An unplugged, damaged, or noisy Ethernet cable is the most likely reason for the disconnection.
You've also been posting about I/O module errors, and your facility is evidently a pretty inhospitable environment. If it were my system, I would spend some time improving the ground connections and the physical mounting of the controllers and cables.
For example, that MicroLogix you posted a picture of. There are little plastic covers that have been removed from both the serial ports, so because there are no cables plugged in, they're getting full of dust and other contaminants. When you finally do plug in a cable, you have a much higher likelyhood of poor physical and therefore network performance. At least get some electrical tape or foam rubber to cover those ports.
The same is true for Ethernet ports. Even ones that are plugged in are not really dust-tight. And I've seen a lot of Ethernet switches with unused ports that get badly contaminated with oil and dirt, then we plug in and wonder why we're getting network errors. Rubber plugs are widely available for RJ45 ports to keep the dirt out.
Now that I think about it, you were also posting about faults on the MicroLogix. Does this communication error occur at the same time as the fault on the MicroLogix ?
