MAPNET was replaced by MAPNET II in the 90s and then that was replaced by IDNET in the very late 90s, with the 4010.
System Sensor’s “CLIP” protocol and its newer high speed versions work off this same principle of alternating the power and the data. If you put a multimeter on an SLC loop it is possible to read both DC and AC voltage. The SLC loop specifications say that it puts out 30 to 32 VDC and about 10 VAC. I normally get 15 VDC and 10 VAC on a loop when I put my meter on it. The reason for only getting half is because of the alternation that it does.
I wish I had an oscilloscope that I could hook up to it to see exactly what is happening here though.