That pesky Jupiter Ace *still* won't work...t

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Jun 2 14:20:33 2003

> > 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!

I thought it might be... Yuck (or Bletch!) is right :-)

> > > 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...

Well, the stack is needed for CALL (or more strictly RET, in that a CALL
will get to tthe right place without the stack, but you'll never get back)
instructions, certainly. Does it call any subroutines (or use RSTs)
before loading the chargen?

>
> > 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...

4000 series CMOS may well not be fast enough for this -- you should get a
74LS32 or 74LS02, I think. You don't wnat problems caused by the test set-up.

In cany case, just chack that the CS/ line is not always asserted
(grounded). It might well be for something like the chargen or video
memory, since those are read out continuously to produce the display. If
that is the case, you just need to trigger on the Wr/ signal at the RAM
(which will be a combination of the Z80's Wr/ signal, the output from an
address decoder, and so on).

>
> > 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...

Well, if one bit is loading correctly it'll be a start...

>
> > [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.

Indeed :-)

>
> > [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.

Oh, if you have a Tektronix with 2 timebases and the delay time
multiplier control you'll be fine ;-)

-tony
Received on Mon Jun 02 2003 - 14:20:33 BST