The HART protocol is applied in the intelligent electromagnetic flowmeter to realize

One, the preface in line with the HART protocol of the intelligent electromagnetic flowmeter can not only realize all kinds of traffic detection and local display, you can also through the upper meter remote setting, change the zero range flowmeter, and complete the self-diagnosis function, so more convenient in the use and maintenance of flowmeter. Second, the introduction of HART protocol HART protocol follows the ISO for OSI open system interconnection reference model, using the * layer of the OSI model, the second and seventh floors, namely, physical layer, data link layer and application layer. 1. The physical layer specification HART protocol used American telephone communication system Bell202 frequency shift keying ( FSK) Standard in 4 ~ 20 ma simulation & plusmn; 0. 5 ma of sine wave, baud rate is 1200 BPS. Average because of the superposition of sinusoidal signal is 0, and continuous phase frequency shift keying technology requirements on the baud rate of 1200 hz boundary of 1 s and 0 s phase Angle of the data bits is continuous, so digital communication signals will not affect 4 ~ 20 ma analog signals. 2. Data link layer specifications of this part of the agreement the HART frame format, implementation, maintenance and data link communication function, HART protocol based on redundancy check wrong words. if information, through error detection mechanism and using the automatic retransmission request protocol ( ARQ) , the realization of communication data error-free transmission protocol related data transmission is implemented in the form of the frame. Frame is the user to control and addressing information encapsulation of user data, only the frame of the byte count and east and the error tested receive correct or notify the physical signal transmission end ( If detect carrier) Before he can identify frame. HART protocol in data link layer is a kind of & other; The master/slave & throughout; The agreement. Has been in the reply message from equipment, in addition to receiving the confirmation of order, also contains the main equipment of many news about the data. HART protocol allows the system to have two main equipment operation, at the same time in a basic master device, a pair of main equipment. From equipment according to the two main different address to distribute their response command messages. 3. The rules and regulations of application layer the HART command message in class 3, * class is gm's command, applicable to all products comply with the HART protocol, to comply with the HART protocol devices function description: the second category is the ordinary operation command, applicable to comply with the HART protocol for most of the products; The third kind is the special equipment command, suitable for special products comply with the HART protocol. Three, the intelligent electromagnetic flowmeter based on HART protocol communication module design with HART communication protocol of the intelligent electromagnetic flowmeter hardware system circuit USES modular design, mainly includes the sensor module, MCU module, HART communication module and man-machine interface module four parts. Low-frequency excitation by the single chip microcomputer control to the excitation coil of the sensor output current, tiny electric potential at the same time from the electromagnetic flow sensor signal by AD modulus conversion after preamplifier circuit into the single chip microcomputer to. Single-chip computer for data collection, these signals analysis processing to the LCD screen display of the instantaneous flow and cumulative flow. And single chip microcomputer is obtained by calculation formula for flow value compensation, the compensation of traffic signal remote by HART communication module and the upper machine communication. This article only introduces HART communication module design: HT2012 chip is used to implement HART protocol communication signal demodulation and modulation process. HART signal demodulation process: the pulse signal output by a band-pass filter after entering HT2012, modems, respectively, the pulse signal of 1200 hz and 2200 hz demodulation for number 1 and 0, then the digital signal by HT2012 ORXD output port, the microprocessor ( CPU) Received data to determine and implement the corresponding task. HART digital signal specification is the average of the peak of 1 ma, as the OmA sinusoidal alternating current signal, the signal line impedance into sinusoidal ac voltage signal, and HT2012 input and output signal is 0 ~ 5 v square wave signals, thus between HT2012 and external HART signal also need to have a filter circuit and shaping circuit. Composed of TLC27 filter is a bandpass filter, used to reduce the noise interference signals. Filter is used to eliminate the waveform of the peak, so that the received signal flattens out. TLC37 Numbers of square wave generating circuit after TLC27 filtering of the sine wave into a corresponding square wave, so that the 74 hc126 HT2012 receive buffer role is to make the square wave of rising and falling along the leveling off, satisfy signal HART, the physical specification required by the signal waveform rise along and down along the request, because more gentle falling edge and the delay time can reduce the crosstalk between with other networks. Provide 460 HT2012 need external. 8 KHZ the clock signal, and require the error of the clock frequency is & plusmn; 0. A 460-1%, if direct input. 8 KHZ crystals will not only cause the increase of system power consumption, and the crystals as part of the non-standard crystals, need to be customized.

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