How does a transmitter communication device work

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mull982

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We have several transmitters that use a 4-20mA loop for transmitting signals to a PLC input. We also have a field device that you can hook up to the transmitter field wires to either program the transmitter, or view realtime data and paramaters.

The field device works by connecting a converter to one end or your computer and and the other end of the coverter has two wires that connect in parallel with the signal loop.

Does anyone know how (current/voltage etc...) how this converter communicates with the transmitter by being in parallel with it? Also wouldn't this device being in parallel cause current to move through it and provide a parallel path for the 4-20mA signal?

The device I am referencing is a Hart communication device.
 
100523-1214 EST

mul982:

From the perspective of your question the written material from Hart was difficult to extract the essence of the system.

Basically their system is a digital communication system that can be piggybacked on an analog 4-20 MA current loop system without adversely affecting the accuracy of the analog system. One assumption is that the digital communication frequency superimposed on the current loop is high compared to the analog frequency range of the current loop.

A digital transmitter in the Hart system superimposes a +/- 0.5 MA carrier frequency modulation on the current loop current. A current source has theoretically an infinite source impedance. Thus, an added parallel current source can add or subtract from the current in the loop. If an equal number of added and subtracted current pulses are injected into the loop, then the average current is unchanged. Thus, no change in the analog signal.

Hart uses FSK (Frequency Shift Keying). One frequency represents a binary 0 and another a 1.

Since the current is changing in the loop it is possible to detect this signal as a voltage or current change at one or more receivers.

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Just a note for others out there using Honeywell transmitters. They are not Hart compliant and they will affect the signal from the transmitter to the controller if you put the communicator on the system. The handheld communicator specifically tells you to put the loop in manual prior to communicating.
 
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