View Single Post
Old 12th Apr 2021, 12:24 am   #1573
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Non-working Commodore PET 3016

Sorry, perhaps I should have made it clearer. The inputs to UC9 are absolutely spot on. That means the fault must lie in or downstream of UC9, but the only things between there and UC7 again are UC9 and the keyboard.

The keyboard is just a bunch of switches, so it is very unlikely to be at fault. Much more likely is the outputs of UC9 going low at the wrong times, and the only way that can happen is if UC9 itself is faulty.

If you want to prove it, connect each of UC9 pins 1-7 and 9-11 in turn via a 10K resistor to +5V, and put your probes on whichever pin of UC9 has the resistor and pin 15. If UC9 was working properly, you should see each output going low in time with a different one of the 10 states of pin 15 (low, high, low, high, low, high, low, high, low, hiiiiiiiiiiigh). This selects a group of just 8 keys from the keyboard, whose states will show on UC7 pins 10-17 as "low" if depressed or "high" if released.

But I think one of the outputs of UC9 is going low out-of-turn. This will cause a different group of keys to be read back than what the computer thinks it is asking about. It thinks it is asking about the group with W in it, but actually takes the common line low for the group containing Q. You press Q. It sees a key was pressed corresponding to W in the group it asked about, and so treats it as a W.

Is that any clearer?
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline