A "DUAL" RESET SWITCH FOR THE SX64 There are two versions possible, a "warm" and a "cold" reset. latest additions and corrections 3-9-2013 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 peripheral "warm" reset which will clear the screen if a BASIC program is running. However, if running a ML (machine language) program, the screen will freeze and the computer will lock up, requiring it to be turned off and on again to reset it. To eliminate that need, additional circuitry can be installed to enable a "cold" reset with ML programs, returning the computer to a near normal bootup state with a clear screen. Both modifications will be described here. NOTE: use of an EPYX FastLoad cart will enable a full system reset, even with a ML program running, with the warm reset modification only. Three transistors and a few other common components can be wired into the computer reset circuit without pulling any boards, cutting board traces or changing any wiring, and without any more disassembly that removing the top cover of the SX. There are several places that these added components can be located. 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. With the modification in place, holding the drive reset button down for one second or more will reset the system. You will see the screen "shrink" and freeze for a second, then redraw the opening screen with flashing cursor. If all you need is a warm reset or you use the EPYX FL cart, that's all that needs to be done. A "cold" reset circuit can be added to clear the screen of a ML program back to a startup state without the EPYX FastLoad cart (which doesn't work with this cold mod installed) although it is compatible with JiffyDOS and other carts. Although it doesn't completely erase the ML program from memory (press RESTORE key to see it), it does work well enough to allow another program to be loaded and run after the cold reset. I will show diagrams and photos of both modifications. THE WARM DUAL-RESET CIRCUIT, 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 peripherals, if present) reset. A dual-transistor "darlington" arrangement was used as the pulse driver because each peripheral 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. Note that two transistors are required if the warm reset circuit only is needed and the darlington collector is tied to the "downstream" system reset line. 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 peripherals 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 component values chosen should not be changed since that is what determines the timing of the delay when the switch is pressed and the duration of the reset pulse. FIRST AND SECOND PROTOTYPE INSTALLATIONS I selected the top of the rear panel connector PC board as the easiest place to mount the components (see photo). 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. The second prototype has all components of the mod on a small perforated 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. Photos show both types. NOTE: some SX computers don't connect the metal chassis to the circuit ground, so a more reliable ground point should be used. Pin 2 (the red wire) of either rear panel serial port is a good ground point as is pin 1 of the drive reset connector P22. Four connections need to be added to the SX circuits to complete the installation of the warm 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 source for the added circuit, 2. +5VDC is taken from user port pin 2, 3. the mod output line connects to pin 6, the reset line (purple wire) of either 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 a reliable ground. All connection points are accessible without disassembling the SX, except as stated above. As an alternative to mounting the mod on the rear panel PC board, all components can be mounted on the FDD board (see photos) but of course this requires that board be removed. The connections for +5V, mod input, mod output and ground are near the point chosen to mount the components by their leads, without a separate PC board for those parts. SX64 COMPUTER DUAL 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 ***************************************************************************** THE COLD RESET CIRCUIT, HOW IT WORKS: A positive going pulse is taken from system reset IC UG7 pin 3 on the CPU board, fed through an isolation diode and used to charge a 10uF capacitor which in turn goes to the base of a transistor through a 10K resistor. When that transistor turns on during system reset, its collector is pulled logic low for a fraction of a second longer than reset (due to the charge on its base drive capacitor) and pulls the /EXROM line low for that short time. In normal operation, the pulse from UG7 pin 3 on the CPU board appears during power up to reset the system. To enable a cold system reset after power up, the output wire (collector of the transistor) from the added warm dual-reset circuit is routed to the trigger line (pin 2) of that IC. The system is reset by holding down the drive reset push button for more than one second. SX64 COLD RESET CIRCUIT MODIFICATION: PARTS LIST 1 TRANSISTOR, NPN, GENERAL PURPOSE, (2N2222 or equiv.) 1 DIODE, GENERAL PURPOSE (1N914, IN4848, 1N4001, ETC.) 1 CAPACITOR, ELECTROLYTIC, 10uF @ 16V 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!