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Still Fighting the War
There is a logical problem somewhere and I am still trying to find it. The
symptom is junk data that is junk because the memory is being loaded at the
same time that it is being read out. The reason that it is still being
loaded is that the signal that is supposed to stop the horizontal scan does
not stop it and it just keeps scanning.
The problem is that mostly the circuits work properly. After a long run,
it may start hanging. For example I started running at bout 8 PM tonight
and it failed around midnight. Then I went out and poked around with a
scope probe and it quit failing. Sigh! The most probably cause is an
unterminated CMOS imput. These inputs are of such high impedance that they
can float slowly from one level to another over hours. It just depends on
the leakage paths on the board. I have looked and looked, but I cannot
find the problem. But I am getting close.
Those of you with Mark IVs beware. If you get junk data consult with me
and I will tell you where to look.