A RESET SWITCH FOR THE SX64 2-16-2011.
A RESET SWITCH FOR THE SX64 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 HOW IT WORKS: 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 leads. 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. SX64 COMPUTER RESET MODIFICATION: PARTS LIST 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 2 CAPACITORS, ELECTROLYTIC, 10uF @ 16VDC 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!