Abstract: First introduces A5191HRT the HART modem’s principle. A5191HRT internal integrated has conformed to the Bell202 standard complete modulation demodulation electric circuit, was convenient with the microprocessor connection, is advantageous for the construction intelligence scene measuring appliance’s HART agreement connection module electric circuit. Finally gives some intelligent scene measuring appliance based on A5191HRT and the AD421 electric current ring content, the mold switch’s HART agreement connection module design.
Key word: HART agreement; Serial communication; Frequency-shift keying; Modem
1 introduction
In the mid-1970s, industrial control measuring appliance development for unified two-wire system 4 mA~20 mA electric current link standard simulated signal system. After the 80s, digital technique’s development caused the massive intelligent control measuring appliance to use micro controller (MCU), the information content which between the measuring appliance and the control device transmitted increased, needed one kind of entire digital two-way communication standard urgently. American Rose-mount Corporation formulates the HART agreement is compatible present 4 mA a 20 mA analogous system’s transitional field bus standard, obtained the very widespread application, becomes intelligence industrial control domain international standard in fact.
AMI Semiconductor Corporation’s A5191HRT the HART modem uses in constructing the intelligent scene measuring appliance’s HART agreement connection module, has the peripheral circuit to be simple and operational reliability higher characteristic.
2 HART agreements
Of HART (Highway Addressable Remote Transducer) the agreement was American Rose-mount Corporation promotes in the mid-1980s, mainly used in digital communication between the scene intelligence measuring appliance and control system’s. The HART agreement simulated signal superimposes the audio frequency digital signal in low frequency 4 mA,20 on the mA, carries on the bidirectional digital communication. Has simultaneously carries on point-to-point 4 mA~20 the mA simulation and the digital communication, the multi-station mailing address, the multiple message transmissions, the opening architecture may free obtain and so on merits.
The HART agreement refers to the ISO/OSI model (open system interconnection model), uses simplified 3 model structure, namely first physical level, second data link layer and the seventh application layer. The physical level stipulation signal’s transmission method, uses based on Bell202 the standard FSK frequency-shift keying signal, simulated signal superimposes the audio frequency digital signal in low frequency 4 mA on a 20 mA to carry on the two-way communication, the digital signal scope is 0.5 mA, the data rate reaches 1200 b/s, represents logic by 1200 Hz…1 ‘, 2200 Hz represents logic…0 ‘. FSK frequency-shift keying waveform as shown in Figure 1(a).
The data link layer stipulates the HART agreement frame form. Realizes the establishment, the maintenance, the end link communication function. The application layer is the HART protocol command collection, uses in realizing the HART instruction.
After HART agreement FSK frequency-shift keying signal and 4 mA~20mA electric current link simulated signal superimposition, simultaneously transmits schematic drawing as shown in Figure 1(b). Because the FSK signal’s mean value is 0, therefore does not affect transmits for control system’s 4 mA a 20 mA electric current link simulated signal size. Has guaranteed with the existing analogous system’s compatibility.
3 A5191HRT principles of work
3.1 ASl91HRT performance and pin function
A5191HRT is applies in HAftT the scene measuring appliance’s monolithic CMOS modem, its main performance is as follows:
(1) monolithic, half-duplex 1200 b/s speed frequency-shift keying (FSK) modem, may substitute SYM20C15;
(2)Bell202 the standard FSK frequency-shift keying signal, the carrier is 1200Hz and 2200 Hz;
(3) internal integrated the receive bandpass filter electric circuit and the sending signal wave shaping electric circuit:
(4) external connection 460.8 kHz crystals or the ceramic filter internal clock oscillator or uses the external input clock;
(5) operating temperature scope is 140 ℃~ 85℃;
(6) supply voltage is 3.O V~5.0 V;
(7) satisfies the HAftr agreement physics level the request;
(8) power loss is low.
Moreover, to save the power loss, A5191HRT when carries on the transmit operation may the accepting circuit closure, carries on when the receive operation may also transmit the electric circuit closure. This working suits in the HART agreement half-duplex operation way.
A5191HRT uses 28 pin PLCC and 32 pin LQFP seal, pin arrangement as shown in Figure 2.
Main pin’s function as shown in Table 1.
3.2 A5191HRT internal structures and principle of work
A5191HRT interior including the transmission data modulator and the wave shaping electric circuit, the carrier detect electric circuit, receives the filter and the demodulation electric circuit, the control logic and the clock oscillator electric circuit. A5191HRT internal structure as shown in Figure 3.
