Giáo trình Chuẩn truyền tin Hart trong đo lường và điều khiển tự động mạng công nghiệp

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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 
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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.
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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 
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The 20C12 has been discontinued, but has been cloned by Smar. The Smar version is the 
HT2012. 
The 20C15 is a 20C12 with increased functionality. This was designed by Gabriel 
Maalouf of Rosemount Inc., David Tetzlaff of Rosemount Inc., and Stephen Anderson of 
Analog Services, Inc. Symbios was recently acquired by LSI Logic. There seems to be 
some question as to whether the 20C15 will survive. A spokesman for Rosemount Inc. 
says that they (Rosemount) have designed a new chip to replace the 20C15; and that it is 
pin-compatible with the 20C15. Again, this new chip will be available to HART 
designers through a major semiconductor vendor. 
Schoppe & Faeser makes a HART Modem chip (no model number available) that is 
similar to the 20C15. That is, it has filters and waveshaping and on-board oscillator and 
consumes about 0.5 mA at 3 V. 
The June, 1998 issue of Control Magazine estimated that there were about 5 million 
HART-compatible instruments in use. At about $4 to $5 per chip, this represents modem 
chip sales of 20 to 25 million dollars. 
Because Bell-202 is still being used for Caller ID and other special telecom applications, 
other Bell-202 chips are also available. An example is the Silicon Systems 73K302. 
These were not designed for HART. They are not low-power and are somewhat 
cumbersome for use with HART. The Silicon Systems chip, for example, is intended to 
be connected to the micro-controller data bus and has several registers that must be 
written to configure and control it. 
A recent article [Holland, S., "Low-Cost Software: Bell-202 Modem," Circuit Cellar, pp. 
12-19, June, 1999] describes the implementation of a Bell-202 Modem entirely in the 
software of an embedded controller chip. The software is also apparently available for 
download from Circuit Cellar's web site. This software modem uses a very fast (50 MHz 
clock) microcontroller chip made by Scenix Semiconductor. The author says that there is 
still some processing power left over. We suspect that chips running much more slowly 
could also do the job. 
On-Line Data Sheet for 20C15 
By Analog Services, Inc. 
The 20C15 HART Modem IC is made by LSI Logic. Unfortunately, they provide only 
paper copies of the 20C15 data sheet. It does not show up on their web site (at 
 We have created here our own data sheet for the 20C15 to 
make it easier for HART designers to get this information on the internet. To purchase 
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the 20C15 integrated circuit or to get a complete copy of the LSI Logic data sheet, contact 
LSI Logic at 
LSI Logic Corp. 
1551 McCarthy Blvd 
Milpitas, California 95035 
800.433.8778 (in the United States) 
408.433.8000 (outside the United States) 
Fax 408.433.8989 
Description 
 The 20C15 is a low-power, single-chip, 1200 bps modem specifically designed to 
satsify HART Protocol requirements. It contains nearly all of the circuitry needed to add 
HART communication capability to analog 4-20 mA field instruments. It is specified for 
use at supply voltages of 3.3 volt and 5.0 volt and over the industrial process control 
temperature range of -40 C to 85 C. For lowest possible power consumption the 
modulator portion of the chip is shut down while the demodulator is operating and vice-
versa. A crystal or ceramic resonator operating at 460 kHz may be used with the chip to 
generate a 460 kHz clock. Or a 460 kHz clock may be supplied to one of the 20C15 
pins. An external bandgap voltage reference is required to set up reference voltages and 
the device operating current. The 20C15 includes transmit waveshaping and part of the 
receive filtering required by HART. The 20C15 is supplied in a 28-pin PLCC package. 
 Pinout 
Pin 
Number Name In/Out Description Comment 
1,5,7,8,9,14 Test Connect To Ground 
2,3,4,25,28 Test Leave Open 
6 /RESET In Reset (Low To Reset) Connect To R & C 
10 OTXA Out Transmit Out 
11 IAREF In Reference Voltage 
Connect to 1.235 volt for 3.3 
volt supply,
Connect to 2.5 volt for 5.0 
volt supply. 
12 ICDREF In 
Carrier Detect 
Reference 
Voltage 
Set at 80 mV below IAREF 
13 OCBIAS Operating Current Set Connect 500k to gnd for 3.3 volt supply,
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Connect 1 Meg to gnd for 5.0 
volt supply. 
