May 3, 1987

         This will describe how to add a hardware reset switch or button
      to your IBM PC, XT, or AT.

         The most stressful time on you computer and hard disk is when
      power is first turned on.  Every time your computer locks up and
      you have to reach for the BRS (Big Red Switch) you add 1 more
      stressful time for your computer and also 1 more time (power going
      off) when you might write to whatever track is under the head of
      your hard disk.

         Adding a button or switch to apply a FULL reset to the system
      would allow power to remain to the system but get you out of any
      lock up situation.  If you have any experience with a electronics
      and know which end of the soldering iron to grab, the hardest part
      of adding the button is deciding where to mount it.  That part I
      will leave up to you.

         This procedure has been done to many IBM PCs, XTs, and ATs and
      even a few Tiwain clones. The description will assume IBM computers
      but it should work on any clone which has a power supply that 
      provides a POWER_GOOD signal and any computer that uses it.  I have
      found 1 Tiwain clone that didn't so check first!

         On the IBM computers, the power good signal is on the connector
      which plugs in the rear-most of the 2 power connectors.  In the
      computers I have modified, it was orange.  This wire must be cut
      at least 1 inch from the connector.  Solder a 1/4 watt resistor
      whose value is between 1K and 2.7K (exact value not critical but
      NOT less than 1K) in between the cut ends of the orange wire and
      also a wire to the end closest to the mother board connector.  This
      wire goes to the button or switch.  A second wire from ground is
      needed.  This may come from a mounting screw on the mother board
      or you can tap on to one of the black wires in the power supply
      to mother board cable (there are 4 black wires available - all are
      grounds).  Since your are already cutting one of these wires (the
      orange one) it will make a neater job if the ground comes from the
      black wire in the same cable.


      Mother board connector (1 of 2)                 ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
             ÚÄÄÄÄÄÄÄÄÄÄÄ¿                            ³
      orange ³ Power goodÃÄÄÂÄÄÄÄÄÄresistorÄÄÄÄÄÄÄÄÄÄÄ´
             ³ (unused)  ³  ³    (1K to 2.7K)         ³
      yellow ³ -12 volts ÃÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´    Power  
      grey   ³ +12 volts ÃÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´    Supply
      black  ³ Ground    ÃÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
      black  ³ Ground    ÃÄÄÅÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
             ÀÄÄÄÄÄÄÄÄÄÄÄÙ  ³        ³                ³
                            ³    /   ³                ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
                            ÀÄÄ+/ +ÄÄÙ
                           Reset button
                             or switch


      Technical explaination:

         The signal to preform the reset is a ground (0 volts) applied to
      the 8284A (82284 in an AT).  This signal is provided by the power
      supply in the form of a POWER GOOD signal.  The reason for adding the
      resistor is that the signal is provided by a PNP transistor with its
      emitter connected to the +5.  Without the resistor, appling ground
      to the power good signal would burn out the transistor and render the
      computer useless.  I had occassion to delve very deeply into the
      power supply of the IBM PC and know how the supply was origionally
      designed.  I am ASSUMING the same or similiar circuitry exists in
      newer supplies used in the XT and AT since the signal hasn't changed.
      Even if the circuitry has changed the resistor can do no harm since
      the input to the 8284A (or 82284) is high impedance.

         As with most things in this world, I assume no responsibility for
      anything that happens, either good or bad, and give this to the world
      to do with it what it will.  Any comments can be left on
      Mikes C board  (619) 722-8742.
      

      Tom Oppenheimer