AI523 detect disconnected 4-20mA sensor

We have some AI523's in our system (controller is PM583). Most of the sensors have the 4-20mA style interface.

Is there a way to detect that the sensor has malfunctioned (such as the input wire disconnected or sensor is not powered)? My thought is that if the sensor is bad, it would probably generate 0 mA, which is outside the valid range.

Anything I can do in my PLC code to detect this condition?

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asked 1 year ago




during configuration in Automation Builder, when you define the type of the AI card, you can choose if the single channel is used and how the CPU react when you have, for example, broken wire in HW.

In the attached file one screen shoot of Automation Builder

Hope this help you


Flavio Mussolin   

answered 1 year ago


By rdb9879 on 5/25/2017 | Like (0) | Report

I see that you can set it to "plausib, cut wire, short circuit" using Automation Builder, but I don't understand what this effects? Will the CPU not boot, do a reset, or is there a memory address in code I can access to interpret the status?

By Flavio Mussolin on 5/26/2017 | Like (0) | Report

If an analog wire is interrupted during operation, the cpu will give you an error signal. As the lack of signal could be dangerous, you also have to set behavior the reaction in the cpu when it detects an error in I/O bus

By rdb9879 on 5/26/2017 | Like (0) | Report

"you also have to set behavior the reaction in the cpu when it detects an error in I/O bus". How do I do this?

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Sorry for the delay in the answer. In the cpu configuration from Automation Builder you can decide how the CPU will behave in case of an error, defined according to classes E2-E3-E4.
The manual shows you which error appears when an analog loses the signal, after defining how the CPU should behave.
See attached file


Flavio Mussolin   

answered 12 months ago


By rdb9879 on 5/31/2017 | Like (0) | Report

Thank you this is very helpful. So the CPU generates an error code if the signal is disconnected...Is there a way to read these error codes in software? We want to transmit these error codes over RS-485 so the main user can be informed.

By Flavio Mussolin on 5/31/2017 | Like (0) | Report

AFAIK, if the CPU comes in stop mode, it's no possible transfer the information between communication.
Generally the behavior of the cpu (error --> stop mode) is set to guaranted the safety for the equipment and people.
I'll investigate if exist one other mode to obtain that you desired. Do you have a bit patience?

By rdb9879 on 5/31/2017 | Like (0) | Report

Thank you for the input. I think our application is more like a car: If a sensor dies while driving, we want to inform the user (check engine light), but will still allow the user to continue driving to find a good mechanic. We have a good understanding of which sensors are critical and which sensors are optional, and double redundant safety hardware which will safely shut itself down at the correct situation. As of now, the little error LED on the PM583 is difficult to access, which is why we included the serial information. We want the CPU to continue running as it can respond to these scenarios more elegantly than to just hard stop everything at once. That is our scenario.

By Flavio Mussolin on 5/31/2017 | Like (0) | Report

Sure, your descrition is really clear. A small tip: you can obtain probably the same results, without set error in AI card but, with one SW comparator sequence in your program, you can verify if the analog signal is lower then the minimum you request for one "safe" values. In this case che CPU remain in run mode but you can set one alarm..what do you think about?

By rdb9879 on 5/31/2017 | Like (0) | Report

That sounds like a solution that could work. It would be just a little bit better if I could distinguish that a sensor is reading very high/low because it is broken or if it is reading very high/low because the temperature really is high/low. But if there is no way to do that, I will live with just "the sensor is too high/low"

By Flavio Mussolin on 5/31/2017 | Like (0) | Report

Waiting for different solution, you could try...:-)

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