CompactLogix PLC does not send output bits to 1734-AENTR/B

I have two 1734-AENTR/B remote I/O racks attached to a 1769-L33ERM processor.


Both racks are communicating; the inputs update in the PLC as the sensor states change.


The problem is that when output bit states change in the PLC, the states of the output bits on the rack do not change.


However, if I force any bit on or off, all of the output bits on that rack are updated.


So I know that the output bits on the rack are accessible to the PLC.


It seems that the PLC is not sending output data under normal conditions—only in the event of a bit force.

Any suggestions on how to fix this?

That is a weird one. The first thing that comes to mind is conflicting logic. One rung turns the output on but another rung is tuning it off. A force would override the program and just turn the output on no matter what the program is telling it. I have seen this several times with my own programs. Do a cross reference and see if you can find other places where the output or bit is used and go from there.

Program validates ok; the bits in question do not have multiple destructive writes.

It is not just one or two bits that do not update as expected. No output bit on either rack updates during the normal scan.

I'm requesting manual operations from the HMI. Requests that are accepted in the rung logic result in OTE to a local tag. At the end of the application program, all local tag bits are copied to the output module Data arrays.

The Data array bits are updating correctly in the processor. But the LEDs on the remote racks do not change.

When I force any single bit on, ALL outputs for that rack are updated to match the values already showing in the PLC.

Are you scanning the logic where the output instruction is located?

Yes, as described above. The Data bits change as a result of the scan of the logic.

Solved, with help from Rockwell Tech support. In the task that writes to the outputs, in the Properties dialog under the Configuration tab, be sure that "Disable Automatic Output Processing To Reduce Task Overhead" is not checked.