In message <m19Mb5L-000IyHC_at_p850ug1>
ard_at_p850ug1.demon.co.uk (Tony Duell) wrote:
> Sounds as though the timing chain is fine, but it can't hurt to check it.
Agreed.
> 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!
> In fact have you checked you have some activity at all the address and
> data pins of all the RAMs? If you see a pin that looks 'stuck' it would
> be worth investigating.
OK.
> > It looks like I misunderstood the code - it stores 0xFC in every byte,
> > starting from the bottom of the main RAM, until the byte it reads back is
> > not equal to 0xFC. It then rounds the RAM count down to the nearest
> > kilobyte.
> Right. What does it do if it finds no valid RAM?
I have no idea... Looks like it'll set RAMTOP (?) to 0, thereby screwing up
any and all further RAM calculations.
> > 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...
Er... No. My Z80 is rusty. VERY rusty...
> 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.
Later.
--
Phil.
philpem_at_dsl.pipex.com
http://www.philpem.dsl.pipex.com/
Received on Mon Jun 02 2003 - 08:46:00 BST