CompactLogix v20 PLC Connection - Tag Bad Quality

I’m trying to connect to a CompactLogix PLC running v20.19 firmware.

I am able to connect to the PLC successfully and browse the tags. However when I try to read tag I get a Value of null and a Quality of bad. I tried this using the Quick Client in the Gateway and also creating a tag in the Designer.

My device connection is showing it is connected, and I did use the Allen-Bradley CompactLogix (Legacy) driver for this connection.

The log entry for this error is shown in the image below.

Any thoughts?

You need to use the new Logix Driver for any firmware version 20.12 (I think) and greater.

Rockwell disabled the direct memory access that is used in 20.11 and older firmware versions.

Unfortunately unless you can either downgrade to 20.11 (and keep using the legacy driver) or move to 21+ you’re going to be stuck with some pretty poor performance.

Thank you Kevin.

Why will I have poor performance with my current firmware? It seems you are implying the newer Logix Driver is more optimized for v21 and higher.

Yes, exactly. When you hook the newer Logix Driver up to a v20 or prior device it works, but it accesses tags by their symbolic name, which is very slow. v20 and older devices do not implement the newer API that v21+ devices do.

Do you know the minimum support version within the major release of v20 that is supported by the legacy driver?

I know that 20.11 works with the legacy driver.

I used the new driver and that solved the problem. Thank you for your input!


From reading this and other threads it seems like, the NEW Allen bradley (+v21) driver is recommended Contrologix firmware version 20.11 and above.

I was using firmware version 20.19 with the new Allen Bradley driver and recently had to downgrade to version 20.18 due to other external issues. Now, the tags are having issues with firmware 20.18 and new Allen Bradley driver. In the tag explorer and in the project the tags blink error temporarily for a second and then back to good state for a second.
I am able to read the tags from the QuickClient but the project is having trouble with the tags.

Which driver should I use for Firmware version 20.18? Is there a workaround in my situation?

Same issue here using 20.19, no luck so far
I tried all the Logic drivers, running Ignition 8.07

Being on firmware 20.xx where xx > 11 (or maybe 15?) is worst case scenario for performance.

It means you’re on a version where Rockwell turned off direct memory access, so you have to use the new Logix driver meant for v21+. But you also are on a firmware version that doesn’t support the new v21+ protocol, so every tag has to be read using its full symbolic address. This is very slow on the PLC side and means the driver can read a lot less tags per request.

That being said, the v21+ driver should work for these firmware versions, so if it’s just flat out not working at all I’d suggest calling support.

If you’re stuck on a later version of v20 and simply cannot get enough throughput for your application, you might consider adding my Ethernet/IP Class1 Communications module to the mix–put your most critical information to read into producer tags and consume them in Ignition. Logix processors have an order of magnitude more capacity handling I/O than handling messages. It does require PLC code or data structure revisions to make it work, though.

(Also true for other AB firmware versions, but at different throughput thresholds.)