Non-CRTC PET video circuitry...
derschjo at gmail.com
Thu Nov 13 00:46:00 CST 2014
Yet another "oh, let's spend an evening doing a small project" that's
ballooned into a major undertaking:
I thought I'd try my hand at getting my old PET up and running again;
this is an original "chiclet" keyboard model (with 6550 RAMs and no CRT
Controller chip). There are a multitude of problems with it (the CPU
isn't resetting, I'm sure there's bad RAM and ROM) but I thought I'd
start with the most obvious fault -- the video generation.
Here's a picture of what it's currently doing (sorry it's not very
good...) : http://yahozna.dyndns.org/scratch/pet/oneline.jpg
It's only drawing a single line of characters (garbage with some noise
due to a very terrible socket holding the character ROM), centered
vertically. I initially thought this was a VRAM addressing issue (a
stuck counter or whatnot) but it turns out the VSync signal is much too
fast -- it's only about 1.2ms long (it's supposed to be about 16ms).
I'm actually surprised the monitor can deal with that without damage, so
I've been leaving the monitor disconnected while debugging. The HSync
period is correct (63.8us or so), and most of the addressing / character
display logic (which is driven by earlier portions of the video timing
chain) seems to be running correctly, it's just only getting a chance to
draw one line of characters before the next frame starts.
I'm not having much luck tracing down the fault; I have a schematic
but it's not making it easy to work out exactly how everything ties
together. I can't find anything resembling a service manual for this
model, but perhaps I haven't looked hard enough yet.
The initial clock (output of E2 pin 6) is correct at 8Mhz; this is
divided down to 2Mhz by the counter at C9 (pin 8, the divide by 4
output), which is passed to the 74LS107 at C7 and from here it gets
murky and I don't quite see how things fit. I did replace C7 to no
effect (ok, I jumped to conclusions...). The divided clock outputs (Q,
pins 3/2) from this have a period of about 256us; the LS107 at B6
divides this down to the final period of 1.2ms which is used for the
Has anyone here any experience with this hardware? Any tips?
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