15,22 Power Pin Connect to 3.3 volt or 5.0 volt.
16 IRXA In Input to Receive Gain Stage See Circuits 
17 ORXAF Out Output from Rcv Gain Stage Low Impedance Point 
18 IRXAC In Receive Filter Continued 
19 OXTL Out Crystal Oscillator Output 460 kHz 
20 IXTL In Crystal Oscillator Input Or External Clock Drive 
21 Ground 
23 INRTS In Request-to-Send Selects between XMIT and RCV 
24 ITXD In Transmit Data 
26 ORXD Out Receive Data 
27 OCD Out Carrier Detect 
Operation 
 Modulator (Transmit): 
 The modulator is operating (and the demodulator is shut down) whenever /RTS 
(INRTS) is low. When TXD (ITXD) is high the modulator output at TXA (OTXA) will 
be a trapezoidally shaped wave at nominally 1200 Hz. When TXD is low the modulator 
output is nominally 2200 Hz. TXA (OTXA) is usually AC-coupled to an amplifier or 
buffer stage. 
 The output voltage levels at TXA depend on the reference voltage applied at IAREF. 
Let VQ, VMIN, and VMAX be as defined in figure 1. 
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Figure 1 
That is, VQ is the quiescent (no modulation) voltage at TXA, and VMAX - VMIN is the 
pp signal voltage swing. VQ, VMIN, and VMAX are given by 
 where VREF is the voltage at pin IAREF. For VREF = 1.235 volt, VQ is 0.5 volt and 
the signal swings from 0.25 volt to 0.75 volt. In a HART Master this is the correct pp 
voltage to be applied to the network. But the OTXA pin does not have enough drive 
capability to drive a HART network directly. A buffer amplifier is usually needed. In a 
field instrument, the 0.5 volt pp OTXA output is usually converted to 1 mA pp. 
 Demodulator (Receiver): 
 The demodulator is operating (and the modulator is shut down) whenever /RTS 
(INRTS) is high. The received signal is first applied to a bandpass filter. Part of this 
filter is off-chip. This is necessary to reduce interference to a level that is within the 
20C15 supply rails. The bandpass filter consists of a 4-pole highpass filter and a single-
pole lowpass. When used with the suggested passive components the overall filter has a 
gain of 1.6 in the passband. Other pins that are part of the receive filter are IRXA, 
ORXAF, and IRXAC. 
 The filter output is applied to two comparators -- one to slice the signal and produce a 
logic-level version of the received FSK and a second to act as a carrier detect. The 
reference for the first comparator is the reference voltage applied at IAREF. The 
reference for the second comparator is applied at ICDREF and is normally set to be 80 
mV below the level at IAREF. The carrier detect comparator is therefore tripped if the 
filter output swings low by 80 mV peak or more. Or, since the filter has a passband gain 
of 1.6, the carrier detect comparator will trip if the input swings by 50 mV peak (100 mV 
pp) or more. These values are chosen to satisfy the HART requirement that the carrier 
detect be OFF for input signals 120 mV pp. 
 Logic circuits following the carrier detect comparator are used to decide whether 
carrier is present. If the input has sufficient amplitude to trip the carrier detect comparator 
on each of 3 or 4 consecutive cycles, then carrier is present and OCD goes high. Once 
carrier is present, if the input signal fails to trip the comparator for a time of about 3 or 4 
cycles, then carrier is no longer present and OCD goes low. 
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 The logic-level FSK out of the first comparator is applied to a logic block that 
generates a high level if a frequency below 1700 Hz is present and a low level if a 
frequency above 1700 Hz is present. The output of this logic block is gated with OCD to 
form the output RXD (ORXD). If carrier is not present the RXD output is always low. 
If carrier is present, then RXD equals the output of the logic block. 
 RXD (ORXD) is normally connected to a UART. The UART sees a low level as an 
assertion of RXD. Therefore, when no carrier is present (no reception occuring) RXD is 
asserted. In some cases this will appear to be an extra character at the end of a message. 
The software that services the UART must handle this condition appropriately. 
 Clock: 
 A 460.8 kHz clock is required. This can be generated by connecting a crystal or 
ceramic resonator across pins OXTL and IXTL (see Circuits). An external clock source 
can also be used. Connect the external source to OXTL and connect IXTL to ground. 
 Parameters 
 Clock: 460.8 kHz, Tolerance 1%. 
