CompactLogix v20 Bad_CommunicationError

I’m having an issue reading tag values from a CompactLogix using v20 firmware. I can connect to the PLC fine and read all the tag structures. The diagnostics show a connection. Everything looks good, except I cannot read any values form any of the tags.

When I try to read a value from the OPC Quick Client I get the following:
Read complete
Status: Bad_CommunicationError
Value: null
Quality: [Bad_CommunicationError] A low level communication error occurred.

Any help would be appreciated.

What’s the exact firmware version of the device? Although the device drivers short-hand to v21 as the cutoff, some AB devices past a certain v20 revision aren’t fully compatible with our legacy drivers. Try the newer Allen Bradley Logix driver and see if it connects successfully.

Firmware Version 20.19

Using the v21+ driver seemed to solve the problem.

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I am having the same problem. My customer has firmware version v20.19 and it seems I should stick to this firmware version. Is there a solution for this error?

You must use the v21+ driver for firmware v20.19, due to Rockwell cutting off the high-performance but totally insecure memory access services that the legacy driver relies on. See this topic:

See also this page in the user manual:

January 7th 2024.
This saved me today. Final day of an outage and after an Ignition version upgrade, this one showed its ugly face!

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But the firmware version for many 1769-Lxx devices updated to V20.19 maximun. dont have V21.

As noted in the documentation, use the V21 driver for V20.19 firmware. Rockwell backported its closure of the security hole from V21 to the last releases of V20. You must use the V21+ driver for this case. Or flash your devices backwards to ~v20.12, IIRC.