The solutions are:
1: Ignore
2: Put the MOSFET on the source instead
3: Use the 5th 7W2 wire for MOSFET control
Option 2 is the most robust, but having the MOSFET on the source key means the load key's MCU must be powered from before that MOSFET. In other words, it must get its power from the source key via the 4 comms lines. This would not work for series sources, at least not without power isolation as well as signal isolation.
But if a diode is tied from the source's negative rail to the optocoupler's negative rail it should get power to the load key's MCU.
So long as the load tells the source what to do with its supply, such as shut off once it hits a certain voltage or just keep regulating, the system should be self sustaining.
But having series MOSFETs is a bit sketchy, they'd still need to be rated to the total voltage.
The solutions are:
1: Ignore
2: Put the MOSFET on the source instead
3: Use the 5th 7W2 wire for MOSFET control
Option 2 is the most robust, but having the MOSFET on the source key means the load key's MCU must be powered from before that MOSFET. In other words, it must get its power from the source key via the 4 comms lines. This would not work for series sources, at least not without power isolation as well as signal isolation.
But if a diode is tied from the source's negative rail to the optocoupler's negative rail it should get power to the load key's MCU.
So long as the load tells the source what to do with its supply, such as shut off once it hits a certain voltage or just keep regulating, the system should be self sustaining.
But having series MOSFETs is a bit sketchy, they'd still need to be rated to the total voltage.