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!  And to get out of that nasty loop you had to turn the computer off and then back on to recover.  Well, in doing so you lost your program, right?  That would not be the case if you could have used a reset switch to regain control of your computer.  However, Commodore was not intelligent enough to put a reset switch on the C-64 or the 64-C (however, they did include the rest switch on the C-128).  I will detail how you can install such a switch on your 64, whatever the model number.

Throughout the computer is a circuit called the RESET line.  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.  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.  The good thing here is that any program held in RAM is not destroyed.  You can recover right where the problem began.  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!

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.  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.

Now, to begin.  The RESET line is found at the User Port, pin 3; the Expansion Port, pin C; and the Serial Port, pin 6 (maybe).  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.  This is where the modem attaches.  Pin C of the Expansion Port is on the bottom, third from the right.  This is where a game cartridge or the RAM Expansion attaches.  Pin 6 of the Serial Port is the center pin.  This is where the cable for the disk drive and printer attaches.  If you found that the Serial Bus is connected to the RESET line then you can connect to pin 6.  If not, then the next best place is pin 3 of the User Port.  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.  Using a 22 gauge wire, you can simultaneously melt the solder and insert the wire in this hole.  Using another 22 gauge wire, connect it to the GROUND path that runs along the back side of the PCB.  This ground is rather large in area and can be connected to in quite a few places.  Take the other ends of these two wires and attach them to a momentary SPST contact normally-open push button switch.  Radio Shack sells one that works very well.  It mounts in a 0.25" hole.  Their P/N 275-1547.  Mount this switch wherever in the case of the computer you like.  I installed mine in the right rear, just above my RAM Expansion.  This switch will reset the computer alone if there is no connection between the RESET line and pin 6 of the Serial Bus.  If you would like to reset your peripherals, as well, install a second switch to pin 6 of the Serial Bus and to ground.  If your computer has a connection between RESET and pin 6 you will reset everything when you use the reset switch.  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.

Having such a reset switch will potentially lengthen the life of your computer.  Turning it off and then back on again is not the best thing for an electronics item.  It is much nicer to simply push a switch to regain control of the computer.  Again, attempt this task only if you are skilled enough to do the job right!  Don't risk damaging your computer with improper techinques.  I will assume no responsibility for your losses.  Have fun resetting!
