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AUTO-REPEAT FOR THE H19/H89
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For those interested in a low-bucks approach to auto-repeat on
your H19/88/89, take a gander at this. Word processing aficianados will
appreciate the ability to move the cursor around without having to
hold down the REPEAT key.
THEORY OF OPS
Diodes D1-D10 monitor the outputs of keyboard plug pins
14,16,18,20,22,24,26,28,30,32 and clamp the outputs at 5 volts. This
is input to a dual retriggerable multivibrator(C1), which sets up the
delay. If, after a delay, the output at pins 4 and 5 are still low,
they feed a PNP transistor which puts a low out on the repeat
line (keyboard pin 31).
ASSEMBLY
The circuit uses 1 chip, 1 transistor, 4 resistors, 12 diodes, 2
capacitors and 1 variable potentiometer.
Point to point wiring was used, with the entire circuit mounted on a
1.5" by 3" piece of perfboard attached to the underside of the
keyboard with stand-offs and sheet metal screws used to secure the
board. The inputs to the diodes were soldered directly to the lands on
the keyboard (plug pins 14,16,18,20,22,24,26,28,30,32), as was the
output going to the repeat function (plug pin 31) using small gauge
wire. The +5 volts and ground were taken directly off the 5 volt
regulator using alligator type connectors in the event keyboard
removal is necessary. The delay time is established by potentiometer
P1. Adjust P1 until you find the optimum delay for your typing speed.
There are times this feature may be a liability, especially in game
playing. Some games have you hold down a key to move a piece on the
screen, this circuit will not inhibit any functions but the keyboard
clicking will drive you crazy. Since I have my +5v attached with
alligator clips, I just pull off one lead to stop auto repeat. An
alternative would be a small switch located on the back of the case.
Be careful when attaching the circuit board to the underside of
the keyboard. There are lands on the reverse side that are difficult
to see.
I have found a "bug" of sorts. After I turn on my system it
takes a minute or two before the circuit warms up and starts
repeating. However, this is usually no problem because it takes me a
few minutes to go through the bootup procedure.
There are numerous ways to implement auto-repeat in your
computer, each with varying degrees of sophistication. This circuit
was chosen as the most direct solution to the problem. The design
credit goes to Ronny Toups, an inveterate hardware hacker who would
rather tinker than eat, drink, or sleep. If you have any questions, try
reaching me at home (214-385-1367), or leave a message on the DF/W HUG
BB. (214-742-1380)
ANDY PALUCH
Dallas/Fort Worth Heath Users' Group
EOF AUTORPT.DOC
the DF/W HU
