From: dmccorm1@students.wisc.edu (Al McCormick)
Subject: Repost: Williams pause hacking
Date: 1996/04/08
newsgroups: rec.games.video.arcade.collecting


OK Duncan, since nobody else has reposted your hack, here it comes straight 
from Virtu-Al's RGVAC archive...

(donning a flameproof suit and a kevlar flack jacket for good measure...)

Is there a similar hack for the Blaster HW? I have a working boardset and am 
in the process of JAMMAtizing it (hey, I don't have the cab) and was just 
wondering. I'm too chicken to even think of slicing a trace on this gem. Our 
group's Berzerker-fanatic would probably have the Unabomber send a little 
care package to me if I did... :)

Thanks for the great hack Duncan!

Virtu-Al
****************************************************************************
From: brown_du@eisner.decus.org (Duncan Brown)
Subject: Re: Projects for bored minds....
Date: Sun, 3 Mar 1996 03:29:24 GMT

In article <1996Mar2.165753.1@eisner.decus.org>, 
brown_du@eisner.decus.org (Duncan Brown) writes:

> This is definitely possible with the Williams 6809-based games, as I have done
> it during development of Alien Arena.  It requires you to disable the watchdog
> circuit (this should be a funny in-joke for a few people...) but then you
> basically just toggle the normally unused ~HALT line to the processor.  The
> video circuitry keeps cranking, so all you do is freeze the game processing,
> and the last known contents of teh frame buffer stay on the screen.  (This
> would be significantly trickier with an Atari vector game!)  A few trace cuts
> on the board, use a spare gate to buffer the switch, and you're in business.

Following up my own post, after some prodding by He Who Mows Lawns:

These instructions are not intended for those unfamiliar with circuit board
modifications/repair.  If what I describe here doesn't make enough sense
without additional pictures and words, you probably shouldn't be trying this
anyway...

These instructions cover modifying a Robotron CPU board to add a halt circuit. 
Games like Joust, Sinistar, Bubbles, etc. should be virtually identical to
this.  Stargate may well be the same; later games probably can make use of the
same concept, but the chip locations all change.  Stargate probably doesn't
strictly speaking require this extensive a mod....it's the fact that the
Special Chips (BitBlt chips doing DMA transfers instead of the processor, and
therefore needing to halt it) use the ~Halt line that makes it more work for
these later games.

First off, you need to disable the watchdog circuit.  Williams provides a handy
"split pad" for just this purpose.  It's located just to the right of chip 5H,
connected to pin 10.  Cut the skinny trace between the two halves of the circle
and you have disabled the watchdog circuit.  (Because of this, you may want to
consider extending the ~Reset line out to another switch to go along with your
halt switch...)  This circuit resets the processor if it gets "off in the
weeds" and doesn't reset a counter frequently enough.  Halting the processor
means it wouldn't reset the counter, and would thus get reset by the watchdog
circuit, pretty much ruining any benefit of halting it!

Next, we need to set up a circuit that will halt the processor if the Special
Chips need to halt it *OR* if we want to halt it via a switch.  First, sever
the ~Halt line between the processor and the connector going to the ROM board. 
The trace for this is on the back side of the board, and leads away from pin 40
of the CPU heading towards the connector.  Be sure to leave it connected both
to pin 40 and the trace running the other way, because that one's a pullup
resistor, which it needs.  The cut should be right beside R26, but on the BACK
of the board.  This means the ~Halt input to the processor is now isolated. 
Let's hook it back up to something.

Our needed spare gate is part of 6J.  Pins 3, 4, 5, and 6 are what we need, but
like good little engineers, Williams' designers have tied 3, 4, and 5 to
ground.  On the back side of the board, underneath 6J, cut the tiny traces
connecting those pins to the ground bus (which runns next to the chip and down
to pin 8.)  Solder a piece of wirewrap wire to connect 3 and 4 together and then
connect that all the way up to the FEEDTHROUGH that was taking the ~Halt line
over to the ROM board connector.  It's just to the connector side of the cut
you made in the ~Halt trace.  That gets the ~Halt signal from the ROM board
into our new logic gate.  Next, use a piece of wirewrap wire soldered on to pin
5 of 6J and tie that to your halt switch (more details later).  That gets our
user-controlled Halt request into our new logic gate.  Now solder a piece of
wirewrap wire on to pin 6 of 6J and solder the other end to pin 40 of the CPU
socket.   This is all on the BACK side of the board, of course...

We have now put a NOR GATE in the circuit.  If the ~Halt signal from the ROM
board is low *OR* the input from our halt switch is low, then the ~Halt signal
to the processor is low and it halts.  Otherwise it runs freely.  Meaning that
if our halt switch is deactivated, the normal ~Halt signal from the ROM board
controls the processor's halted state.  If our switch is active, the processor
stays halted completely.  Bulletproof design would require a 4.7Kohm resistor
between pin 5 of 6J and the 5V rail...but TTL has internal pullups on inputs,
and I didn't put one in, and it worked fine.  I was trying not to add parts
other than wire!

About hooking up a switch.  Whatever works for you.  Here's what I did, but
it's a lot of work:  Using a razor blade and a drill, or a Dremel tool, or some
such, we can add a new Molex connector at location 0A or so.  I used a .156
straight pin connector, you can use whatever you like.  On top of the board,
cut a rectangular "hole" in the ground plane ringing the board.  You want it to
not be broken completely, you're just trying to keep it from shorting against
the pins of your connector.  Mark off and drill holes through the board to
mount  your connector on the top side.  On the bottom side, cut slots around
the pin holes to electrically isolate most of them (leave at least one
connected to the ground trace to allow for a ground pin for your switches.) 
You're trying to manufacture pads for the pins out of the existing copper clad
material!  Once those are all isolated, scrape the solder mask from the bottom
side of the new pads (including the area where your new conector's ground pins
are) and solder in your new connector.  Then connect up the new conector pins
to the aforementioned new halt input (pin 5 of 6J), and the reset input if so
desired.  Run a not-too-long wire with a mating connector, and on the far end
put some switches.  Momentary N.O. for reset, click-on/click-off N.O. for the
halt circuit.  Signal on one pin, ground on the other, for each of the
switches.

Looking at this board, I seem to have also wired up a one-shot circuit using a
spare 7474 gate, that feeds into the ~NMI circuit.  I have no recollection why,
but I suspect it was to allow single-stepping the program somehow (amazing what
you can forget in 10 years...)  That will all be a dissertation for another
day.

Hope this helps someone somewhere.  It's kinda fun to step through a Williams
game, halting right in the middle of an exploding alien or something, to get a
better look at how they do things.

Duncan, hack-from-the-past