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What if hart protocol communication doesn't work?
Dec 07, 2018


What if hart protocol communication doesn't work?



1、 Introduction

HART protocol is the conditions of the intelligent pressure transmitter communication loop resistance p 250 Ω, working voltage within the scope of its communication requirement, such as 3151 communication requirements of pressure transmitter is a communication minimum voltage loop resistance is 250 Ω to 18 V D C, the power connection in the right direction.


2 、Check the circuit

(1) the communication cable is not properly connected. Check whether the cable plug is plugged tight and the screw of the wiring terminal is loose to ensure good contact.

(2) incorrect cable position. The handheld terminal can be connected to the terminal A and B or the load resistance R at both ends of CD, but it cannot be connected to A and D.Between power supply and hand-held terminals need a 250 or higher Ω resistance, its role is to prevent the low impedance of the power of the communication signal shunt, but the R value is also not too larger to ensure the minimum operating voltage transducer, such ability ensure the handset to read information from the transmitter and memory will set the configuration parameters to the transmitter.Since the internal resistance of the low impedance power supply is very small, it can be regarded as a short circuit. Connecting the AD end is equal to connecting the two ends of the communication cable at one point, so the communication cannot be conducted, so the information cannot be read and the configuration parameters cannot be set, as shown in the figure.

 

What if hart protocol communication doesn't work?

If the above two conditions are met, still do not communicate, you can replace a transmitter to troubleshoot the communication device fault, so you can confirm whether the pressure transmitter itself fault.


3、Check the instrument

(1) over 200 3151B intelligent transmitters in Qingdao guangyuan development project with too low power voltage did not communicate in the pilot production process, resulting in output errors and flashing of the meter head, which seriously affected the production plan of users.After searching and measuring the power supply voltage is only 15V D C.Because the power supply is not independently supplied to the safety grid, the voltage of the safety grid supplied to the transmitter is too low, so that the transmitter cannot communicate and the output is abnormal. After changing the power supply, the transmitter can communicate and the instrument output is normal.

(2) the cable is too long. A university in xi 'an ordered a 1151 intelligent remote transmission pressure transmitter for oil project test, and the user reported that the transmitter did not communicate.The measured terminal voltage is normal, but the power supply cable is very long, which is 600 m. Through analysis, the reason is that the cable is too long, and the cable capacitance is greater than the maximum capacitance of communication equipment H, A, R and T.The communication frequency of HRT is high, ranging from 1 to 2k to hz, so the network T=RC (time constant) is not greater than 0.65 microns.Therefore, the maximum capacitance of HART communication equipment and network limits the maximum communication distance.HART communication maximum frequency calculation formula:

Maximum cable length m=(160000-cmaster)/Ccable

Where, the constant obtained from the calculation of the time constant allowed by HART communication network is 160,000, which reflects the maximum capacitance allowed by communication network, unit PF;

Cmaster is the capacitance of the control system or HART filter, PF;Ccable is the distributed capacitance of the cable, PF/ m.

About 6 0 0 m cable length, cable C = 4 0 0 F P/m, Rcable: 0.025 Ω/m, convenient for calculating assumption Cmaster = 0.

The maximum cable length Lm a x=160000/400=400 < 600m.The actual Cmaster does not equal 0 (Cmaste changed to Cmaster) = the actual maximum allowable cable length < 400m.

The network time constant T = R C =(0 .0 2 5×6 0 0) ×(400×600)/1000000=3.6μs>0.65μs .

Due to the cable, the network time constant is too large, which leads to serious distortion of communication waveform, and communication cannot be achieved. Therefore, it can be solved by using 2.5sq cross-linked cable (Ccable is 90PF/m) (Cmaster is changed to Ccable).

(3) electromagnetic interference

Xi’an Huaheng Instrument Is equipped with 6 measuring instruments of materials, 4 of which are 3151 differential pressure transmitters.1 HART protocol vortex flowmeter, only 1 differential pressure transmitter can work normally when put into operation, the rest are not communicated.When the flow rate is 0, the indication of 1 differential pressure transmitter is 48%, and the cycle change of 1 transmitter is 0~25%. The other displays faults, the on-site power supply voltage is normal, and the measured output and communication in the laboratory are normal.It is normal to discard the original power supply cable on site and use temporary power supply indicating communication.It can be confirmed that the instrument does not communicate and misindicate due to line interference.It is confirmed that the interference frequency is 2 kHz, which affects the communication frequency of 1200Hz and 2200Hz of the h-rt communicator.Frequency is generated by motor/pump. In order to adapt to 50Hz power supply, a frequency converter is used. The frequency setting of 2k Hz is very close to the communication frequency of H ART, which makes it unable to respond to the communicator correctly.Increase the distance between instrument cable and power cable groove frame to solve the problem.

What if hart protocol communication doesn't work?

Electromagnetic interference, usually in the power supply voltage, loop resistance, the attachment is correct, smooth lines, there is no communication problems need to consider the electromagnetic interference problem, the problem is usually laboratory are difficult to repetition, despite the pressure transmitter through all kinds of electromagnetic compatibility test in field may also be interference appear all sorts of problems.If these interferences are not well handled in the measurement process, the measurement results will be distorted and even the instrument or control system will not work at all if they are serious.In order to avoid electromagnetic interference, signal line should be twisted, shielded, grounded, filtered and other methods, the distance between the signal line of the instrument and power cable is increased, the wire is shortened, parallel wiring is avoided as far as possible, magnetic shielding of transformer and other interference sources.

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