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.

1 Like

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: