2020.12.23
- studying schematics

  - vacuum sensor board has 3 sensors with 3 states each (2 bits) output,
    all using 24V logic

    8-pin connector to the rest of the system
	1	24V
	2	head A bit 0
	3	head A bit 1
	4	head B bit 0
	5	head B bit 1
	6	head C bit 0
	7	head C bit 1
	8	GND

  - limit sensors seem to have 3-pin connectors .. these must be optical
    sensor modules or something and not just switches?
	1	24V
	2	output
	3	GND