SWTPC ASCII, Manual

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4
This keyboard uses EVEN parity in both the upper case only and the upper
plus lower case modes. The parity bit should be low when the number of zeros in
the output code is odd and should be high when the number of zeros is even. For
example the ASCII code for X, 0001101, should make the parity bit high. The
repeat function should be checked by holding any one key down for more than
about 1 sec. The keypressed strobe should switch on and off until the key is
released. If a faster or slower repeat rate is desired the value of C6 can be
changed slightly.
Circuit Description
As shown in figure 1 the keyswitches are arranged into a matrix with the X
lines from IC-1 outputting test pulses in time sequence and the Y lines sensing
their presence. If no key is pressed, IC-1 continuously scans the keys, at a
rate of 50,000 keys per second. Whenever a key is closed, the scanning action
stops and that location is held by the internal circuitry of the encoder. This
location is called an address. This address is routed to an internal fixed
memory called a read only memory. In exchange for an address and some
information on the status of the shift and control keys the read only memory
gives the proper 8 bit ASCII output code, complete with parity.
As soon as the scanning stops, a time delay is started and controlled by
R1 and C1. This takes into account any switch closure bounce or noise and makes
sure the key is firmly down and not just brushed on the way by. After the delay
time, an output "keypressed" strobe is made available. This tells whatever you
attach to your keyboard that the code is ready for use and valid.
If a key is released the scanning action starts up again and goes on till
a new key is pressed. If two keys are pressed at nearly the same time the first
key pressed provides its output code after a debounce delay. When the first key
is released, the scanner starts up but only goes around till it hits the other
key's location. After a new debounce delay, the second key's code is output.
Keys can continue to be pressed in sequence two down at a time forever, with
always the right code being output in sequence, and nothing missed or out of
order. This most handy feature takes care of sloppy typing and "burst" rate
typing where keys are hit fast and furious in sequence.
IC2 and IC3 are used for the automatic repeat function. IC3 is setup as an
astable multivibrator that continuously runs at a frequency of approximately 8.5
Hz. When a key is first pressed the KP line of IN goes high causing an immediate
low output of IC2-C. If the key is immediately released no repeat function is
initiated. If the key is held down for more than about 1 second C7 will charge
to a point where IC2A is triggered. At this time the output of IC3 is NAND'ed
with the KP strobe therefore causing the KP signal to be chopped just as if the
key was continuously pushed and released. This action will continue until the
key is released.