I took some resistance readings of the harness and I finally got to put that Turing Machines course I had to take in college those many years ago to use.
This is what I came up with so far:
This is turning out to be about as basic as it can get.
One strange thing though, I did an ohms test both ways (swapping plus and minus test leads), and I was getting different readings, which suggested the possibility of a diode. But I was getting low readings in one direction, and slightly higher readings in another. This does not quite act like a diode, unless it has a very low PIV... but I never heard of a diode with a 1.5V PIV.
Since I had 4 harnesses (the monitor I bought is a 4 channel monitor) and I only need one channel for testing, I did some destructive testing and tore a harness apart, and sure enough, I confirmed my schematic was correct.
There was no diode, and after I tore the wires out of the potting compound, the goofy readings I was getting by reversing the test leads went away. I can only surmise that the potting compound they use must be very low quality.
The potting compound seems a bit strange and kind of rubbery feeling too. It was kind of like cutting into a bar of soap. Potting compound is usually made of epoxy and very brittle and hard to cut.
Now all I need to do is to figure out how this sets up with the input to the comparators, and if the switches turn on the entire system or just the ladder.
The manual states that the system draws no power unless a button is depressed, so I want to look into that as well.
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