Design of Intelligent Transmitter based on HART Protocol

An Guochen, Meng Zhiyong, Ma Hongtao, Sui Bingdong Institute of information science and engineering

Henbei university of science and technology Shi Jiazhuang, China

Angch@hebust.edu.cn

Abstract—In this paper, on the HART (Highway Addressable Remote Transducer) protocol and the system project gives a brief introduction, and then expatiates the hardware and software design of the intelligent transmitter based on HART protocol. Proposed a set of practical intelligent transmitter solution. The ultra-low-power consumption microcontroller is adopted as the control core of the system. The power consumption demand is considered continuously during device selection, circuit design and program design process. The system low power consumption design is realized, and meet the demand of the loop powered intelligent transmitter. while the system realized all of the common commands and part of the special commands of the HART protocol, matched the general characters of the HART intelligent transmitter.

Keywords-HART protocol; intelligent transmitter; microcontroller; low power consumption

I. INTRODUCTION The intelligent transmitter is normally provided by a

combination of sensors and microprocessors. It takes full advantage of the microprocessor computing and storage capacity, can process sensor data, including the condition to the measured signal such as filtering, amplification or A / D conversion. the data show, automatic calibration and automatic compensation. With the rapid development of science and technology, intelligent transmitter also in the process of fast upgrade. The current transmitter will achieve all-digital, which means that will abolish the traditional analog signal transmission, and require each have a intelligence and digital communications capabilities, and also real-time access to all aspects of the field devices , such as measured values, environmental parameters, equipment operation and equipment calibration, self-diagnosis, alarm information, fault data. In addition, the control algorithms completed by the main controller can also be distributed to each field device, which greatly improved system reliability and flexibility.

The key of field-bus technology system lies in its openness, emphasis on consensus and to comply with the standard. Breaking the traditional manufacturer independent standard situation to ensure that products from different manufacturers can be integrated into a single field bus system. HART protocol is widely recognized industry standards, the technology spread very quickly, in fact, all the major equipment vendors offer products HART protocol. HART protocol is the only backward-compatible with the

smart meter solutions, a cable can also pass on 4 ~ 20mA analog signals and digital signals.

In this paper, on the HART protocol and the system project gives a brief introduction, and then expatiates the hardware and software design of the intelligent transmitter based on HART protocol. Proposed a set of practical intelligent transmitter solution.

II. SYSTEM PROJECT DESCRIPTION A. HART Protocol Description

HART Field Communications Protocol extends the 4 ~ 20mA standard in intelligent measurement and control instruments based on the enhanced communications capabilities. As an important step in the evolution of process control, HART protocol promoted a significant revolution in process instrumentation functions. HART-smart devices, without interfering with 4 ~ 20mA analog signals, while allowing both-way digital communication. 4 ~ 20mA analog signal and HART digital communication signal can be transmitted at the same time in a pair of lines. The main variables and control information is provided by 4 ~ 20mA analog signal transmission, while the other measurement, process parameters, device configuration, calibration and diagnostic information are accessed through the HART protocol in the same line, the same time. The main difference between HART protocol and the other open digital communication technologies is that HART compatible with existing systems.

HART protocol uses Bell202 frequency-shift keying (FSK) standard, the low-level digital signal acts as the superposition onto the 4 ~ 20mA analogy signal. Digital continuous phase FSK signal will not affect the 4 ~ 20mA signal. The frequency of 1200 Hz is representative of logic ‘1’, while 2200 Hz for logic ‘0’. The HART communication signal is shown in figure 1.

Figure 1. HART communication signal

2010 International Conference on Intelligent Computation Technology and Automation

978-0-7695-4077-1/10 $26.00 © 2010 IEEE

DOI 10.1109/ICICTA.2010.167

40

HART FSK signal mode allows two-way digital communications, which makes more information read from or write to the intelligent field meter can be transmitted. HART protocol has 1200 bits / s communication speed, which allows the main station updating the digital data from the field devices twice per second or more without interruption to 4 ~ 20mA signal.

