PLC analog input

[Aruck]

Hi eevblog,

I've lurked for a long time, but I finally got around to making an account to ask a question. I have a basic understanding of A to D and D to A converters, sample rate, bit depth, Nyquist frequency, etc.

I'm using an Allen Bradly compact logix plc, and my plc has what's described as four "Universal Analog Input Points", each point having a terminals labeled "V in" "I in" and "V/I in".

Can someone explain to me the function of each terminal? I've tried my best to hunt through the pdf's provided by Allen Bradly but I simply can't find what I'm looking for.

Thanks in Advance.

[128er]

Hi Aruck. Welcome to the forum!

First ...

Which exact model do you use? If It's a PLC with seperate CPU and I/O modules, the type of your analog input module would be good.

Dont know the allen bradley stuff exactly (more used to Siemens). But industry standard is 0-10V (sometimes 0-5V) for analog voltage inputs. And 0/4-20 mA for analog current inputs. The "V/I In" terminals are probably universal and you select the function you want to use in your PLC hardware configuration.

EDIT:
Seems that you can configure their analog input modules in every way you like. Each input independently for V or I, or even for thermocouples and RTDs

[Aruck]

The PLC that I'm using is not a chassis / card type, everything is all built together as one unit.

You are correct about the inputs being configurable. After several hours of searching I think I found what I'm looking for. I don't understand why a voltage signal uses 2 terminals and a current signal uses 3, but I guess I'll just roll with it.

[Paul Moir]

Betcha the middle pin provides the current source for the current loop.

[Zero999]

Can you post the full data sheet for the PLC?

[Rerouter]

measure across the middle pin and the negative for the current input, and you will likely find a 250 ohm resistor, (4mA * 250 ohm = 1 Volt, but it still needs to measure that voltage so you bridge the voltage sensing pin, and the resistor to measure current