Author Topic: PLC analog input  (Read 2432 times)

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Offline Aruck

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PLC analog input
« on: June 02, 2015, 01:20:04 am »
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.

Offline 128er

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Re: PLC analog input
« Reply #1 on: June 02, 2015, 01:39:44 am »
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.

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
« Last Edit: June 02, 2015, 01:49:09 am by 128er »

Offline Aruck

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Re: PLC analog input
« Reply #2 on: June 02, 2015, 02:30:44 am »
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.


Offline Paul Moir

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Re: PLC analog input
« Reply #3 on: June 02, 2015, 06:00:48 am »
Betcha the middle pin provides the current source for the current loop.

Online Zero999

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Re: PLC analog input
« Reply #4 on: June 02, 2015, 08:42:08 am »
Can you post the full data sheet for the PLC?

Offline Rerouter

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Re: PLC analog input
« Reply #5 on: June 02, 2015, 09:04:45 am »
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

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