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The Controls leader board takes in a 12V line from BPS called POW_2ND_CONN and is routed back out to BPS Power Systems (Power Board) as BPS_PWR_OUT. The 12V line is on the 12V power domain and is either cut off from ground or connected to GNDPWR, which is the 12V ground. This line goes through a rotary switch which determines whether current goes through the photocouplers or not. Photocouplers allow for logic to be done across two power domains, as seen here. If a photocoupler has current, then the transistor on the other side allows current to conduct, creating a short circuit. On the other side of the photocouplers, is a GPIO input that is pulled up to 3.3V (3V power domain). The MCU reads HIGH if the photocoupler has no current going through it since there is an open circuit to ground. Once the photocoupler receives current, it creates the short circuit meaning the node which the GPIO input sits on is now GND and the MCU reads LOW.
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