latest additions and corrections  2-16-2011

There is a way to add a computer reset to the SX64 without drilling any 
holes in the case for a new switch. The SX already has a drive reset switch on 
the front panel inside the preference control door. With the addition of less 
than a dozen small components, that button can be made to perform a dual 
function: the original drive reset and, if the button is held down longer than 
one second, a full computer, drive and external periferal reset. You will find 
more information about the SX as well as pictures and schematics of this 
modification at http://staff.washington.edu/rrcc/uwweb/SX64/

     Three transistors and a few other common components can be wired into the 
computer reset circuit without pulling any boards, cutting any board traces 
or changing any wiring, and without any more disassembly that removing the top 
cover of the SX. The transistors and other components form simple delay and 
momentary pulse circuits that take the momentary logic low of the existing 
drive reset switch, modify it, then feed it back into the main computer reset 
circuit. The schematic of the added circuitry is listed as RESET.GIF
     Since there are so few components used, I decided against fabricating a 
PC board for my initial modification, but one can be made with perforated 
board if desired. On my first prototype, all added components were suspended 
by their own leads in a convenient spot that was easily accessible and close 
to the connection points needed. It's listed as sxmod.jpg on the site.
     My second prototype used "perf" board and was installed at the SX rear 
panel PC board by removing the existing screw and mounting the added board 
with it, providing a ground return for the board. See modboard.jpg

     The on-board disk drive in the SX has a stand-alone drive reset switch. 
The +5VDC on that switch when it's open (not pressed) is used to charge a 
small capacitor. The charge on the cap keeps the first transistor of this
modification (Q1) turned on with forward bias on its base, so its collector 
is held at zero volts. When the reset button is pressed for less than one 
second as is normally done to reset the drive, that first capacitor remains 
charged and keeps Q1 turned on, so the rest of the circuit does nothing. If 
the drive reset button is held down for longer than one second, the charge 
on that first capacitor bleeds off and allows Q1 to turn off. As its collector 
voltage rises, that positive going voltage is sent through a second capacitor 
to the base of transistor Q2, turning it on and driving Q3 on as well. As the 
charge on the second cap rises, Q2 and Q3 turn off again about half a second 
later. The collectors of Q2 and Q3 are wired into the main reset circuit of 
the computer at pin 1 of the serial port. The resulting nagative-going pulse 
triggers a full computer and drive (and external periferals, if present) 
reset. A dual-transistor "darlington" arrangement was used as the pulse driver
because each periferal such as a 1541 adds a pull-up resistor to the serial
reset line. If many drives are connected, that represents a greater load that
must be driven reliably. I tested my prototype circuit with four external 
drives connected and all were reset along with the computer. 
     The circuit modification is such that one and only one pulse is sent to 
the reset circuit if the button is held down longer than one second. Except as 
stated, the added components do not change the operation of the computer, 
internal drive or connected periferals in any way, so there is no need for any 
kind of added "bypass" switch or reset disable circuitry. 
     The components used are all common devices available at such outlets as 
Radio Shack (www.radioshack.com) or Mouser Electronics (www.mouser.com). The
three transistors are switching NPN types with a generic number of 2N2222, but 
any small signal NPN type would work as well. The diode can be any signal or 
power diode. I used a generic 1N914 type but a 1N4001 power supply diode would 
also work. The resistors used for the prototype were 1/4 Watt carbon but any 
wattage can be used as long as they will fit physically. The two capacitors 
are small electrolytics, 10uF at 16 volts or higher. Polarity is critical for 
the diode and the capacitors. Higher wattage resistors and higher voltage 
electrolytic caps will also work in this modification but will be physically 
larger. The capacitance values chosen should not be changed since that is what 
determines the timing of the delay when the switch is pressed and the length 
of the reset pulse. 
     For my 1st prototype installation, I selected the top of the rear panel 
connector PC board as the easiest place to mount the components. There is an 
open and tinned spot on the ground plane of that board between the user and 
serial ports. The two transistors emitter leads were first soldered to the 
ground foil, then the other components were wired to them suspended by their 
     Four connections need to be added to the SX PC boards to complete the 
installation of the mod: 1. pin 2 of connector P22 on the FDD board (comes 
from the drive reset switch on the front panel) is used as the trigger source 
voltage for the added circuit, 2. +5VDC is taken from a solder point near the 
user port that goes to pin 2 of that port, 3. the mod output line connects 
to pin 6, the reset line (purple wire) of the rear panel serial port, and 4, 
the  ground return line for the modification. If you fabricate a board, make 
sure to include a line to chassis ground. All connection points are 
accessible without disassembling the SX, except as stated above.
     With the modification in place, holding the drive reset button down for
one second or more should reset the system. You will see the screen "shrink" 
and freeze for a second, then redraw the opening screen with cursor flashing. 
That's true for running BASIC programs, but with some ML programs running, a 
"soft" reset will freeze the screen but not completely erase the program. In 
that case, the computer will have to be turned off and on again to clear the 
memory. However, a soft reset via this mod circuit works better to clear the 
memory if you have JiffyDOS installed... as if you needed any more reasons to 
get that upgrade. 


 3     TRANSISTORS, NPN, GENERAL PURPOSE (2N2222 or equiv.)
 1     DIODE, GENERAL PURPOSE (1N914, 1N4848, 1N4001, ETC.)
 3     RESISTORS, 100K OHM, 1/4 OR 1/2 WATT
 1     RESISTOR, 10K OHM, 1/4 OR 1/2 WATT

Ray Carlsen CET
Carlsen Electronics... a leader in trailing-edge technology. 

Please let me know if you spot any errors here or if you have any feedback
(good or bad) to report. Questions and comments are always welcome. Thanks!