Date   : Tue, 29 Jun 2010 22:58:11 +0100
From   : samwise@... (samwise)
Subject: BBC Master Compact key ghosting

Hi,

I've managed to convince a rather talented individual, Paul - creator
of the Retro Adapter [http://retro.world3.net/], to work on the rather
cool project of converting a dead Master Compact into a PC USB
keyboard that can be used with emulators.  It even has a retro adapter
inside which will allow usage of beeb joysticks with the PC.

After getting over most hurdles, he's stumbled across some issues with
ghosting which he'd like to compare with what an actual BBC does with
them (as the Compact was dead when it arrived, so couldn't be tested
beforehand).  I don't have a beeb to hand and, tbh, probably don't
have the hardware know-how to answer properly either so thought I'd
post here to see whether anyone more knowledgeable would be inclined
to comment.

Take for example the "Z X / : Return" combo. If you press Z+X+/+: it
gives you a ghost A and space bar.  Some games like Chuckie Egg don't
allow you to have the space bar  in some combinations as one of the
assigned keys, perhaps because it is so prone to ghosting?

Does a real BBC ghost in that way too?  Paul has looked at the PCB and
the schematic for the Master Compact and can't see any diodes or
anything like that so it seems that it must have ghosting. The
question is how does it deal with it. There are three potential
options:

1. Ignore it, allow ghosting
2. Prevent ghosting by not reporting newly pressed keys when it happens
3. Lock up until keys are released

The PC USB keyboard does (3), and currently it looks like the BBC
keyboard does (1).

We're missing some BBC expertise but Paul theorises that there are ROM
calls responsible for handling this sort of keyboard input, although
the suspicion is that any program could bypass those and access the
hardware directly.

Can anyone confirm how a real beeb/master compact behaves wrt ghosting?

Thanks for any insight,

Peter.