Thanks, all, for your help. Replaced PLM as other died after 2 years 2 months. Was able to restore system (after following procedures listed here in the Forum) by rebooting ISY and restoring each device individually. Took awhile, but all is well now.
Michel - did a "Restore Modem", and after 2+ hours rebooted ISY, and did a restore on each device. No communication errors that I am aware of. Maybe I was impatient, but in spite of my blundering the system is now operational.
Now I'll tackle the capacitor replacement on the old PLM, for a spare......thanks again!