I am attempting to set up a block transaction group with six values. Five of them are reading directly from a PLC, but the sixth one is a calculation including two tags. When I set it up how I expect it to work, the calculated value always inserts NULL into the database, instead of the sum I am expecting. Are expressions not allowed inside a block item?
When I set it up like this, I get the result below. As you can see, meter 4 is inserting NULL.
I tried to set up a separate expression item, but it won’t let me insert the values into the same column. Is there a way to build a transaction group so that an expression like this can be inserted as part of the block?
Thanks for the tip. That is a good solution, but the limitation I see is that changing the transaction group’s execution rate will not change the poll rate on the tags. So if we set up the tags and transaction group to fire off every minute, then later change it to every 10 seconds, the rate will need to be changed on all tags involved. Is that the case?
You are correct that the scan rate of the tags are different than the execution cycle of the group. So possible options would be:
[ul][li]–Scan class on the tags way faster than the exec cycle of the group.[/li]
[li]–Instead of trying to trying to make an expression in the group, store the individual values separately, and then do the math through your query.[/li][/ul]