Questions about HART communication!
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When using 375 or 744 communication differential pressure transmitters or pressure transmitters, it is necessary to connect a 250Ω resistor in series in the circuit. Why is this necessary? Another thing is that a mistaken understanding of mine led me to use a convenient method of communication; since the internal resistance of the PLC is 250Ω, I once used the 744 communication protocol to establish a connection. However, after further discussion, it became clear that this communication could work regardless of the PLC’s internal resistance, as I was communicating from the input side of the distributor. The input and output sides of the distributor are isolated from each other, meaning that even if there is no connection at the output side of the distributor, a circuit is still formed at its input side. Thus, the PLC is not part of the circuit through which I am communicating! So I can simply use a clip to connect the positive and negative terminals at the input of the distributor, and that way communication is possible. Why is that?
Hey~~~ I’ll vote for myself! Why isn’t anyone responding? If you know, let me know! Thank you! If you don’t know, just reply! Solve it together!
This.....:L I really don’t understand
The purpose of using a resistor is to ensure that HART can receive sufficient voltage. If the transmitter is connected directly to a 24V power supply, the low internal resistance of this supply prevents HART from picking up signals from the device. By inserting a resistor, HATR can obtain sufficient voltage across the resistor for communication. If the entire circuit is equipped with a safety barrier, it can be connected directly to the transmitter for communication, as the resistance of the entire circuit is sufficient for the HATR to acquire the signal. The typical resistance ranges from 200 to 650 ohms; too low a value results in insufficient amplitude, while too high a value affects the operation of the transmitter.
Hart uses frequency-modulated signal transmission; if the resistance of the current loop is below 250 ohms, the frequency-modulated AC signal will attenuate to such an extent that it becomes unacceptable for the communicator, preventing communication.