[BBC-Micro] Electronics Advice Needed for Beeb Project

[David Hunt]

> 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.
> 

My electronics is pretty rusty... but it looks like a potential divider

When SDO is low (0V) it brings CB2 to (0V)

When SDO is high (+3V3) it lifts CB2 to (+3V3), probably enough to represent
a logic 1. 

Why not ask Martin ? He doesn't mention the reason for the diode on the
site, just the need for level conversion between +5V and +3V3.

Cheers

Dave ;)