designed so that they need only a 5 byte preamble; and the requirement for a variable preamble length may now be largely historical. The status field (2 bytes) occurs only in replies by HART Slave devices. If a Slave does not execute a command, the status shows this and usually indicates why. Several possible reasons are: 1. The Slave received the message in error. (This can also result in no reply.) 2. The Slave doesn't implement this command. 3. The Slave is busy. 4. The Slave was told to do something outside of its capability (range number too large or small, for example). 16 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
17. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer 5. The Slave is write-protected and was told to change a protected parameter. A Slave Device will often be equipped with write-protect capability. This is often implemented with a two-position shorting block on the device's circuit board. With the shorting block in the write-protect position, parameters can't be changed. A Slave that is commanded to change a protected parameter will not act on the command and will reply that it is write protected. Commands are one of 3 types: Universal, Common Practice, and Device Specific (Proprietary). Universal and Common Practice commands implement functions that were either part of an original set or are needed often enough to be specified as part of the Protocol. Among the Universal commands are commands to read and write the device's serial number, tag, descriptor, date; read and write a scratch memory area; read the device's revision levels; and so on. These parameters are semi-permanent and are examples of data that is stored in EEPROM. A Device Specific command is one that the device manufacturer creates. It can have any number from 128 to 253. Different manufacturers may use the same command number for entirely different functions. Therefore, the Master must know the properties of the devices it expects to talk to. The HART Device Description Language is helpful in imparting this information to a Master. The command value 255 is not allowed, to avoid possible confusion with the preamble character. The value 254 is reserved probably to allow for a second command byte in future devices that may require a very large number of device-specific commands. The checksum at the end of the message is used for error control. It is the exclusive-or of all of the preceding bytes, starting with the start delimiter. The checksum, along with the parity bit in each character, create a message matrix having so-called vertical and longitudinal parity. If a message is in error, this usually necessitates a retry. Further information on HART error control is given below in the section HART Message Errors. One more feature, available in some Field Instruments, is burst mode. A Field Instrument that is burst-mode capable can repeatedly send a HART reply without a repeated command. This is useful in getting the fastest possible updates (about 2 to 3 times per second) of process variables. If burst-mode is to be used, there can be only one bursting Field Instrument on the network. A Field Instrument remembers its mode of operation during power down and returns to this mode on power up. Thus, a Field Instrument that has been parked will remain so through power down. Similarly, a Field Instrument in burst-mode will begin bursting again on power up. HART Protocol puts most of the responsibility (such as timing and arbitration) into the Masters. This eases the Field Instrument software development and puts the complexity into the device that's more suited to deal with it. A large amount of Protocol information, including message structure and examples, is given in [1.6]. 17 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
18. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Overview: Addressing Each HART field instrument must have a unique address. Each command sent by a Master contains the address of the desired Field Instrument. All Field Instruments examine the command. The one that recognizes its own address sends back a response. For various reasons HART addressing has been changed a few times. Each change had to be done in such a way as to maintain backward compatibility. This has led to some confusion over addressing. Hopefully, this somewhat chronological presentation will not add to the confusion. Early HART protocol used only a 4 bit address. This meant there could be 16 field instruments per network. In any Field Instrument the 4-bit address could be set to any value from 0 to 15 using HART commands. If a Master changed the address of a Field Instrument, it would have to use the new address from then on when talking to that particular Field Instrument. Later, HART was modified to use a combination of the 4-bit address and a new 38 bit address. In these modern devices, the 4-bit address is identical to the 4-bit address used exclusively in earlier devices, and is also known as a polling address or short address. The 38 bit address is also known as the long address, and is permanently set by the Field Instrument manufacturer. A 38- bit address allows virtually an unlimited number of Field Instruments per network. Older devices that use only a 4-bit address are also known as "rev 4" Field Instruments. Modern devices, that use the combined addresses, are also known as "rev 5" instruments. These designations correspond to the revision levels of the HART Protocol documents. Revision 4 devices are now considered obsolete. Their sale or use or design is discouraged and most available software is probably not compatible with revision 4. So, why the two forms of address in modern Field Instruments? The reason is that we need a way of quickly determining the long address. We can't just try every possible combination (2 to the 38th power). This would take years. So, instead, we put the old 4-bit address to work. We use it to get the Field Instrument to divulge its long address. The protocol rules state that HART Command 0 may be sent using the short address. All other commands require the long address. Command 0, not surprisingly, commands a Field Instrument to tell us its long address. In effect the short address is used only once, to tell us how to talk to the Field Instrument using its long address. The long address consists of the lower (least significant) 38 bits of a 40-bit unique identifier. This is illustrated in figure 1.13. The first byte of the unique identifier is the manufacturer's ID number. The second is the manufacturer's device type code. The 3rd, 4th, and 5th are a serial number. It is intended that no two Field Instruments in existence have the same 40-bit identifier. 18 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
19. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.13 Unique Identifier and Long Address There is an another way to get a Field Instrument to divulge its long address: By using its tag. A tag is a 6-byte identification code that an end-user may assign to a Field Instrument. Once this assignment is made, Command 11 will provide the same information as command 0. But command 11 is one of those that require a long address. This seems to present a chicken-and-egg dilemma: We want to use command 11 to learn the long address. But we need to know the long address to use command 11. Obviously, there is a way around this. It is to use a broadcast address. The broadcast address has all 38 bits equal to zero and is a way of addressing all Field Instruments at once. When a Field Instrument sees this address and command 11, it compares its tag against the one included in the command. If they match, then the Field Instrument sends a reply. Since there should be only one Field Instrument with a matching tag, only one should reply. The short address in either the older or modern Field Instruments has one other purpose: to allow parking. A parked Field Instrument has its analog output current fixed. Usually it is fixed at some low value such as 4 mA. Parking is necessary for multi-dropped instruments to avoid a large and meaningless current consumption. A Field Instrument is parked by setting its short address to a value other than 0. In other words, the short address of the parked Field Instrument can be any value from 1 through 15. Some HART-only Field Instruments have no Analog Signal and are effectively parked for any short address from 0 through 15. There are potential problems with the HART addressing scheme. These are discussed in the section entitled Addressing Problems, Slave Commissioning, and Device Database. Overview: Conclusion Although some of the details and variations are left out, this is basically how HART works. The complete topology rules and device requirements are given in HART specifications, which are sold by the HART Communication Foundation [1.5]. The information presented here should 19 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
20. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer not be considered a substitute for the actual specifications. A current list of the specifications and their HCF designations is given in the section entitled Table of Current HART Publications . Some circuit designs and more detail on selected HART topics are covered in the HART Application Note. Why So Slow? A common question or complaint about HART is its relatively low speed of 1200 bps. In an age of DSL, HART is clearly a snail. One has to keep in mind the time period in which HART was developed (early 1980's) as well as the relatively small amount of available power in 4-20 mA analog instruments. In the early 1980s, a 300 bps modem for a personal computer was considered pretty good. And when 1200 bps modems came out, they sold for $500 to $600 each. The power to run personal computer modems has always been watts. The power to run a HART modem is often only 2 mW. Not only is there very little power available in analog instruments, but it keeps shrinking! Demands for greater functionality keep shifting the available current into more powerful processors, etc. Some of the issues/problems involved in a higher speed HART are: 1. Many of the protocol functions must be moved into hardware. A single low-power microcontroller in a Slave device would otherwise be hard-pressed to keep up. 2. Backward compatibility with devices/networks that run at the current speed and and use the existing bandwidth. If the bit rate is to be higher than the existing bandwidth of 3 or 4 kHz, this generally means that spectrally efficient techniques are needed. This loosely translates into complicated modulation methods and digital signal processing. Thus, there is a quantum leap in current consumption. 3. The cost of a larger and more complex HART chip. 4. Burst type operation, which is used in HART becomes difficult to achieve at higher bit rates, because of the need for long equalization periods and other receiver start-up activities. The HART Communication Foundation has actively sought and invested in the development of a higher speed HART. But so far the hardware has not materialized. For information on the theoretical upper speed limit for a HART network, see the section entitled How Fast? Too see our proposal for a higher speed HART, click here. 20 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
21. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer What's In A Bell-202 Standard? If you've searched through the various Bell-202 Standards and wondered where the FSK modulation and the shift frequencies appear, the answer is they don't. Not even the bit rate of 1200 bps is stated, although it is the recommended upper limit for PSTN (dial-up lines). The bit rate (1200 bps), type of modulation (CPFSK), and the shift frequencies (1200 Hz and 2200 Hz) are all de facto values used in Bell-202 modems. Apparently, just as J.S. Bach never put dynamic markings in his music, believing that it would never be performed other than under his direction; Ma Bell never put in this vital information, thinking that she would forever have a monopoly on modems. Process Receiver HART was originally conceived to augment process transmitters. However, specifications were later revised to cover process receivers (typically valve positioners), as well. Here, we will briefly examine the electrical characteristics of a HART process receiver. In a conventional process receiver loop, the controller generates a 4-20 mA current that is applied to (passed through) the process receiver. The desired characteristic of the receiver is that it have an impedance of almost zero. This is the opposite of the process transmitter, which ideally has infinite impedance. Thus, the two types of Field Instruments are electrical opposites. To add HART communication to the process receiver loop, we could perpetuate the existing impedance situation and require high-impedance Masters and low-impedance Field Instruments. This would require a new set of HART Masters that would transmit using a current source instead of a voltage source. In fact there would be a duplication of most of the HART elements that already exist for process transmitter loops; and possibly a separate specification and separate products for process receiverdom. Another approach a more practical one is to devise a process receiver with nearly zero impedance at DC and a high impedance at HART frequencies. Using this approach, a single type of HART Master is able to talk to either a process transmitter or a process receiver. It is easier to make such a HART process receiver if the "high impedance" doesn't have to be too high. About 300 to 400 ohm is about as high as it can easily go. Since this is still relatively low, the HART specifications permit this device to set the network resistance. That is, the impedance of this device at HART frequencies replaces the current sense resistor. Note that a current sense resistor wouldn't normally be present, anyway, in a process receiver loop. The complete process receiver loop with HART components is shown in figure 1.14. The frequency-dependent impedance in the process receiver is represented by the small graph of |Z| versus frequency. 21 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
22. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 1.14 Process Receiver Loop Circuitry Although the figure shows the transmit source in the process receiver as a current source, this could probably also be implemented as a switched voltage source. There are actually two types of process receivers. The second type is electrically the same as a process transmitter, except that it draws a fixed current and the position is set by writing a setpoint with a HART command. This allows the process receiver to be multi-dropped with other similar receivers or other HART devices. There are also smart positioners that incorporate both types of HART interface for maximum versatility. Other Books About HART? As far as we know there aren't any. A search of amazon.com (on-line bookstore) turned up nothing. The Instrument Society of America (ISA) publishes a variety of books on process control, but has nothing with "HART" in the title. The Virtual HART Book is a catalog of HART products. The entire field of data communication in process plants and on the factory floor began in the 1980s. There is a book entitled "Industrial Data Communications: Fundamentals and Applications" - Second Edition, 1997; that appears to deal with several different networks, including HART. Undoubtedly, there will be others of a general nature that examine and compare the various types of communication that have become available. 22 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
23. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Alternatives To HART There is no exact alternative to HART in the sense of a competing open standard that augments analog signaling in an industrial process control setting. There are, however, similar proprietary methods that have been developed by companies such as Honeywell, Foxboro, and Elsag-Bailey. There are also many process control devices advertised that have RS232 and/or RS485 ports built-in, along with proprietary protocols, for the purpose of configuration, calibration, etc. The H1 Physical Layer (Voltage Mode Low Speed) of Foundation Fieldbus [1.7] is an open standard for process control instruments that supports only digital signaling. It is similar to HART in its support of 2-wire Field Instruments and its superposition of signal onto the DC instrument power. Its raw data rate at the Physical Layer is 31.25 kbits/second much higher than HART. However, it also has much higher overhead so that a full 26X increase in transaction rate is not realized. A much higher level of circuit integration and far more software are generally needed to support it. At present Foundation Fieldbus devices typically use 3 to 5 times as much power as HART devices. The network topology of Foundation Fieldbus is similar to HART but more restricted. Table of Current HART Publications Document Number Title HCF-SPEC-11 HART - Smart Communications Protocol Specification HCF-SPEC-54 FSK Physical Layer Specification HCF-SPEC-81 Data Link Layer Specification HCF-SPEC-99 Command Summary Information HCF-SPEC-127 Universal Command Specification HCF-SPEC-151 Common Practice Command Specification HCF-SPEC-183 Common Tables HCF-SPEC-307 Command Specific Response Code Definitions HCF-SPEC-500 HART Device Description Language Specification HCF-SPEC-501 Device Description Language Methods Builtins Library HCF-SPEC-502 Device Description Language Binary File Format Specification HCF-TEST-1 HART Slave Data Link Layer Test Specification HCF-TEST-2 HART Physical Layer Test Procedure HCF-TEST-3 HART Universal Application Layer Conformance Tests HCF-PROC-1 HCF Entity Control Procedures HCF-PROC-12 HCF Quality Assurance Program HCF-LIT-1 Application Layer Guideline on Building HART Commands NCR 20C12 Modem Application Note: A HART Master HCF-LIT-2 Demonstration Circuit NCR 20C12 Modem Application Note: A HART Slave HCF-LIT-3 Demonstration Circuit 23 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
24. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer HCF-LIT-5 Application Layer Guideline on HART Status Information HCF-LIT-8 Data Link Layer Slave, Structured Analysis HCF-LIT-9 Data Link Layer Master, Structured Analysis HCF-LIT-11 HART Slave Library Software Design NCR20C15 Modem Application Note: A HART Master HCF-LIT-14 Demonstration Circuit NCR20C15 Modem Application Note: A HART Slave HCF-LIT-15 Demonstration Circuit HCF-LIT-17 HTEST Application Manual, HART Master Simulator HCF-LIT-18 Field Device Specific Specification Template HCF-LIT-21 HART Communication Foundation Tokenizer User Guide HCF-LIT-24 HART Telecommunications Guideline Table 1.2 HCF Publications About HART Part 2 Part 2: Practical Stuff A Caveat: HART and Current Consumption Adding HART to an analog 2-wire transmitter eats into the available current in two ways. First, there is the current consumed by HART functions. And, second, there is less current to start with because of the superposition of the HART signal. If the analog output is 4 mA, then the instantaneous output during HART transmission can typically drop to 3.5 mA. This often means that there is only 3.5 mA available to power circuitry. Alarm conditions and guard bands can further erode this number, as illustrated in figure 2.1. Energy storage methods can prevent the loss of 0.5 mA, but might be unsatisfactory in an intrinsically safe device. 24 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
25. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 2.1 Available Operating Current With HART Modem Sources When people talk about modems, it's not always clear whether they mean an integrated circuit that can be designed into their product or a completed, network-ready unit (HART Master). For information on HART modems of either type, see Romilly Bowden. The HART Communication Foundation is another source of information. For just the integrated circuits, you might also want to check out our paper entitled HART Chips: Past, Present, Future. HART Library Software For PC HART Device Drivers are available from Borst Automation . This allows you to put buttons, etc. on your screen that read and write HART parameters, put them into spreadsheets, etc. Also, see the section entitled HART and PCs . HART and PCs The combination of a Personal Computer and Serial Port HART Modem is often used as a HART Master. In the days of DOS this was easier because you could write software that would take over the whole computer and generate the proper timing. Nowadays it isn't so easy. The very enhancements, namely Windows and buffered UARTs, that make PCs more useful for other 25 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
26. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer applications have made them less useful for HART. Windows 95 and 98 have no provision for real time activities, and delays of 20 to 100's of milliseconds are reported [2.1]. Windows NT has provision for "Real-time Threads". But experiment shows [2.2] that it can still devote 100% of CPU time to a task and ignore I/O events completely. Since one character time in HART is 9.2 millisecond, the delays involved can be several character times. This is enough to destroy HART Master arbitration. So-called RTOS extensions to Windows NT are available, that can make Windows NT appear to be more of a real-time operating system. But this is extra software to buy, install, and understand. Worse yet, the HART application software may depend on whose RTOS is being used, so that it becomes tied to the RTOS instead of the PC. In some applications, where it is known that there will be only one HART Master and where HART burst-mode is not used, a Windows-based PC and simple Modem can still be used. The only timing consideration is how long to wait for a Slave to reply. Such applications don't follow HART Specifications and don't allow Master arbitration. Nevertheless, they are useful and probably represent a fairly large subset of HART software. You can download source code (in C) that works in this manner and does a lot of general HART activities, such as extracting device information, calibration, etc. It runs in a DOS window under Windows. Click here to download. To address the real-time requirements of HART, some systems put another processor between the PC and the Modem. This can take the form of either a single-board-computer or an embedded microcontroller that is part of the Modem. The single-board-computer or embedded controller forms a buffer between the Modem and the PC and takes care of all of the HART timing. A recently introduced integrated circuit, the "P51", from Cybernetic Micro Systems, Inc. addresses the timing problems of PCs. It appears to have almost everything you need to make a full HART modem that plugs into a PC's ISA bus. It is based on an 8051 and contains a complete interface to a PC or PC104 bus, including dual-port RAM and interrupts. In this case the 8051 does the HART communication and provides the timing. Even a newer computer running DOS won't always perform as expected, because of the way that the UARTs work. Most modern UARTs in PCs are usually equipped with built-in FIFOs, to avoid frequent interrupts of the CPU. This is a swell idea, except that the UART doesn't put error information into a FIFO. If there is an error, such as a parity error, there is no way of knowing which byte in the FIFO had the parity error (or whether more than one byte was in error). Consequently, there is no way of weeding out the initial HART preamble bytes that are in error because of carrier start-up. (See the section entitled Start-Up Synchronization in HART for details.) Fortunately, the UART can often be programmed so that the FIFO is disabled, allowing you to associate error status with each data byte. Commercially available software packages and libraries for data communication are another source of trouble for the would-be HART Master. Most of them are geared toward telecom modems and have no concept of burst modems. They invariably turn on RTS (request-to-send) and assume it should be ON forever. (HART Modems require RTS to be on only during transmit.) They also are good at losing error status, just like FIFOed UARTs. They let you set up a receive buffer, for example. But they don't let you set up a corresponding buffer of error 26 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
27. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer status. You receive a notice telling you there's a HART message in the buffer, and another notice saying that some of the bytes are in error. But you don't get to know which ones are in error. Finally, one other nasty thing the software package will do is to make sure that your UART FIFO is turned ON. Another UART caveat is that if you read a PC-based UART status, the status is automatically cleared. If you need to use the status word more than once, make sure that you store it after the first read. Timing is Everything HART allows two Masters. Arbitration is used to determine which one will use the network. The arbitration is based on monitoring of network traffic and implementation of timers. A Master that is aware of what has recently transpired is said to be synchronized. An unsynchronized Master is one that has either lost synchronization or has recently been connected to the network and has yet to become synchronized. Loss of synchronization occurs if the processor in the Master must temporarily stop monitoring network traffic to do other things, or if there is no network traffic, or if there are message errors that prevent it from knowing what's happening. If two Masters are present and both are synchronized, then they will use the network alternately. This assumes, of course, that both have something to say. If one of them doesn't it can give up its turn but still remain synchronized. This is illustrated in figure 2.2. The Slave Response in each case may be from a different Slave. 27 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
28. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 2.2 Master Alternation During this process a given Master knows that it is free to use the network when it sees the end of the Slave response to the other Master. If a Master doesn't take its turn, the other Master can 28 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
29. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer have another turn, provided it waits a length of time called RT2. The time interval RT2 is illustrated in figure 2.2. The Master whose turn it is to use the network has this much time in which to start. Otherwise the Master that last used the network may start. This is how things role merrily along when there are no problems and when both Masters have almost continuous business to transact. Although not explicitly shown in figure 2.2 and subsequent figures, both Masters start their timers at the end of any network activity. Any fresh activity cancels the timers. Also, it is implicit in these explanations that a Master will not begin talking if someone else is talking. Now suppose that, as a result of a message error, a Slave doesn't respond to Master 1. Master 2 must now wait a length of time called RT1 before it tries to use the network. Master 1, while waiting for the Slave response, sees the Master 2 command instead. It then waits until Master 2 is done and then it can retry. This is illustrated in figure 2.3. Figure 2.3 Master Alternation with No Slave Response to Master 1 Here, Master 2 has lost synchronization because it did not see a Slave Response to Master 1. It regains synchronization at the end of RT1. Suppose, in figure 2.3, that the Slave finally did respond to the Master 1 command before the end of RT1. Then things would have proceeded normally. RT1, which is longer than RT2, is approximately the length of time that a Slave is allowed to respond. Actually, the Slave maximum response time, which is designated TT0, is slightly shorter than RT1. This ensures that a Master and Slave will not start transmitting simultaneously. 29 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
30. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer If a Master is new to the network, then it must wait a length of time RT1 before it tries to use the network. At the end of RT1 it has become synchronized and may use the network. Or else, if it sees and recognizes a transaction of the other Master before RT1, then it is immediately synchronized. In another scenario suppose that a Slave has responded to Master 1, but the response appeared garbled to Master 2. Figure 2.4 shows what happens. Figure 2.4 Alternating Masters with Master 2 Failing to Recognize Slave Response to Master 1 Since Master 2 didn't see a good Slave Response, it begins waiting a length of time RT1 from the end of the Slave Response. Master 1, which saw a good Slave Response and is still synchronized, starts RT2. At the end of RT2, Master 1 sees that Master 2 isn't using the network and decides to use it again. Master 2 sees this new transmission by Master 1 and becomes resynchronized. Had Master 1 not wanted to re-use the network again, then Master 2 would have become resynchronized at the end of RT1 and could have begun its transaction then. If neither of the Masters needs to talk, the two Masters become unsynchronized. In effect, either Master knows it has waited a time RT1 and can begin again whenever it needs to. Suppose that both Masters are new to the network or are both unsynchronized and try to use the network at the same time (after waiting for RT1). The respective commands will be garbled and there will be no response. Both Masters will start RT1 again at about the same time. And both will collide again at the end of RT1. To prevent this from going on endlessly, the Primary 30 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
31. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer and Secondary Masters have different values for RT1. The Primary Master uses a value designated RT1(0). The Secondary Master uses a value designated RT1(1). How do things work if there is a Slave in burst-mode? Arbitration is simple if there is a Slave in burst-mode. To see this, recall that the bursting Slave will be the only Slave on the network. Following each burst it must wait a short time to allow a Master to use the network. The Protocol requires that the bursting Slave alternate information in its bursts, making it appear that alternate bursts are Responses to alternate Masters. Masters watching the network will see a burst that is a Response to Master 1, followed by a burst that is a Response to Master 2, followed by a burst that is a Response to Master 1, and so on. A given Master knows it can use the network following a burst that is a Response to its opposite. That is, if a given burst was a response to Master 2, then Master 1 knows that it may use the network at completion of the burst. In this strange turn of events, the Slave gets to decide who is next. Values of the timers are given in table 2.1. Value (character Timer Description Symbol times) Master Wait Before RT2 8 Re-Using Network Primary Master Wait RT1(0) 33 from Unsynched Secondary Master Wait from RT1(1) 41 Unsynched Slave Max time to TT0 28 Respond Slave Time Between BT 8 Bursts Table 2.1 Timer Values TT0, the length of time in which a Slave must respond, is deliberately made quite large to accommodate less capable hardware and software that is likely to be found in a Slave. RT1(0), in turn, has to be larger than TT0. And RT1(1) has to be larger than RT1(0). The various timer values have been carefully set to account for various hardware and software latencies. It would probably have been possible to omit RT2 and just force Masters to resynchronize (using RT1) after every Master or Slave Response. However, since RT2 is much smaller than RT1, the existence of an RT2 allows much faster arbitration. The Beginning, End, Gaps, and Dribbles 31 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
32. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer The previous section on arbitration shows the importance to a Master of knowing when a message ends. In fact, both Masters and Slaves need to be aware of when a message starts, stops, or is present. This is not entirely straightforward, and depends on a combination of (1) carrier detect, (2) UART status indications, and (3) monitoring message content. Carrier detect indicates that a carrier of acceptable amplitude is present. It tells a device that it should be examining its UART output and UART status. In the UART status, a "receive buffer full" (RBF) indication will occur once each character. Whether a message is present is determined by the combination of carrier detect and the RBF indications. Many devices don't directly monitor carrier detect. Instead, they use it to qualify (gate) the UART input. This bypasses the additional step of having to look at an I/O line. The presence of RBFs indicate that a message is present. But they don't necessarily indicate the end of a message or the start of another. Back-to-back messages can occur (see box below), which means that a new message starts simultaneously with the end of a previous message. The transition from one message to another can only be detected by monitoring message content. The start of a message is indicated by a 3-character start delimiter. This delimiter is a sequence that isn't likely to occur anywhere else in a message. It is more completely described in the section entitled Start-Up Synchronization in HART. A device will normally be looking for this start delimiter sequence unless it has already seen the sequence and is simply parsing the rest of the message as it arrives. But, what if the start sequence does appear in "normal data"? This is a weakness of HART, but probably not a very important one. The reason is that Slaves are probably the only devices that do not fully parse each message. Therefore, if a start sequence occurs in mid-message, only a Slave will be fooled into thinking that it sees the start of the next message. This Slave will then look for its own address, a command, etc. The chance that the rest of the byte sequence will contain the Slave's 38 bit address is probably almost non-existent. Therefore, the Slave will not see its own address and will resume the normal activity of looking for the start sequence. Back-to-back Messages and Temporary Collisions: A device will often parse the entire message and know, upon receipt of the checksum, where the message ends. A Slave may do this, for example, if the message was addressed to it. Masters do it as part of arbitration. The Slave that is supposed to respond may immediately assert its own carrier upon seeing the checksum. Similarly, the Master may realize that it will have the next use of the network and assert its own carrier upon seeing the checksum. The new carrier may be asserted before the previous one has been removed and before the incoming RBFs stop, leading to a temporary collision. During this time carrier detect never drops. A temporary collision may sound like something terrible, but it has the same effect and is no more of a problem than carrier start-up alone. Carrier start-up is more completely described in the section entitled Start-Up Synchronization in HART. 32 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
33. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer If a message should become garbled, then devices that have been parsing it must revert to waiting for the RBFs or carrier detect to stop, or for a new start sequence to appear. Ideally, RBFs would occur at a constant rate of one every 9.17 millisecond and the last one would correspond to the checksum character. But received messages can have two peculiarities called gaps and dribbles. A gap can occur between two characters of the same message. It is a delay from the end of the stop bit of one character until the start bit of the next character. It will appear to be an extension of the stop bit (logic high at UART input). A dribble is an extra character that appears at the end of a message, just after the checksum character. A dribble isn't transmitted and doesn't appear on the network. It is manufactured by the receive circuit/demodulator/UART, possibly as a result of the carrier shutting OFF. It will be shown here that these really don't affect anything, except to slow down communication. Gaps occur when a Slave is not able to keep up with the 1200 bits/second data rate. In theory there could be a gap between every two characters of the received message. During a gap the carrier is present but no information is being sent. Most modern Slaves are probably able to transmit without gaps. But we still must assume that they can occur. The HART specifications seek to limit gaps to insure maximum throughput, but are ambiguous as to how large a gap can be. One bit time and one character time are both specified. The ambiguity probably reflects the fact that a gap size on the order of 1 character time or less doesn't matter much. In the following we assume a maximum of one character time. Normally, RBFs occur at a rate of one per character time throughout the message. If there is a gap, then there could be up to two character times between RBFs. A device that is trying to decide whether a message has ended will normally restart its timer on each RBF. The timer must be at least two character times (18.33 millisec) to account for the possible gap. Masters will start RT1 and RT2 timers, both of which are much longer than a gap time. Therefore, arbitration will not be affected by a gap. A Slave that is being addressed may also implement a timer, so that it can detect truncated messages. This timer must also be longer than two character times. A dribble generates an extra RBF. It occurs so soon after a preceding character that it simply restarts timers and does not affect arbitration. A device that creates this extra RBF will have to read and discard the phantom character. And, since it will not be able to tell the difference between the phantom and a real transmitted character, it may have to check the character to see whether it is part of a start sequence. To summarize, the presence of a message is indicated by the combination of carrier detect and RBFs. Since back-to-back messages can occur, it is not acceptable to look for carrier detect drop-out as an end of message. Devices must look for the 3-character start sequence. Gaps and dribbles can also occur in a received message. Provided that device timers are longer than 2 character times, gaps and dribbles have no effect except to slow down communication. Start-Up Synchronization in HART 33 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
34. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer HART is a type of data communication in which devices assert carrier only for the time it takes to send a message. (The modems are also called "burst modems.") When it's time to talk, a device starts up its carrier and begins modulating it with the desired data. When it is finished talking the device drops its carrier. Devices that are listening must determine where the data starts. If a listener fails to locate the starting point, then the message will generally appear meaningless. The chosen protocol must, therefore, provide some way for listeners to reliably locate the start of the data. A common way to do this is to send an initial pattern of bits or symbols a preamble or start delimiter that is known to all listeners. A challenge is always to make the preamble and/or start delimiter short to keep overhead low. Another challenge arises because of the start and stop of the carrier. There is generally no way to insure that this happens in a "clean" fashion. Initial bits will appear to change randomly as the carrier rises to full amplitude, filter circuits settle, etc. There may also be "dribble" bits at the end. The originators of HART faced this problem: If carrier start-up causes random bits to be applied to the UART, how do we create an unambiguous start of data? The UART uses a start bit (logic zero) to synchronize reception of one character. If it's been sitting idle for a time or if it thinks that the last thing it got was a stop bit, then the UART must assume that the next zero bit or the next transition to zero, is a start bit. Any initial random zero bit will be considered a start bit. Then, after 10 more bit times the UART will take the next zero bit as the next start bit. Data containing a normal mix of ones and zeros will confuse the UART by presenting it with zeros that it thinks are start bits. The solution in HART is to start the message with a string of characters whose only zero bits are start bits. The UART may be confused at first. But after one or two characters it becomes synchronized. There is only one character that is all ones. It is formed by adding odd parity to 0xff. The start-up process is illustrated in figure 2.5. 34 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
35. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 2.5 Start-Up Synchronization Here the modems have caused a delay between the transmit and received UART signals. At carrier start-up there are some garbage bits at the receiving UART's input. This causes the UART to begin assembling a character. When it has finished it will present this garbage character to the processor. Then it will wait for the next start bit. It won't find one until after the "gap" has passed. Then it will begin assembling the first good character. The processor looks for a 0xff byte (good character) and discards the initial non-0xff bytes. The receiving processor looks for a sequence of 3 contiguous bytes: preamble, preamble, start delimiter. Thus, at least two good preamble characters must be received and they must be those that immediately precede the start delimiter. HART requires that a minimum of 5 preamble characters be transmitted. This allows for the loss of up to 3 characters by the process just described. Typically 1 or 2 characters are lost. Repeaters typically cause a loss of preamble character because they must listen for carrier at both ends and then "turn the line around." The fact that there is only about one character to spare means that a HART repeater must do this in under one character time. Usually this is enough time. Another possible way around the start-up problem would have been to have transmitting devices turn on their carrier and force it to 1200 Hz (logic 1) and wait for a few character times before loading the UART to begin data transmission. If the transmitting UART is simply left empty before transmission the output will be 1200 Hz, equivalent to a stop bit or idle condition. This is the same as creating a deliberate gap of a few character times. At the receivers the respective UARTs would all collect an initial garbage character, as in figure 2.5. But then there would be a gap, followed by the start bit of the first transmitted character. This method has the drawback of requiring transmitting devices to implement a gap timer at the start of the message. A weakness of HART is that message start sequence of (preamble, preamble, start delimiter) can occur in data. A device looking for a start sequence must look at context to determine whether these 3 characters represent a delimiter or data. This makes HART somewhat less robust than it could be if there were a non-data type of start sequence. Slave Receive Algorithm Figure 2.6 below shows an example Slave Receive Algorithm. If the receive data stops prematurely, then there must also be a branch to "dump message, no reply." To provide the quickest possible reply, the Slave usually has to parse the message as it arrives, instead of waiting until it's done. Note that the Slave has to read every incoming byte and possibly just toss it. "Can I Do This?" generally means "is the parameter that I received within an acceptable range?" 35 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
36. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer 36 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
37. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 2.6 Slave Receive Algorithm The software that performs these functions is sometimes called a "stack". Data Compression HART makes limited use of data compression in the form of Packed ASCII. Normally, there are 256 possible ASCII characters, so that a full byte is needed to represent a character. Packed ASCII is a subset of full ASCII and uses only 64 of the 256 possible characters. These 64 characters are the capitalized alphabet, numbers 0 through 9, and a few punctuation marks. Many HART parameters need only this limited ASCII set, which means that data can be compressed to 3/4 of normal. This improves transmission speed, especially if the textual parameter being communicated is a large one. Since only full bytes can be transmitted, the 3/4 compression is fully realized only when the number of uncompressed bytes is a multiple of 4. Any fractional part requires a whole byte. Thus, if U is the number of uncompressed bytes, and T the number of transmitted bytes; find T = (3*U)/4 and increase any fractional part to 1. As examples, U = 3, 7, 8, and 9 result in T = 3, 6, 6, and 7. The rule for converting from ASCII to Packed ASCII is just to remove bits 6 and 7 (two most significant). An example is the character "M". The full binary code to represent this is 0100,1101. The packed binary code is 00,1101. The rules for conversion from packed ASCII back to ASCII are (1) set bit 7 = 0 and (2) set bit 6 = complement of packed ASCII bit 5. Note that, with some exceptions, HART Slaves don't need to do the compression or know anything about the compression. They simply store and re-transmit the already compressed data. Again, this is an instance where the more difficult software is placed in the device (Master) that is more capable of dealing with it. Device Description Language As stated earlier, a HART Slave device can have its own unique set of commands. It can also have a unique sequence of commands to accomplish some goal, such as calibration. A Master must know about these commands and sequences, if it is to use the Slave Device to the fullest extent. One way that the Slave Device manufacturer has of disseminating the information would be as text in a manual for the Device. Then software engineers and system integrators could write specific code for the Slave Devices used at each installation. Another way is to write a Device Description for the Slave Device using the Device Description Language (DDL). The Device Description is similar to the Electronic Data Sheet (EDS) used for DeviceNet. The HART Communication Foundation provides a specification for DDL and also provides training in how to write the DDL files. 37 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
38. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer The Device Description is a file that can be read by a compiler, and converted into an end-user interface. A program running in a HART Master reads the output of the compiler and is able to produce a complete sequence of menus and help screens that guide the plant engineer through whatever procedures the Slave Device can do. In principle, using the DDL avoids writing code to talk to a given Slave Device. Writing the DDL also forces the Slave Device manufacturer to critically examine how his device is supposed to work. So far it seems as though DDL hasn't seen widespread usage, except in hand-held communicators made by Rosemount Inc. Unfortunately, most of the software associated with DDL is apparently centered around the hand-held communicator. In effect, the HART Communication Foundation and Rosemount Inc. still jointly distribute software needed to use DDLs. There do not appear to be any 3rd party vendors of DDL compilers or the Master software that uses the compiler output. We would suggest, as a remedy to this situation, that the HART Communication Foundation start giving away the DDL specification and that manufacturers of Slave Devices publish the actual DDL files via the Internet. The device description, using the HCF device description language, is a text file with an extension ".DDL". It is a series of compound statements that start with an identifying word and a name. It looks something like this: VARIABLE variable_name_1 { structured info about variable_name_1 } VARIABLE variable_name_2 { structured info about variable_name_2 } COMMAND command_1 { structured info about command_1 } MENU menu_1 { structured info about menu_1 } METHOD method_1 { structured info about method_1 } etc. etc. These do not imply any flow control and can appear in any order. Each VARIABLE, COMMAND, etc. has its own structured information that must be included. A VARIABLE is any quantity or index that is contained in the device or is used by a host to interact with the device. In a device such as a pressure transmitter, one of the VARIABLEs (and probably the most important one) would be the pressure. Others might be upper and lower range limits. Another would be the device tag. The structured information for a VARIABLE might include, for example, a format specification that tells how the VARIABLE is to be displayed to the end user. 38 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
39. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer A COMMAND is a HART command. The DDL has one of these statements for every HART command recognized by the device. The structured information for a COMMAND is essentially everything related to the command including its number, request bytes, response bytes, and the returned response codes. A MENU is a presentation to the end user. It can be used to present VARIABLEs or other MENUs or general information to the end user. A METHOD is a sequence of operations that the host is to perform on the device. Examples would be installation or calibration. METHODs are the least similar to the other example entities because they contain C-language statements. When a METHOD is invoked, usually through some MENU choice that appears on the host display, the statements are executed in the order they appear. "For" and "while" and "do", etc. can all be used to perform looping operations. The DDL language provides a large number of built-in functions that are essential for METHODs. An example is "send(command_number)", which sends a HART command. There are also a large number of built-in functions related to aborting the METHOD. This is essential to allow the end user to understand what is happening with the device and the host when things don't go as planned. In addition to VARIABLEs, COMMANDs, MENUs, and METHODs, there are about 5 or 6 other possible entities. These are described in HCF documents. IMPORT is one of them that deserves special mention. IMPORT is a means by which an existing DDL can be re-used without having to enter its entire text. This allows, for example, the HART Universal COMMANDs, VARIABLEs, and tables, to be used by any device without having to enter them all. IMPORT provides a mechanism for re-defining any entity in the imported DDL. If, for example, a new field device does not use one of the Universal VARIABLEs, this can be indicated in one or two lines of code after importing all of the Universal VARIABLEs. Perhaps the most important use of IMPORT is fixing an existing DDL. The revised DDL is simply an IMPORT of the existing one with one or more entities re-defined. Among the various available HCF and Fisher-Rosemount hand-held documents, one that is seriously lacking is a document to explain how the DDL relates to what is displayed on the hand- held communicator. In other words there is nothing that says that if I write code 'ABC' I will see 'XYZ' in the hand-held display. Similarly, there is no way of knowing what hand-held functions (soft-keys at the bottom of the display) become available in a given situation. We strongly encourage Fisher-Rosemount to come up with an app note that covers this. But until then DDL writers are probably stuck with trial-and-error. A few general guidelines or caveats are as follows. Keep in mind that this applies to an existing version of the hand-held and that a future version might be different. 1. The display is very small. Almost all text, except for help messages, must be abbreviated. 2. Help messages can be quite long because the hand-held allows 'page-up' and 'page-down'. 3. Help messages and labels are defined in the called METHOD or VARIABLE; not in the calling entity. In other words, help messages associated with a given MENU are not defined in that MENU. 39 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
40. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer 4. Help for a MENU is not allowed. Thus, an end-user cannot know ahead of time whether he wants or needs the next MENU. 5. In the DDL source there is no way to define long messages on multiple lines. To print the source code, it is necessary to invoke some printing method that has automatic wrap- around. 6. You cannot define any of the hand-held soft keys. Everything you do must occur through numbered or ordered lists that you program into the display. 7. Format specifications in a METHOD over-ride those in a VARIABLE. 8. A HART communication defined in a METHOD occurs automatically. Others often require the end-user to push a button labeled 'send'. 9. During execution of a METHOD, there is no convenient way of having the hand-held repeatedly execute a loop in response to a key being held down or even repeatedly pressed. 10. During execution of a METHOD, an 'abort' will automatically be available via soft-key. There is no need to program this. 11. It is possible to define a MENU named "hot_key". This MENU becomes available when the user presses the hand-held hot key, which is one of the available function keys. There are two problems associated with the hot key. First, there does not appear to be any way of informing the user that the hot key is available and what it does. Second, the hot key MENU is unavailable during execution of a METHOD. 12. Most "pre-" and "post-read" METHODS that you might want to associate with a VARIABLE won't work. You will get a "non-scaling" error message. Apparently everyone has seen these and nobody knows why they happen or what can be done about it. 13. METHODs are generally not checked for errors during compiling of the DDL. They don't show up until you run the hand-held simulator. Slave Development Steps Suppose you make smart (microcontroller-based) analog X-meters (where X = flow, temperature, etc.) and now you want to make a HART version of the X-meter. Here are the steps to take. You might also consider joining the HART Communication Foundation as a first step, since you will eventually have need of them anyway. 1. Make a list of things your customers do with the existing X-meters. 40 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
41. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer 2. Of these things make a smaller list of things that are difficult to do because someone has to be sent to the X-meter site. Determine whether one or a series of HART transactions with an X-meter could reduce or eliminate this activity. 3. Determine whether any existing HART Universal or Common Practice commands could be used to implement or partially implement these transactions. If not, define one or more new commands (Device-Specific commands) that will be needed. Writing a DDL may help at this point by obviating missing commands. If there are more than a few new commands, write a specification that spells out in detail (down to the individual bits) what each one does. 4. Determine whether the HART communication will place too much demand on existing X-meter resources (memory, etc.) and what kind of resources will need to be added. You will need EEPROM to store things like the Slave's address. 5. Start hardware and software design of new HART X-meter. Devise or otherwise obtain a HART Master that can be loaded with your new HART commands. Begin assembling equipment for HART Conformance verification if it's not already available. Or else locate an outside company that is set up to run the tests. 6. Decide how your customers will talk to the X-meter (DDL and hand-held master, info in User Manual, Complete Software package that runs on PC). If software must be written, start now. 7. Complete the design, set up some HART networks in your own manufacturing area and start banging away on prototypes. The HART Communication Foundation has code to help you run tests. For example, it can send your device a message with a bit error to see if you catch it. 8. With HART X-meter apparently functioning as intended, run HART Slave Conformance tests. Or have tests run by outside company. Note: Some tests such as bit error rate are of questionable value if you have followed a relatively standard design spelled out in application notes and have not seen any reason to suspect non-conformance. Bit error rate tests are also difficult to do and are not adequately addressed in the HART Conformance document [2.3]. 9. Contact HART Communication Foundation for assignment of (and payment for) a Slave Manufacturer's code. This is a byte that becomes part of the long frame address of every HART Slave that you manufacture. You may not legally be able to claim HART compliance or use the HART trademark without this step. 10. Obtain other product approvals, do EMI tests, etc. 41 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
42. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer 11. Sell more X-meters. If this all seems obvious, that's great! It means your halfway there. Addressing Problems, Slave Commissioning, and Device Database The existing HART addressing scheme has several potential problems that are examined here. Only modern (rev 5 or later) Field Instruments (using 38-bit address) are considered. First, a Field Instrument can have any of about 275,000,000,000 possible addresses, which makes it impossible to determine the long addresses of Field Instruments after the network has been built and the Field Instruments installed. Even if there are only 2 Field Instruments on the network and Universal Command 0 is used to try to read the long address, the procedure can still fail because short addresses might be duplicated. This makes it almost a necessity, in all but point-to- point applications, to commission a Field Instrument prior to installation and to maintain a database of long addresses. Commissioning means powering up the new Slave on a bench network where it is the only Slave, and asking it for its long frame address. (Commissioning usually includes other things as well, such as assignment of a tag.) The long frame address is then added to a database of devices. The database will later be used when the Slave is put into service. Eventually, most such databases probably expand to include virtually all the Slave characteristics available through HART Universal Commands. Initially, however, they need only contain the address or the tag for each Slave. In most end-user applications commissioning and building the device database is probably done anyway. Therefore, the need to do it to satisfy HART is probably not much of a burden. Another problem involves the unique identifier. The first byte of the unique identifier is the manufacturer's ID. Since only one byte is allowed for this ID, it means that only 256 manufacturers can be so identified. Undoubtedly, if the number goes higher than this, some change to the protocol will be devised. Another possibility is that vendors will share an ID number and devise a serial number (last 3 bytes of unique identifier) scheme such that neither will ever duplicate the other's unique identifiers. Another problem, also involving the manufacturer's ID, is that only the lower 6 bits of this byte are used in the long address. This means that, even though each Field Instrument can have a unique identifier, it can't have a unique long address. If device vendors were to begin numbering their product lines at 0 or 1 and their serial numbers at 0 or 1 (which seems entirely reasonable), then there is a pretty good chance that the long addresses might be duplicated. The HART standards attempt to reduce this likelihood by requiring the product line byte (the second byte of the unique identifier) to be numbered in a specific way from higher to lower numbers for half of the possible manufacturer's IDs (first byte of unique identifier) and from lower to higher numbers for the other half. There are also 4 ranges of product line numbers and 4 ranges of manufacturer's IDs. Each range of manufacturer's ID numbers has a specific range of product line numbers. This is illustrated in Table 2-2 below. 42 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
43. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Manufacturer ID Range Of Acceptable Range Product IDs 0 to 63 (decimal) Start at 0 and increase Start at 239 and 64 to 127 decrease Start at 127 and 128 to 191 decrease Start at 128 and 192 to 255 increase Table 2-2 Allowed combinations of MFR IDs and product line numbers Finally, the addressing scheme creates a need for device vendors to register their devices with a registration body the HART Communication Foundation. This is a costly bookkeeping adventure that could just as well be done by end-users. The end-user already decides which vendor's device to buy, maintains an address database, and assigns a tag to each device. He could as easily assign the address and determine which DDL he needed, based on the device vendor and serial number. Bell-202: Bad News in Europe Telecommunication systems (phone utilities) use certain well-defined tones for administrative purposes. These tones fall within the voice band but are only effective if there is no energy present at other frequencies. The tones used in Europe are different from those of the United States. Unfortunately, one of those used in Europe falls into the range of 2130 Hz to 2430 Hz (see [2.4] for example). The Bell-202 frequency of 2200 Hz could appear as a pure tone within this forbidden band. Consequently, if Bell-202 equipment were to find its way onto a European phone grid, it could cause problems. Normally, European telecommunication regulations prevent the sale and distribution of incompatible equipment, so that this doesn't happen. So what does this have to do with HART? If HART communication is confined to private industrial networks, as is usually the case, then there is no association between HART and telecommunications. HART should be just as acceptable in Europe as anywhere else. It should, but it isn't always. We have experienced problems when using the term "modem" in instruction manuals, etc. Apparently this term has become almost universally associated with telecommunications. And in some instances this has led to seizure of HART equipment by authorities who did not understand its purpose. We found that we had to remove the term "modem" from some literature. Under certain circumstances it is possible to do HART communication over European phone lines. This is further discussed in the section entitled HART Bridges and Alternative Networks. 43 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
44. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Grounding and Interference An industrial environment can produce a variety of powerful electric and magnetic fields, as well as significant voltages between different "grounds". Most often they are at 50 Hz or 60 Hz and don't pose much of a threat to HART. However, sometimes they or their harmonics fall into the HART band (about 900 Hz to 2500 Hz). Since HART signals have a rather small amplitude, there is a possibility that these higher-frequency fields will disrupt signaling. Interference that would have no effect on an analog 4-20 mA signal (because of lowpass filtering in the controller) might be enough to destroy HART messages. Here we look at how to protect against the interfering fields and how ground potential differences can still manage to cause trouble. The circuit of a HART Field Instrument is typically contained inside of a metal case that is at earth ground. The circuit is isolated from the case, except for feedthrough EMI filters and an inevitable small capacitance from the circuit to the surrounding case. There is no single wiring scheme that is best at reducing interference in all circumstances. But for Field Instruments of the type described, the following is usually recommended: The Field Instrument and Controller (Master) are connected by a shielded twisted-pair cable. The shield is grounded at the Controller end and left open at the Field Instrument end. The shield prevents electric fields from coupling into the signal conductors, while the twisting attempts to reduce the effects of magnetic coupling by forcing equal coupling into both sides of the pair. The shield is left open at the Field Instrument end to avoid the conduction of ground currents. A ground current on the shield couples magnetically to the conductors of the twisted pair. These ideas are more thoroughly explained in [2.5]. A ground current can result from making a connection between two "grounds" that are at different potentials. The different potentials can arise from huge currents (amps) flowing through cables that separate the grounds. A ground current also arises when a conductive loop is formed in the presence of a strong (varying) magnetic field. Even when the wiring rules are followed, this difference in ground potentials can cause interference. To see this, consider the circuit of figure 2.7. The cable shield is connected at the Controller ground, according to recommended practice. A voltage Vin exists between the Controller ground and Field Instrument ground. The circumstances that create Vin usually also cause a relatively large impedance in series with it. This is represented by Zs. 44 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
45. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 2.7 HART Circuit Showing Ground Potential Difference At HART frequencies we can make the following assumptions: 1. The cable can be considered a lumped circuit consisting mainly of capacitance. 2. The Circuit-to-case Capacitance inside the Field Instrument is negligible. 3. The impedance of the box labeled "Circuit" across the Field Instrument terminals is practically infinite. Note that one side of the twisted pair is grounded at the Controller. Then the equivalent circuit is as in figure 2.8. 45 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
46. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer Figure 2.8 Equivalent Circuit for Effect of Ground Potential Difference The interference is the voltage Vout developed across the Field Instrument terminals. It is related to Vin by If we assume that the EMI feedthrough filters have nearly equal capacitance so that C1 = C2, this becomes Quite often Zs is large (small capacitance, large resistance). Then sC2Zs >> 1 at HART frequencies and the expression becomes approximately In a properly constructed HART network the pole frequency in this latest expression will be above the HART band so that within the HART band the expression reduces to 46 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
47. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer As an example, suppose that Zs is due primarily to a capacitance with a value of 1000 pf. And let Vin = 100 volt at 2 kHz, R = 250 ohm. Then the magnitude of Vout = 0.31 volt. This would certainly destroy the HART signal. One way to remove the effect of Vin is to connect the cable shield to the instrument case at the instrument end. This gets rid of or reduces the effect of Vin, but may cause other problems. Some experimenting may be necessary. A better way is to use a separate ground conductor between the controller and instrument grounds. This new ground conductor can be separate from the network cable. Or it can be built into the network cable. Special cables, having both an inner and outer shield, are made for this purpose. HART and Intrinsic Safety HART was always intended to be retrofitted to existing process loops, including those that are intrinsically safe (IS). The relatively low HART signal level and superposition of the HART and analog signals are the result. Also, an individual HART Field Instrument is not too different from a non-HART Field Instrument in terms of power consumption and equivalent capacitance and inductance. If we look only at signaling and Field Instrument design, we might conclude that combining HART with IS is not a problem. But when we look at HART in its broader context, there are problems. They are, or result from, (1) the need to transmit HART through IS barriers, (2) topology differences between HART networks and conventional current loops, and (3) the existence of a new device the hand-held communicator. The implications of combining HART and IS and their effect on each other will be considered next. The simplest form of IS HART network is one with just a single 2-wire Field Instrument (a Point-to-Point HART network). The Field Instrument and cable are located in the hazardous area and a barrier separates the safe and hazardous areas. This is illustrated in figure 2.9. Figure 2.9 Simplest IS Arrangement 47 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng
48. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer We assume for the moment that nothing else will ever be connected to the network at the hazardous side. However, there may be a HART Master at the safe side. There are two things to consider here: First, the barrier, Field Instrument, and cable must represent a compatible combination. That is, the Field Instrument must consume less voltage and current than allowed by the barrier and the cable+Field Instrument must represent less C and L than allowed by the barrier. A HART Field Instrument consumes about the same voltage and current as a non-HART Field Instrument. And the addition of HART components (such as receive filter, etc.) don't add much C or L. Thus, the single HART Field Instrument is comparable to a conventional process instrument and presents no unusual difficulties in achieving IS or in compatibility with other IS components. (Note: We will neglect heating effects on small components, even though this can be an important consideration in the design of the Field Instrument. We assume that the added components needed for HART affect IS only through added C or L or changes in circuit topology.) Second, the barrier must pass the HART signal with a minimum of attenuation and distortion. Barriers can affect the HART signal in a variety of ways, depending on the type of barrier and on associated components. The HART Physical Layer Specification prescribes several tests that barriers must satisfy. These tests are all related to insuring that there is sufficient signal passed in each direction through the barrier; and that the signal is not distorted by the barrier. In a conventional resistor-zener diode barrier, it is theoretically possible to operate too near the zener clamp voltage so that the HART signal excursions are clipped. Usually, however, the peak HART voltage is on the order of 0.25 volt; and barrier ratings are conservative enough that clipping doesn't occur. (Note that if the barrier working voltage is too near the clamp voltage, there would be too much leakage current. Analog signaling would become inaccurate.) Another effect of a resistor-zener barrier is a flat attenuation of a voltage signal. A barrier with a 300 ohm resistance, used with a 250 ohm current sense resistor, creates a divider that will attenuate by 0.45. The Controller will see a HART signal voltage that is 0.45 of the voltage across the network. Yet another effect of the standard resistor-zener barrier is a lowpass filtering caused by junction capacitance of the zener diodes. Capacitances of several thousand pf are possible. However, this often just adds to a much larger cable capacitance and doesn't need to be taken into account. An important consideration for active or repeater barriers is that they do not remove the HART signal with a lowpass filter designed primarily to pass the analog 4-20 mA signal. Another is that they do not chop (to create AC) at a frequency that is in or near the HART band. These special barriers must often be specifically designed to work with HART. The next step up in complexity is an IS HART network identical to that described previously, except that it contains two or more multi-dropped Field Instruments, as illustrated in figure 2.10. 48 Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng