2 Jun
2003
2 Jun
'03
7:20 a.m.
IIRC
there's some kind of multiplexer between the timing chain and the
video RAM address lines (I can't remember if this is a real mux, or one
of the Jupiter Ace type ones [1]). Maybe worth checking that's OK.
Will do.
You're right about it not being a real mux - a couple of 1k carbon
resistors and a tristate buffer. YUCK! Obviously, if it thinks the RAM isn't set up
right, the stack is going to
get pretty badly fudged. That shouldn't stop it from loading the CHG
though...
Are you sure no stack or data memory is needed for this... For it to load the chargen, the Wr/ and CS/
signals to the chargen RAM
must be asseted at the same time, right? Can you attempt to detect this
with the 'scop, maybe with an external OR (or NOR) gate?
I'll have to
snaffle a 40-series gate from my junkbox but it should be
possible. I'll need to find a set of probes as well... If you can trigger on that [2] (assuming a good
'scope [3]), then maybe
you can look at the data going into the chargen, etc.
On a dual-channel scope?
Hmm... That's going to be fun... [2] A useful trick, I don't rememebr where I
learnt it, is to pulse the
reset line at a few 10's of Hz. Slow enough that the machine does
something after each reset, but fast enough that you get repetitive
signals to look at on the non-storage 'scope.
Right. I'll grab a 555 from
the junkbox as well. [3] A delayed timebase is very useful here....
The 466 has a dual timebase with a Delay mode so I guess there's no problem
there.