 Power Supply Current: 400 uA max. (3.3 volt supply), 600 uA max. (5.0 volt 
supply). 
 (NOTE: This is the current during receive. It is about 100 uA to 200 uA less during 
transmit. 
 This can sometimes be used to advantage.) 
 Operating Temperature Range: 0 C to 70 C. (NOTE: This would seem to 
preclude using 
 this part in industrial control applications, which usually require -40 C to 85 C. 
However, this 
 specification is apparently the result of problems in testing the part outside of 0 C to 
70 C; and 
 doesn't reflect the part's true capability. It is designed for and is being widely used 
in equipment 
 specified for -40 C to 85 C.) 
 Reset Minimum Pulse Time: 2 microsecond. 
 Transmit Output Drive Capability: Needs minimum of 30 kohm. 
Circuits 
 Field Instrument, 3.3 Volt Supply: 
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Figure 2 
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 Field Instrument, 5.0 Volt Supply: 
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Figure 3 
Package 
Figure 4 
Package -- 28 Pin PLCC 
 References 
1.1. Control Magazine, "You Gotta Have HART ...", June, 1998, Putman Publishing Company 
555 Pierce Road, Suite 301, Itasca, Illinois 60143 
1.2. Control Engineering Magazine, "4-20 mA Transmitters Alive and Kicking," October 1998. 
1.3 ARC Study as reported in I&CS Magazine, "Pressure Transmitter: A Unit For Every 
Application," November, 1999. 
1.4. Rosemount Inc., 12001 Technology Drive, Eden Prairie, MN 55344. 
1.5. HART Communication Foundation, 9390 Research Blvd., Suite II-250, Austin, TX, 78759 
1.6. HART Field Communications Protocol: A Technical Overview, HCF LIT 20, rev. 2, 1994, 
HART Communication Foundation. 
1.7. Fieldbus Standard for Use in Industrial Control Systems, ISA SP-50, Instrument Society of 
America. 
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 2.1 Questions on real-time programming in Programming Questions, 
2.2 Ramamritham, K., et. al., "Using Windows NT for Real-Time Applications: Experimental 
Observations and Recommendations," 1998 IEEE Real-Time Technology and Applications 
Symposium. 
2.3 HART Physical Layer Test Procedure, HCF_TEST-2, Revision 1.0, 1995, HART 
Communication Foundation. 
2.4 BS 6305:1992, "General Requirements for Apparatus for Connection to Public Switched 
Telephone Networks Run By Certain Public Telecommunications Operators," British Standards 
Institution, Linford Wood, Milton Keynes, MK14 6LE. 
2.5 Ott, H.W., Noise Reduction Techniques in Electronic Systems, Wiley, 1976. 
2.6 EN50082-2, "Electromagnetic Compatibility Generic Immunity Standard, Part 2: Industrial 
Environment". 
2.7 ENV50141:1993, "Electromagnetic Compatibility -- Basic Immunity Standard -- 
Conducted Disturbances Induced by Radio-Frequency Fields -- Immunity Test." 
2.8 Stahlings, W., ed., Tutorial: Local Network Technology, IEEE Computer Society Press, 
1983. 
2.9 DeviceNet Standard. Originally developed by Allen-Bradley, now Open DeviceNet 
Vendor's Association (ODVA) ODVA, PMB 499 * 20423 State Road 7 #F6 * Boca Raton, FL 
33498-6797 USA. 
2.10 NT International, Minneapolis, MN, 612-502-0200. 
3.1 MATLAB, Mathworks, Inc., Natick, MA. 
3.2 ISO/IEC 7498-1:1994(E), "Open Systems Interconnection -- Basic Reference Model: The 
Basic Model," ISO/IEC, Case Postale 56, CH-1211 Geneva 20, Switzerland. 
3.3. Bennet, W.R., and Rice, S.O., "Spectral Density and Autocorrelation Functions Associated 
with Binary Frequency-Shift Keying", Bell System Technical Journal, Sept., 1963. 
3.4 Proakis, J.G., Digital Communications, McGraw-Hill, 1983. 
3.5 Shanmugam, K.S., "Digital and Analog Communication Systems," 1979, John Wiley & 
Sons. 
3.6 Private Communication with Doug Arntson of Rosemount Inc. 
3.7 Private Communication with Jay Warrior of Rosemount Inc.