B. System Project Description The intelligent transmitter consists of the following main

parts: MCU part, D / A circuit part, modulation and demodulation circuit parts, A / D circuit part and the keys / LCD display part. The system block diagram is shown in figure 2.

Figure 2. System block diagram

MSP430F149 which is produced by TI corporation was adopted as the control core of the system, which is responsible for the initialization and control functions to various components of the system in order to complete the system's functions. The AD421 was adopted as the D/A converter, which can not only complete digital to 4 ~ 20mA current signal conversion, but also provide the +3 V, +3.3 V, or +5 V power supply of the system because of its internal voltage regulator coupled with a small number of external components. The HT2012 was adopted as the HART protocol modem in order to achieve HART signals to send and receive and realized HART communication. The 16-bit low-power A/D converter AD7714 produced by AD corporation was selected to achieve multi-channel analog signal acquisition work. In addition, temperature acquisition module, keys and LCD display circuit, serial interface and other circuit are design to increase the system functionality and flexibility.

III. SYSTEM HARDWARE DESIGN From several key aspects of the system such as the

system low-power design, microcontroller circuit design, D/A converter circuit design and modem circuit design, the more detailed description will be gave.

A. System Low Power Consumption Design Intelligent Transmitter generally have two kinds of power

supply mode: external power supply or loop-powered. In the first mode an external power supply was needed to power the transmitter module. In the loop power supply mode the power is took directly from the 4 ~ 20mA current loop. In this way, it is more convenient of equipment installation, commissioning and maintenance. Therefore, most of the smart transmitters adopt loop power supply mode. Not only to ensure the system work, and do not affect the loop analog

signal transmission. This requires the transmitter can not be larger than the current consumption of 4mA. In addition, according to the HART protocol, the transmission of digital signals depends on two frequencies of 2200Hz and 1200Hz representing ‘0’ and ‘1’ plus on the analog signal. The signal amplitude of the frequency is 0.5mA. For the positive and the negative half-cycle of the sine wave is symmetrical, so its mean value is zero, so that the analog signal in the loop remains unchanged. When the current is 4mA, the loop on the instantaneous maximum value is 4.5mA and the minimum value is 3.5mA. If the transmitter current is more than 3.5mA, a negative half-cycle AC signal distortion will be result in, thus making the HART communication loop current is more than 4mA, what is not allowed on the transmitter design. To sum up, it is required that the system current consumption can not be larger than 3.5mA.

As noted above, on designing the loop-powered transmitter, its power consumption problem is especially sensitive, so in the system hardware design process the system's power consumption was firstly considered, especially in the choice of the various parts of the device in the system. For example the choice of MCU, the MSP430F149 produced by TI corporation was adopted, which not only have strong internal 16-bit processor core, built-in ROM, RAM, FLASH, timers, parallel port, serial port pairs and the phase-locked loop and other modules, but also industry-recognized ultra-low power microcontroller, the lowest standby current of only 1.6uA. Properly select the CPU clock frequency, the current can be controlled around 0.8mA, which is fully meet the system's low power requirements. The AD421 produced by AD corporation was adopted as the D/A converter, which is composed by internal voltage regulator modules and 4 ~ 20mA current output circuitry. Its maximum quiescent current of only 750uA. The HT2012 produced by SMAR corporation was adopted as the communication modem, which can complete digital signal modulation, the modulated signal demodulation and carrier detection and other functions. And it has very low power consumption, typically to 40μA. The AD7714 produced by AD corporation was adopted as the A/D converter This is a low-power 16-bit serial ADC, with pre-amplifiers and filters, its normal power consumption is very low, about 350uA. The system design can guarantee that the current consumption below 3.5mA and meet the general loop-powered smart transmitter power requirements.

