1 hour ago, paulbates said:

If unplugging the PLM helped, its a "runaway" device sending repeated messages on the powerline and overwhelming the PLM/ISY. A pushed in set button could be the culprit. Pull the air gap out as described above, Plug the PLM back and reboot the ISY. Did you get control back? If yes, push the set tab back to its normal operating position. Test the switch

Paul

Thanks.  I did get control back, and I went through my devices to find one that was misbehaving.  After disabling the device, the PLM/ISY is now working well.

Thank you very much for the suggestions.

1 minute ago, paulbates said:

I believe that the X10 RX is receiving a message. Do you have X10 devices? Any chance you have a "runaway" x10 device?

After the ISY is up and running, uplug the PLM to stop the receipt of messages and see if you get control back.

Paul

Unplugging the PLM was *super helpful*.  I don't believe that I have any X10 devices anywhere, since I thought I upgraded everything (or enough devices) to Dual Link.

Thanks for the help.  

I have but one program: the factory-set "Query All" program, which is not changing color.

I did find a device that was pinging - I think the reset button was being held down by the cover plate, but when I removed the cover plate and reset the device, the System Busy only sped up and is still in the endless loop.

With my ISY994i, version 5.0.13A, I have been in an endless "System Busy" loop.  There is a constant event of "[X10-RX   ] 02 52 C4 80" being sent every second, and I have been unable to get past it.

I have rebooted the ISY multiple times, including rebooting the PLM, noting to reboot the PLM prior to the ISY or the opposite - all with no luck.

How can I find the culprit?  Any ideas?  I am out of them.