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But suppose I also want a reasonable current to flow through the harness, how would change this circuit? As it stands, the current will be very low because of the high input resistance of MOSFETs.
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Simply move to to the receiver end the resistor which pulls up to +12V.
Leave on driver end only the MOSFET. Sorry I was not clear on this point.
You are right: allowing current to flow through the harness is a good idea to make the system more immune to noise.
I guess the switch in your schematic refers to the fpga output driven in open drain mode.
This won't work correctly since gate capacitance would keep the active level indefinitely.
In this configuration the fpga output must work in push-pull mode then a standard output, which drives both low and high level. But this would mean twice fpga pins because you need to route input signals elsewhere, and I think you can't afford it.
If you want to use the original bidir i/o configuration I suggested, you must change the circuit in one of this ways:
- use BJT instead of MOSFETS (the BJT base current will switch off the transistor)
- place pull down resistors on MOSFET gates
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