Any controler capable of reading dual 4-20 mA?


Thread Starter


I want to read to separate 4-20 mA signals (CT's) by way of a PLC or controller, and then the lower value of the two, then send this signal on to a separate alarm feature...

Steve Myres, PE

Almost any PLC with any analog input will be fine for this. You'll need two 4-20 inputs (most analog input cards have at least two points). In software, either compare the two and use the lesser one to trigger the alarm output, or simply perform the alarm test on both values.

I believe an all hardware solution is possible as well. I think you can get a signal conditioner from Action Instruments or similar that takes two signals and selects the lesser (or greater one) and outputs that. You could send this signal to the alarm, if analog is desired, or to a limit alarm SI, to produce a digital alarm signal.

If you'd like more help, feel free to contact me directly.

Steve Myres, PE
[email protected]

Robert Willis

The Schneider Electric TSX Momentum PLC using the Concept Lite Programming package (IEC 61131-3) has a Function Block that will take up to 32 input
signals and output the minmumn signal detected. The output can then be processed as required for the application.

Robert Willis
Square D Company
Almost all controllers will accept more than one analog input. Many have a low signal select block whose output will be the lower of the two (or more) inputs. If this function block is not available, you could program a compare (if A<B)
to switch the lower of two inputs to an output.

Daniel Chartier

Please clarify and give more information about your project. As stated, your problem appears trivial: any PLC with analog inputs and a compare function should be able to process the lower of 2 analog values... So, what is the catch?
Daniel Chartier

Johan Bengtsson

For stand alone PID: I don't know if this is supported
but any PLC (capable of handling analog I/O) could do that if
programmed correctly.

/Johan Bengtsson

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just put both transmitters in series and the lowest signal will dominate.
very dirty but it does work.