C64 Reset Switch - How To

EDITED BY: Steve Peyrot

FROM: MEMPHIS COMMODORE USERS CLUB

NNNN If you're a Commodore 64 user you've probably been in the situation
before where you were either working on or writing a program and the computer
crashed!N And to get out of that nasty loop you had to turn the computer off
and then back on to recover.N Well, in doing so you lost your program, right? 
NNNN That would not be the case if you could have used a reset switch to
regain control of your computer.N However, Commodore was not intelligent
enough to put a reset switch on the C-64 or the 64-C (however, they did
include the reset switch on the C-128).N I will detail how you can install
such a switch on your 64, whatever the model number.
NNNN Throughout the computer is a circuit called the RESET line.N This reset
connects to the 6510A MPU, the 6526 CIA, the 6581 SID, the Expansion Port, the
User Port, and on some 64's to the Serial Port.N When this RESET line is
connected to GROUND the computer performs a Cold Start, which is just like
turning the computer off then back on.N The good thing here is that any
program held in RAM is not destroyed.N You can recover right where the problem
began.N If you're in BASIC then you'll have to use a utility program that
resets BASIC's pointers back to their proper place before the reset occured.
In addition, any programs held in the 1764 REU will still be there!
NNNN I mentioned that the RESET line is connected to the Serial Bus in some
64's. The schematic shows such; however, it appears that early 64's had the
Serial Bus/RESET line connection and late 64's did not.N I found that mine did
not. If you have no connection between the RESET line and the Serial Bus, then
you can install two (2) reset switches; one for the computer itself and one
for any peripherals you have connected.
NNNN Now, to begin.N The RESET line is found at the User Port, pin 3; the
Expansion Port, pin C; and the Serial Port, pin 6 (maybe).N You can use an
ohmeter to determine if pin 6 of the Serial Port is connected to either of the
other two pins mentioned. Pin 3 of the User Port is on top, third from the
left when viewed at the back of the computer.N This is where the modem
attaches.N Pin C of the Expansion Port is on the bottom, third from the
right.N This is where a game cartridge or the RAM Expansion attaches.N Pin 6
of the Serial Port is the center pin.N This is where the cable for the disk
drive and printer attaches.N 
NNNN If you found that the Serial Bus is connected to the RESET line then you
can connect to pin 6.N If not, then the next best place is pin 3 of the User
Port.N This particular pin runs to a feed-through hole that connects a trace
on the bottom of the PCB to a trace on the top.N Using a 22 gauge wire, you
can simultaneously melt the solder and insert the wire in this hole.N Using
another 22 gauge wire, connect it to the GROUND path that runs along the back
side of the PCB.N 
NNN This ground is rather large in area and can be connected to in quite a few
places.N Take the other ends of these two wires and attach them to a momentary
SPST contact normally-open push button switch.N Radio Shack sells one that
works very well.N It mounts in a 0.25" hole.N Their P/N 275-1547.N Mount this
switch wherever in the case of the computer you like.N I installed mine in the
right rear, just above my RAM Expansion.N This switch will reset the computer
alone if there is no connection between the RESET line and pin 6 of the Serial
Bus.N If you would like to reset your peripherals, as well, install a second
switch to pin 6 of the Serial Bus and to ground.N If your computer has a
connection between RESET and pin 6 you will reset everything when you use the
reset switch.N Additionally, if you have the Commodore 1670 modem, you will
reset the modem when you reset the computer. You will then have to command the
modem to not pickup the phone on the first ring.
NNNN Having such a reset switch will potentially lengthen the life of your
computer.N Turning it off and then back on again is not the best thing for an
electronics item.N It is much nicer to simply push a switch to regain control
of the computer.N Again, attempt this task only if you are skilled enough to
do the job right! Don't risk damaging your computer with improper techinques.N
I will assume no responsibility for your losses.N Have fun resetting!
 
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