My serial PLM will stop working after a day and the light is out. I can unplug it and plug it back in, then the light will come back on, however, the EISY still reports communication errors until I reboot. I've had several PLMs go bad, but not in this way before. Could it be the USB to serial cable? Also, why doesn't the EISY auto-recover? I've ordered the newer USB version of the PLM, figuring that will replace both the USB to serial cable and the PLM, but I'd like to know if my current PLM is really toast since I've not seen this kind of failure before.

Yes it has failed. Power cycling and working again for a short time is one of the failure modes we see.

If the eisy behaves like the older ISY994i. It will only check for a PLM at boot time. It may have to be rebooted to again check for a PLM. If it thinks it is not communicating.