[BBC-Micro] Electronics Advice Needed for Beeb Project

[Darren Grant]

I agree, I don't quite get it. I can see the purpose of the diode as the
beeb user port CB2 signal is being held low by the 4k7 resistor and the SDO
signal is being held high by the 47k resistor on the 3.3v line. So to
prevent there being the full current flowing from the 3.3 v through to the
ground the diode has been put there.

Does this mean that the MMC uses a low to signal a 1 ?

But I still can't see how it is possible for the signal to change from a
high to a low.