I hadn't had interface (FTDI) problems since I upgraded to a 2018 Mac Mini, so I foolishly upgraded to 11.3.1 (Big Sur) from ver 11.2.3 and bam. No FTDI com port for the PLM. I tried installing the latest FTDI drivers, but MacOS blocks them. I tried all the old tricks of removing the USB cable and reinserting after boot, connecting via a USB hub, etc, but no luck at all in communicating with the PLM. I guess there's a chance that the PLM is bad, but it's unlikely. If anyone has any other suggestions, I'm open to hearing, but in the mean time be warned about the 11.3 upgrades.
Interestingly, I can access my RFXCOM serial port via the built in FTDI driver. But that's never been a problem.
-Mike
Edit: I had an old 2314S with a serial to USB converter, so I unplugged the 2413U and plugged in the 2314S and it communicated. So I may be too fast in blaming the com port driver. But the 2413 U was working fine prior to the OS upgrade, so it would be a remarkable circumstance if the PLM went bad at that exact time. I guess I've seen weirder, but I'm still not convinced. I'll try it on a different machine and report back.