STAR*TECH Journal
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used as signal blockers so that closed
switches on inactive strobe lines don't
conflict with the switches actually being
polled.
December 1987
(7)
Now let's cause a failure:
Short the diode at (0,1). Strobe 0 still
pulls return O low, but it now also pulls
strobe 1 low -which forward-biases the
diode at (1,1). It conducts and pulls
return 1 low. Now the board thinks (0,0)
and (1,0) are closed, even though (1,0)
isn't really.
If a return fails and gets stuck low the
logic will think every switch connected to
it is always closed, or if it gets stuck
high the logic will think all of the
switches connected to it are always open.
0
j_
2..
TROUBLF.sHOOI'ING
Sometimes trouble shooting a faulty matrix
can get a little confusing, so let's cause
some failures in our 4 by 4 matrix and see
what happens.
First let's examine normal operation.
let's assume the following conditions:
Strobe O is low and the switches at (0,0),
(0,1), and (1,1) are closed.
A strobe that is stuck high will do the
same thing. The logic will think every
switch connected to that strobe is open.
A strobe that gets stuck low can cause all
kinds of wierd things to happen. Let's
assUIIE the same switches that we used in
the first exanple are still closed, and
strobe 1 is stuck low. As the strobes are
sequenced, return O is always going to be
pulled low by the stuck strobe, therefore
the CPU "sees" every switch on return Oas
being closed. Not only that, the closed
switch at (1,1) is going to pull return 1
low also. This means the CPU also assUIIEs
every switch connected to return 1 is
closed also.
In an actual game this can cause a switch,
or corrbination of switches, to do things
like add credits, tilt the game, or score
incorrectly, depending on the strobe and
switches involved.
switch matrixes are something irost of us
will have to fool with sooner or later,
and even the irost bizarre problem can be
corrected with a little understanding, and
sometimes, a lot of patience.
Until next ironth, have fun!
Note: The format used is (return,strobe).
.
'!be diode at (0,0) is forward-biased so it
conducts, pulling return O low. The diode
at (0,1) is reversed-biased so it is
cutoff, and the diode at (1,1) has highs
on both sides so there is no current flow
there either.
Murphy's Law #910
People who love sausage and respect the
law should never watch either one being
made.
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