Analog I/O

[The Sparky]

I have an Vision 570 OPLC, an Allen Bradely micrologix PLC and 1 pressure transducer 4-20mA. Can I wire this transducer to the anlaog input of both plc's and how?

[Simon]

The "plug and play" option is to use a signal conditioner that takes the 4...20mA input and duplicates it to give you two 4...20mA outputs. Both outputs will be identical copies of the input signal. For example Weidmuller produce this type of deivce. This is 99% guaranteed to work, but involves purchasing another module.

IF (and I say IF) certain conditions are met you may be able to feed the signal to both PLCs without the additional module as above. You would wire the sensor output and the two PLC inputs all in series. The situation varies a little depeding on whether the transducer is loop powered or has a separate supply. In general:

1. At least one of the PLC analogue inputs must have full galvanic isolation, that means not even the 0V is connected with the Power or chassis GND. Usually this is not the case for the Unitronics analogue inputs, but I am willing to be corrected.
2. If one of the PLC inputs is not isolated, then the transducer output must isolated from the supply (or be a loop powered trasnducer)
3. Check the loop impendance, which will need to be evaluated differently depending on whether the sensor is loop powered or externally powered. Bascially you need to push 20mA though all the devices without exceeding the maximum voltage available in the circuit.

I hope this helps.