thj@hpcnof.UUCP (Thomas Hjellming) (11/10/86)
When I was in grad school, I rigged up some reset switches for some
Zenith 150's (IBM clone).
I simply connected a momentary contact switch across two of the
pins on the Bus Controller chip (8284A). A schematic may help:
+5 volts
-----
|
|
+-+ +------------+
| | pull up resistor | |
+-+ | 8284A |
| | |
+-----------------------| RESET |
| | |
------- capacitor | |
------- | |
| +------------+
|
-----
---
-
This is a simple reset circuit that will hold the PC system in RESET
until the power supply is sufficiently close to +5v for the system to work
reliably. When the capacitor is charged, the RESET signal (active low) will
go high allowing the system to begin execution.
For a simple reset switch, just connect one end of the momentary contact
switch between the capacitor and resistor and the other end to ground. Closing
the switch discharges the capacitor and forces a cold start. I
actually made a miniature PC piggyback board. The PC board is slightly
bigger than the 8284A chip itself and contains holes for all the pins (16
if my memory serves me correctly). On the RESET and GRND pins, however, the
PC board has copper paths to the bottom of the chip where connections for
the two momentary contact switch wires reside.
Any hobby PC board kit can be used to make the piggyback board. Or you
can just soldier the wires directly on the pins (ouch). As a matter of fact,
I think I still have one extra piggyback board at home. If you want it, send
me some e-mail & I would be glad to mail it to you.
Incidentally, this reset button circuit is exactly what is used on my
AT&T 6300 PC so I know it is safe.
Hope this helps,
Tom Hjellming
Colorado Networks Division
Hewlett-Packard
...!hplabs!hpfcla!hpcnof!thj
(normal disclaimers)