Cannot read some tags inside Rockwell AOI

[Kevin.Herron]

@nminchin

SCADA writes to the tag setting it True, the PLC immediately resets it to False, but SCADA remains True indefinitely.

This is a race condition that can happen if you use the "Read After Write" option in tag groups with an OPC UA subscription. Not sure about other causes.

@Kevin.Herron

Imagine, if you will, a boolean tag in a 30 second scan class.

0. driver polls value=false, next poll in 30s
1. user writes value=true to tag, read-after-write confirms value=true, ignition tag now set to true
2. some seconds later, before the next poll, an external source (script, PLC logic, whatever) sets the value to false again
3. driver eventually polls again, reads value=false, no change since last poll, so no data change is sent to client

The tag is now false in the PLC but true in Ignition.

from Indirect Tag Delay - #9 by Kevin.Herron