4-20ma Read to 2 AI Devices


Thread Starter

John Nguyen

How can I split a 2 wires 4-20ma signal of a pressure sensor to be read by 2 AI devices of 2 separate PLCs without using 2-channels splitter isolator? What happen to signal that change my reading according to parallel or series connection to sensor?

If I cannot connect 2 AI devices to the sensor, what is best method? Please provide a suggestion.

You have several options.
If the analog inputs are isolated, just wire in series.
If they are not isolated, wire to one as 4-20 mA, wire to the other as Voltage in parallel.
Wire to one PLC as a 4-20 mA input, from that PLC wire an analog Output to an input on the other.

Before PLCs became common receivers were wired in series all the time
Transmitter, Controller, Chart-recorder, Alarm unit etc.
Wiring in series you just have to be careful you don't run out of power supply Voltage, study the transmitter manual.
Paralleling the 4-20mA will not work.

You can try running the 4-20mA through both AI's in series. Sometimes it works OK, other times it doesn't because of the ground loop caused by the different ground reference of the devices.

Every commercial 'splitter' that I've seen isolates the input from the two outputs and the outputs from each other, and that isolation is what makes the whole thing work.

There's some risk of not the pressure transmitter not being able to drive the combined loop resistance but a 24Vdc power supply can typically drive about 600 ohms which is two 250 ohm resistors and some wire resistance.

It's the ground loop that's the killer.

You connect to one and it works. You connect to the 2nd device and both signals have an offset or go offscale. You take one away and the remaining one works. Welcome to ground loop school.

Try it and see.

If you get a ground loop, use a splitter.
> Paralleling the 4-20mA will not work.

Perhaps I should have been a bit clearer.

The usual analog input is 1-5 VDC quite high impedance about 10K Ohms or so. To measure 4-20 mA a 250 Ohm resistor is applied across the input.

Across the input {PLC A) you have 1-5 Volts, you can run that to another grounded 1-5 Volt input (PLC B) provided it's not too far away. The signal will not pick up noise because the impedance is only 250 Ohms provided by the resistor at PLC A.

If the PLC Inputs are fixed 4-20 mA type of course you are restricted to series or a signal isolator.

I prefer not to use signal isolators, it can usually be avoided however they do come in handy if you have an emergency shutdown system where the analog transmitter is connected into the ESD but you want to also have the signal in the PLC or DCS. Not all signal isolators can be used for that it requires careful selection.

Please don't just try it and see, design the circuit to work.

Just so you know, my comment about not paralleling 4-20mA was a reply to the original poster, Mr. Nguyen. Your comment wasn't posted when I replied, and my post only shows up after yours sequentially because of the date stamp for when it was submitted.

I was saying the that paralleling 4-20mA to two different devices will not work.

You're right and I thought you were clear the first time, paralleling the 1-5V IR drop to a 2nd AI might work, as long as a ground loop/common mode isn't a killer.