Siemens 4-20mA Input

mad4x4

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Join Date
Mar 2009
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ST CYrus
Posts
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Simple Question - Standard 4-20 ma Input Card (Siemens)
SO if at 4 ma we get 4000counts and at 20ma We get 26800 counts when it goes overscale slightly this will tend to 32768 counts (2^15) But what happens if the 4-20mA signal goes to 22 or 23 mA(out of Scale). Does the count revert back to 0 or does the count flip to -32768?


We are doing an investigation into an Analog signal failure as part of a Root Cause and this question has come up.

We think the signal has gose +++ to Wirebreak and we are trying to understand what the logic would do on receipt of the Over mA signal.
 
You don't mention which specific input card you have.

What you need to find is the data sheet for the card you are using. There should be a table showing what numerical values correspond to which mA levels.

I just poked around and found the data sheet for an
ET 200SP
Analog input module
F-AI 4xI 0(4)..20mA 2-/4-wire HF
(6ES7136-6AA00-0CA1)

The attached image taken from the data sheet gives this type of information.

***** YOUR CARD MAY HAVE DIFFERENT CONVERSION ***** Get the data sheet for your specific card.

4to20.png
 
P.S.

Some cards can be set up for different ranges and different conversions. For instance, when the card I referenced is set for 4-20mA then the number 0 signifies 4mA. But when the card is set for 0-20mA, the number 0 signifies 0mA.
 
Very similar to the S7-1200. Watch for the built-in scaling functions, I have found those can go a bit wonky if you go outside the scaling range. It was near the start of the release of the S7-1200 that I had problems, I haven't checked it recently, I prefer my own scaling function.
 

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