As an electronics Tech, I like to troubleshoot so I can see and understand the issue and solution. With that in mind, I had an Insteon Developers manual from 2009 that I found on line years ago. In there I found the schematic for an early model PLM which I happened to have in my junk drawer. From the schematic, Q2 collector drives the Rx Input of the custom Insteon micro-controller chip.
I defeated the Output driver transistor Q3 by grounding the base lead so transmitted packets would not interfere with Rx packets that I was trying to detect. I then soldered a wire to Q2 collector and a second wire to ground, then hooked my oscilloscope to these wires. Wow, the amount of power line noise was overwhelming in my shop. In my house it was fairly clean and Devices were working in the house.
I started turning everything OFF one at a time in my shop and still had noise. Then I went in the utility room and remembered the in-floor heating controller. When I opened the controller box to pull it's power, I saw the Dog's confinement transmitter wall wart there too. I put the dawg in his kennel and removed his collar so I could power the system down. Then pulled the wall wart for it and the noise was gone!! Plugged it back in again and the noise returned. This equipment is on the same circuit breaker as the Insteon devices in the shop, so I didn't shut that breaker down because I would have no way to know if communications returned when I was just using the devices for troubleshooting.
My make-shift PLM noise detector showed me exactly what the problem was!
FYI, I had gone back to the original PLM and managed to get it working again. And found that when I ran an extension cord from the house and plugged an Insteon Access point into it, I could control the shop devices provided the access point was about 20 feet from the shop. I had then tried to install the new PLM, but still kept getting the Config File Not Found error. Something about the new PLM doesn't agree with the ISY.
But I am just happy to have everything working again!
RF_Burns's post in Replaced PLM, it will not talk to devices>>SOLVED! was marked as the answer