B. Microcontroller Circuit Design The MSP430F149 is the processing core of the whole

transmitter, which not only can satisfy the system low power requirements, but also be very powerful with all kinds of on-chip resources, built-in 60KB of FLASH, and 2KB of RAM. The system storage space requirements can be meet without external memory to be expanded, thus saving system cost and power consumption. A 12-bit precision A/D converter is build in MSP430F149, which can alternate the external A/D converter for saving cost and power consumption when lower precision was required. In addition, double serial ports, PWM output, FLL, and reach to 48 I/O ports are integrated in MSP430F149 for easy system development.

41

The external 1.8432MHz crystal was adopted as main clock of the microcontroller, while an increase of 32.768KHz crystal clock is used as a secondary standby clock, in order to further reduce power consumption. Also the basic manual and power-on reset circuit and JTAG debug interface of the microcontroller system was designed. C. Loop Powered DAC Circuit Design

AD421 is a kind of single chip high-performance digital to analog converter (DAC) produced by AD corporation, which is power by loop current, with serial interface for 16-bit digital signal input and 4 ~ 20mA current output, and enabling low-cost remote intelligent industrial control. AD421 contains internal voltage regulator to provide +5 V, +3.3 V output voltage, which can be used as the system power supply, also contains +1.25 V and +2.5 V reference supply, which can be used for their own or other circuits. AD421 using Σ-Δ DAC structure to ensure 16-bit resolution and the integral linearity error of ± 0.001%, the offset error of ± 0.1%, the gain error of ± 0.2%. AD421 is fully compatible with standard HART protocol or other similar FSK. The standard three-wire serial interface can be running at 10Mbps, and can be connected to the general-purpose microcontroller easily.

Loop-powered D/A converter circuit is shown in Figure 3, the selection of the output voltage of 5V or 3.3V can be achieve by the four jumpers of W1~W4. Q1 using N-channel JFET ND2020L, Q2 using BC337-type transistor in order to achieve voltage regulation. AD421 voltage output of 1.25V just as AD7714 reference voltage input, eliminating the AD7714 reference voltage generating circuit. AD421 voltage output of the 2.5V reference voltage connected to their input as its own reference voltage. Microcontroller control the AD421 for 4 ~ 20mA current signal generation through CLK_AD421, LATCH_AD421, DATA_AD421 three pins. The FSK waveform signal generated by the modem coupling to the AD421 current loop through capacitor C13, which ensure the transmission of the digital signals and the analog signals at the same time.

REFOUT11

REFOUT22

REFIN3

CLOCK6

LATCH5

DATA7

COM9

C112

C211

C310

LOOPRTN 8

COMP 14

DRIVE 13

LV4

VC

C16

BO

OST

15 U3AD421

0.01uFC11

1000pFC121KR3

0.01uFC8

GND

0.01uFC2

GND

DVDD

+1.25V

4.7uF

C9GND

CLK_AD421LATCH_AD421DATA_AD421

2.2uFC1

L1

L2

0.16F

C13

0.5uF

C15

0.01uF

C14GND

HT2012

LOOP+

LOOP－

W1

VCC

W2

W4GNDW3

+5V

Q2NPN

Q1NPN

12

P2

Header 2

LOOP－LOOP+

Figure 3. Loop-powered D/A converter circuit

D. HART Modem Circuit Design HT2012 produced by SMAR corporation is a kind of

CMOS low-power modem, which working in the Bell 202 standard modem. HT2012 chip is used to achieve the HART protocol communication signal modulation and demodulation process, which can provide the HART communications capabilities for process control instrumentation or other low-power devices. HT2012 chip can operation in 3.3V or 5V power supply with crystal frequency of 460.8KHZ. It can be designed as the modulation and demodulation circuits With external amplification circuit and filter circuit together.

The HART modem circuit is shown in Figure 4, the circuit is divided into three parts: a modem, input filter and output shaping circuit. The modem mainly achieves the signal modulation and demodulation. Input filter circuit usually contains band-pass filter circuit and the square-wave shaping circuitry to generate a square wave signal and sent to the HT2012. Band-pass filter circuit is not only used to reduce noise in the input signal, but also used to eliminate high-frequency signal interference. Square-wave shaping circuit is a zero-crossing detection comparator. The comparator make the jump edge of the signal be steep according to the setup voltage, which can make it easy to deal with for digital circuits.

