Ignition Edge Expression Tag Failure

Suddenly all of my expression tags in Ignition Edge 8.0.2 failed with “Expression Right Operand is Null”. These tags are expression tags with an expression like:

100 * (1 - {[edge]Output} / {[edge]Input})

where Output & Input are derived tags with a source of a modbus OPC tag {[edge]PLC1/Raw/MW770}. The Raw PLC tags were reading a value. The derived tags (Input & Output) didn’t have an error and were showing a value. Only the expression tags showed an error.

Also the expression tags and the derived tags are both published using the Cirrus Link MQTT Transmission module.

Here are some logs from when this occurred:

I tried restarting the Modbus-Driver module and the OPC module which did not make a difference.

I eventually fixed this by pointing the expression tags directly to the OPC tags without using the derived tags. Then when I changed the expression back to using the derived tags… they worked.

So it is working now, but was wondering if anyone had an idea why this happened and how we could prevent it from happening again.


It may be best to get in contact with support if you haven’t already.