First, HART agreement synopsis
HART (Highway Addressable Remote Transducer), the addressable long-distance sensor high speed channel’s opening communication protocol, is American Rosemen Corporation one kind which promoted in 1985 uses between the scene intelligence measuring appliance and the control room equipment’s communication protocol.
The HART installment provides has the relatively low band width, the moderate response time’s correspondence, underwent more than 10 years development, the HART technology in overseas was very already mature, and has become the global intelligence measuring appliance’s industrial standard. The HART agreement uses based on Bell202 the standard FSK frequency-shift keying signal, superimposes the scope on the low frequency 4mA~20mA simulated signal is the 0.5mA audio frequency digital signal carries on the bidirectional digital communication, data transfer rate is 1.2Mbps. Because the FSK signal’s mean value is 0, does not affect transmits for the control system simulated signal size, has guaranteed with the existing analogous system’s compatibility.
In the HART agreement correspondence, the main variable and the control information by the 4mA~20mA transmission, in the situation which needs, other survey, process parameter, equipment configuration, calibration, diagnostic message through HART agreement visit.
What the HART correspondence uses is the half-duplex mailing address. The HART agreement refers to ISO/OSI (open system interconnection model), has used its simplification three model structure, namely first physical level, second data link layer and the seventh application layer.
First: Physical level. Has stipulated the signal transmission method, the transmission medium, to realize the analog communications and the digital communication simultaneously carries on and does not disturb mutually, the HART agreement uses frequency-shift keying technology FSK, namely iterates a frequency code on the 4mA~20mA simulated signal, the frequency code uses the Be11202 international standard, the digital signal transmission baudrate hypothesis is 1200bps,1200Hz represents logic “0″, 2200Hz represents logic “1″, signal peak-to-peak value 0.5A, as shown in Figure 1.
The correspondence medium’s choice regards the transmitting range length to decide. Usually uses double twists when the coaxial cable takes the transmission medium, the maximum transmitting range may achieve 1500m. The line total impedance should in 230Ω~1100Ω.
Second: Data link layer. Had stipulated the HART frame’s form, realizes the establishment, the maintenance, the end link communication function. The HART agreement basis redundant error-detecting code information, uses the automatic repeat request transmission mechanism, eliminates, because the line noise or other disturbances cause the data communication makes a mistake, realizes the communication data non-mistake transmission.
The scene measuring appliance must carry out the HART instruction, the operand must conform with the size which assigns. Each independent character including 1 outset position, 8 data positions, 1 parity check position and a stop position. Because the data is whether there is inconstant with the length, therefore the HART data’s length is also is dissimilar, the longest HART data contains 25 bytes.
Third: Application layer. For the HART order collection, uses in realizing the HART instruction. The order divides into three kinds, namely general order, ordinary order and special-purpose order.
Second, scene measuring appliance HART agreement long-distance correspondence hardware design
Some scene measuring appliance’s HART agreement part mainly completes the digital signal to the simulation electric current signal transformation, and realizes visit to the main variable and the survey, the process parameter, the equipment configuration, the calibration and diagnostic message. Figure 2 is the HART agreement connection module structural design diagram.
The HART correspondence part mainly realizes by the D/A transformation and Bell202 MODEM and the adjunct circuit. And, the D/A transformation function is transforms directly the digital signal the 4mA~20mA electric current output, outputs the main variable. Bell202 MODEM and adjunct circuit’s function is to superimposes carries on after the 4mA~20mA ring circuit’s signal the bandpass filter enlarge, if the HART correspondence unit examines the FSK frequency-shift keying signal, by Bell202 MODEM the 1200Hz signal demodulation is “1″, the 2200Hz signal demodulation is “0″ the digital signal, hands over the MCU, MCU receive order frame through the serial port correspondence, makes the corresponding data processing. Then, MCU produces the reply frame which must send back, the reply frame digital signal modulates corresponding 1200Hz and the 2200Hz FSK frequency-shift keying signal by MODEM, after the wave shaping, after the AD421 superimposition on the ring circuit sends out.
The D/A converter uses AD421, it is one kind of monolithic high performance d/a converter which American ADI Corporation promotes, by the ring circuit power supply, 16 digital signals by the serial mode input, may transform directly the digital signal the 4mA~20mA electric current output. It provided the high accuracy, the complete works to become, the low power loss solution, used 16 pin DIP, TSSOP, the SOIC seal, might realize the low cost long-distance intelligence industrial control. AD421 (digital/electric current) transforms including serial input 16 D/A, uses electricity besides oneself, but also provides may choose (5V,3.3 V or 3 V) the constant voltage output uses electricity for transmitting instrument other parts.
HART MODEM uses Smar Corporation’s HT2012, conforms to the Bell202 standard half-duplex modem, realizes the HART agreement stipulation digital communication code or the decoding. This chip is the HART instrumental design specially, internal integrated has conformed to the BELL202 standard modulator, the demodulator, the clock and the timing circuit, the examination control circuit. The performance-to-price ratio is high, 16 foot DIP and 28 foot PLCC seal, when the 5V power supply operating current 80μA. HT2012 with the micro controller exchange digital signal, makes the simulated signal connection with AD421. It asynchronous serial passes at the same time with MCU carries on the serial communication unguardedly, on the one hand will input the digital signal which does not enull to modulate the FSK signal, passes through the AD421 superimposition to output again in the 4mA~20mA return route, or return route signal after bandpass filter, enlargement reshaping takes out the FSK signal demodulation is the digital signal, thus realizes the HART correspondence.
Input clock which needs regarding HART MODEM, uses 7.3728MHz the crystal oscillator to carry on two 4 frequency divisions through two counter 74LS161, obtains 16 frequency division clocks.
As a result of the HART digital communication’s request, has the 0.5mA sinusoidal current signal superimposition on the 4mA electric current, therefore the entire hardware circuit must guarantee below 3.5mA can also the normal work, therefore realizes system’s low power loss design to be important.
Figure 3 and Figure 4 respectively is AD421 which and the HT2012 periphery circuit diagram uses.
Third, correspondence software design
The HART signal procedure is also the HART agreement data link layer and the application layer software realizes, is the entire scene measuring appliance software design key.
In the HART course of communications, the main engine (superior machine) the routing directive frame, the scene measuring appliance receives the order frame after the serial port interrupt, by MCU makes the corresponding data processing, produces the reply frame, by the MCU triggering transmission interrupt, sends out the reply frame, thus completes an order exchange.
First after on electricity or watch-dog replacement, the master routine must carry on the initialization to the correspondence part, mainly includes the baudrate hypothesis, the serial port working hypothesis, the clear correspondence buffer, to open the interrupt and so on.
After the initialization completes the communication part has occupied prepares under the accepting state, once the superior machine has the order to send, HT2012 carrier detect mouth OCD becomes the low level, the triggering interrupt, the start receive, the procedure enters the receiving unit. Then completes the main engine order the explanation and carries out the corresponding operation according to the order, finally presses certain form production reply frame and sends in the correspondence buffer, the start transmission, after completing, closes SCI.
Figure 5 is the serial receive request frame, the reply reply frame procedure flow chart.
After transmission reply frame, enters the waiting status once more, the waiting next main engine order.
Fourth, conclusion
The practice proved that the above method has the structure to be simple, the operation reliable the characteristic, conforms to the HART agreement completely, has the good versatility.
As a result of the HART numerous undeniable merits, enables it to become the whole world to apply the broadest scene communication protocol, has become in the industry the practical standard. Will therefore be very long in the present in times, the HART agreement product in domestic still had the very broad market.