The role of the output shaping filter circuit is like a current to voltage modulator to reduce the visible edge hopping and eliminate the high-frequency interference.

In addition, using the clock frequency programmable features of the microcontroller, sending the 4 divided frequency of the main clock to the HT2012 as the 460.8 KHz frequency input, thereby eliminating the trouble of custom non-standard crystal.

6.2nFC21

INRTS12

OCD3

ITXD14

ORXD8

IRXA 4

OTXA 11

I460k16

VDD1

VSS 9O19.2k2

ITEST0 13

ITEST1 6

U8

HT2012

HT2012

GND

14

83

2U7A

TLC27L2ACD

48

5

67

U7B

TLC27L2ACD

100pFC16

100pFC18 374K

R4

2M2R5

8

3

2

4

1

U6ATLC3702CD

210KR84M7R7

33pFC19

100KR6

LOOP+

374KR11

374KR14 221K

R12

221K

R13

100pFC22

100pF

C20

DVDD

GND

GND

DVDD

I460k

DVDDDVDD

DVDD

GNDGND

GND

DVDD

INRTSOCD

ITXDORXD

Figure 4. HART Modem circuit

IV. SYSTEM SOFTWARE DESIGN The system software can be divided into two parts, which

are measuring and control software part and communication software part. measuring and control part includes data acquisition, data processing, control algorithms, output control and self-diagnosis section. The software determine the type of sampling signal, automatically set the sampling magnification, to select the appropriate non-linear

42

compensation function for different sensor types through the configuration information.

The communication software part is the software implementation of the data link layer and application layer of HART protocol which is the key of the entire software design. And the interoperability of the HART transmitter can be reflected in here. Because HART communication based master-slave mode, so the transmitter takes a response only when the master inquiring (except for burst mode). In order to ensure real-time communication, the serial port interrupt was adopted for the communication software.

The realization of the HART data link layer is primarily through the serial receive and transmit interrupt subprogram. In the HART communication process, usually sending command frame by the master, after the smart transmitter receiving the command frame through serial port, the microcontroller start the corresponding data processing and then send the response frame to the transmitter buffer and trigger the transmitter interrupt. Thus a exchange of a command is completed.

In the HART application layer, the received command frames were translated, and then corresponding command processing was achieved according to their indexes, such as modify the measuring range, modify the units, modify the damping time, etc. Finally, the response frame according to a certain format was send into the buffer and send back through the serial port interrupt.

V. CONCLUSION There is great development between the intelligent

transmitter based on HART protocol with traditional field

instruments or a smart field instrument. The increase of two-way digital communications capability makes it more popular in the industrial process control systems, instrumentation management system.

The ultra-low-power microcontroller was adopted as the control core of the system. In the processes of devices choice, circuit design and software design, the system power requirements was considered at all times. So the system ultra-low-power design was achieved and meet the loop-powered smart transmitter power requirements. The system supports all of the common commands and part of the special commands of the HART protocol, matched the general characters of the HART intelligent transmitter. While a serial interface was added to communicate with PC and realized temperature and measurement upload, which is easy on-site controlling and debugging.

REFERENCES [1] HART Communication Foundation, “Hart Field Communications

Protocol Application Guide”, 2000. [2] Shen Jianhua, Principle and Application of MSP430 Family 16-bits

Ultralow Power Consumption Microcontroller, TsingHua University Press, Bei Jing, 2004

[3] Li Chao, An Guochen, “Hardware Design of Intelligent Temperature Transm itter Based on HART Protocol”, Instrument Technique and Sensor, 2008(5), 55-57.

[4] Li Chao, An Guochen, “Software Design of Intelligent Temperature Transmitter Based on HART Protocol”, Instrument Technique, 2007(11), 53-54.

43