The modulator receive does not enull the digital code and modulates is the FSK signal. Represents logic by 1200 Hz “l”. Represents logic by 2200 Hz “0″, the data rate is 1200 b/s, afterward transmits by the wave shaping electric circuit the FSK signal shaping for the compatible HART agreement request’s signal. Receives the filter to complete the input signal the bandpass filter, gives the carrier detect electric circuit and the demodulator the signal. The carrier detect electric circuit through the comparison receive filter output signal and the exterior reference voltage, when examines the received signal input pin input effective modulation signal outputs the high level. The demodulator when the carrier output is effective after filter’s FSK input signal demodulation for digital signal output. The clock oscillator electric circuit produces 460.8 kHz clock signals, may use the internal neterodyne electric circuit to produce the clock signal. This time needs an external connection crystal oscillator. May also the direct input external clock signal. The control logic circuit uses in controlling a5191HRT interior each electric circuit module the work. For example the transmit operation and receives the operation the cut.
4 HART agreement connection module design
4.1 HART agreement connection module hardware circuit design
Some intelligent scene measuring appliance requests the HART agreement connection module to complete with the main engine (superior machine) exchanges intelligence transmission and so on measuring appliance’s establishment parameter, middle metrical data, calibration parameter. As well as the measuring appliance measurement result’s digital code transformation is 4mA~20 the mA standard simulation electric current link signal output function. The design completes HART connection module structure diagram as shown in Figure 4.
The HART agreement connection module mainly by scene measuring appliance’s in MCU, A5191HRT and AD421 DAC is composed. And .AD421 receives the MCU transmission the digital signal and transforms 4 mA~20 the mA electric current output, transmission measurement result: A5191HRT receive superimposition in 4 mA 120 mA on ring circuit’s FSK signal. Demodulates rear-drive loses to MCU. or the reply frame message modulation FSK signal which produces MCU after the profile shaper ring circuit transmits by the AD421 superimposition in 4 mA on a 20 mA.
Because a5191HRT interior integrated the complete HART modulation demodulation electric circuit. Therefore its peripheral circuit only need the few parasitic elements. A5191HRT peripheral circuit principle as shown in Figure 5.
In Figure 5, A5191HRT and the MCIJ connection signal including carrier detect OCD, the HART demodulation outputs ORXD, the HAIl’r modulation to input ITXD, the request to transmit INRTS:Loop is 4 mA~20 the mA ring circuit input: The HART modulation demodulation’s clock signal stems from the external connection 460.8 kHz crystal birth vibration.
AD421 is ADI Corporation’s high performance monolithic DAC, its compatible HART the agreement FSK communication circuit, suits the low power loss, the high accuracy, the low cost intelligence industrial control application. AD421 by the voltage regulator, ∑ a △ structure DAC nucleus and the current amplifier is composed. May transform the 16bit digital code into corresponding 4 mA~20 the mA simulation electric current.
4.2 HART agreement connection module software design
HART agreement connection module software design including HART agreement software design and MCU to AD421 control procedure design. The former including HART agreement data link layer signal procedure design and application layer software contact surface design. Is the entire module software design main part.
The HART agreement connection module’s course of communications by the main engine routing directive frame start, the scene measuring appliance takes from the equipment use interrupt transfer subroutine completes the receive and the reply. On after scene measuring appliance electricity replacement, signal procedure first initialization HART agreement connection module. After for example establishes MCU the UART working, the serial communication baudrate, the data frame form and the interrupt and so on, enters the waiting status. When main engine routing directive, A5191HRT carrier detect OCD becomes the low level, triggers MCU the UART interrupt. Transfer receive subroutine. After MCU completes the main engine order the receive and processing, the production reply frame and transmits for A5191HRT. modulates the FSK signaling to give the main engine, after completing, establishes again the HART agreement connection module as the waiting status.
The HART agreement connection module uses this kind of interrupt transfer subroutine the method to complete between the scene measuring appliance and main engine’s correspondence, may realize the main engine to the scene measuring appliance’s each operational parameter establishment, measurement result read, instrument mode functions and so on examination, and has the programming nimble merit.
5 concluding remark
The HART technology in overseas was very already mature. And becomes in the intelligent control domain by its own prominent merit to apply the most widespread scene communication protocol. May foresee, will be very long in the present in times, the HART technology will develop in our country scene measuring appliance’s intellectualization and transforms plays the influential role. The design practice proved that uses A5191HRT the design intelligence scene measuring appliance’s HART agreement connection module to have the circuit design to be simple, the operational reliability high merit, the reference value and the use value are high.
