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Flag for loss of communication with an expansion module?

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In this particular case I have a V1210 with an EX-A1 expansion module and two IO-ATC8 modules.

I always write in checks for my analog inputs to make a determination if they are failed or not, but I have a problem. If I lose communications with an expansion module any MI that I am bringing the raw sensor data in on just gets frozen where it was when it last had a good connection.

At the moment of a complete com loss to either an individual IO module or the main expansion module the PLC will lock up and give an indication of an expansion error, but if the customer just ignors this and cycles the power the PLC will just politely come back on with no error indicated and theoretical analog input values that are frozen (but valid).

Is there a flag I can see in either the system bits or system integers? If I lose the entire communication with an IO expansion string I will sii Sb91 turn on as none of the communication is happening at all, but if I loose communication with just one module out of several how can I know this so that I can fault out the entire system. I can not find any SB, SI or anything else that will let me know that I have lost just one module (or if I lose every module after the initial expansion module) after the loss has hapened and the power has been recycled.


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As you mention, when the communiction is lost, the controller stops and give Communication error.

The problem is only when the controller wakes up. If it will not find the expansions, it will run without them and without urging mesage. This is very useful, when you debug your program in the desk.

In your case, you need to find on powwer up if the controller communicates with the expansions. you can use for thhis purpose (and only for this purpose) SB90. You can make a simple net N.O. of SB2 (Power Up) AND N.O. of SB90 to SET any MB (or, in specific cases - Output). This MB will be your flag, that the controller wake up without recognizing the expansions.

I hope this helps.

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