BobO,
After installing the new OS you provided I have not seen any issues until 11/29. It's a little different though, modbus communication will be active for a couple polls than drops out for hours then repeats (I can see this trend from my Historical Trend Charts in my HMI), unlike last OS where the comms would be down and never reconnect until the Re-initialize PLC I/O was enabled .
Today I connected to it remotely over radio with DmD (Same pathway as the Modbus Comms) and watched as the ModbusTCPServer.ActiveSessions = 0 (watched for a couple polls), once I enabled Re-initialize PLC I/O modbus communication instantaneously came back up, I watched for atleast 30 minutes with good comms? Let me know if I can provide any data from this controller that might help.
Drops for *hours* and then eventually comes back briefly, before dropping again? So strange. We have never been able to duplicate this, but as I mentioned, we have heard a few reports of it, always with radios. My theory is that something odd is happening with unstable radio connections that is depleting something in the TCP stack, which eventually prevents the client from connecting, but I have no idea what it is. Very frustrating, for users and us.
Once it's failed, do you see any change in the scan time or any other change in the PLC's behavior?
The one thing that might be useful is to see a Wireshark trace when it's failed. It would also be really helpful if I could see exactly how the TCP session is failing when the radio gets flaky, but it's hard telling how often that happens. In another place this is happening, the comms to the site are constantly erroring out due to the radio being less than perfect, but it still takes weeks/months before the problem shows up, so the offending failure may not happen very often. That might make catching it nearly impossible, or finding it in the trace...since we don't even know what we're looking for.
I have a version of firmware with some diagnostics in it. It isn't much, but it might give more info about what is failing. If you'd like to try it, I'd be happy to give it to you.