Modbus or Direct Logic success and error control relays

[Henryp]

I may have a mistake in my ladder, but it seems like the Modbus or Direct Logic success and error control relays don't work when the communication box is set to a continuous update interval rate.

[Controls Guy]

I think any comm box acts like it's in a queue.  IOW,  on a leading edge on the enable line it then waits for the port to be available, then attempts the transaction.  It also clears the success and failure flags, not sure whether on leading edge or when it gets the port.   Anyway the point is that the flags are only valid between requests and for a continuously-triggered box with no competition, you may have a transaction in process 99% of the time.

Try putting in counters on the success and fail flags.

[BobO]

I may have a mistake in my ladder, but it seems like the Modbus or Direct Logic success and error control relays don't work when the communication box is set to a continuous update interval rate.

They turn on for one scan as the instruction recycles.