Re: PNP versus NPN sensors
 

True, but that's typically the power rails which self protect, not the signal inputs.

Was able to test last night. Unfortunately the web interface had become corrupted and wasn't able to get status messages. I did connect battery voltage (~12V) to the DIO signal line and no smoke occurred. Checked DIO voltage & expected signals afterwards and it was still operating as expected, no apparent damage. Will see if I can get it to talk.