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SLC 5/05 Communication problems
After some network fails, some Allen Bradley PLCs connected to the network lost Ethernet communication.
By Héctor Rey on 21 May, 2009 - 8:51 am

We had experienced some problems using PLC 5/05 connected to an Ethernet network. In some opportunities, we had network problems (because of a switch or because a wrong configuration of a redundant link) and some PLCs lost communication, and the only way to restore it is rebooting the PLC. We try to reproduce it in a laboratory but we couldn't. Also in some cases the PLC had gone to "Fault" state.

These network problems are not too much frequent, but when it happens it's an important problem in the plant.

Can somebody help me?



I've seen a similar condition in a plant here in AZ. We tracked it down to having too many MSG instructions executing too often. The communications memory is limited especially in the older SLC-5/05 models (16K and FRN5 is what I had). To determine the version, look in S:57 thru 59 for data.

By limiting how often the MSG instructions are repeating (I used a timer with a 2 second preset, you may be able to use a shorter time successfully), I kept the memory limit in check.

As was explained to me by Rockwell, the memory has no buffering function and won't prevent additional messages from being attempted. Once the memory is exceeded, a major fault is triggered and unless you have access to the PLC via the Channel 0 port, power cycling is the only way to reset the fault.

One other thing, if you change the model to increase the memory to 32K, the communication memory also is made larger - that might be enough to prevent the communication issue without changing your program.

If you have access to the Rockwell Knowledge Base on thier web site, search for SLC communications to get additional information.


How can I see the free memory? I'm using 64K CPUs.


As far as the communications memory, I don't know that you can see what is being used.

Go to Controller Properties\General to see what is free of the logic and word memory.


By bob peterson on 21 May, 2009 - 4:40 pm

AB had some issues with early implemenatations of Ethernet in the 5/05. It may be possible to upgrade the PLC firmware. You may want to run down your problem with AB tech support.

There are also some known bugs in certain versions of RSLinx that can create havoc under certain conditions.

>There are also some known bugs in certain versions of RSLinx that can create havoc under certain conditions. <

Where Can I find this known bugs? I asked Rockwell but I had no answer.


By Jerry Miille on 28 May, 2009 - 4:32 pm

Go to Rockwell's knowledgebase and search for Answer ID 22199 for a pretty good overview of some of the RSLinx problems.

Jerry Miille