Thread: Mk14 vdu
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Old 26th Oct 2020, 12:03 am   #603
Timbucus
Octode
 
Join Date: Mar 2019
Location: Barry, Vale of Glamorgan, Wales, UK.
Posts: 1,362
Default Re: Mk14 vdu

Quote:
Originally Posted by SiriusHardware View Post
Indeed, maybe have a 470R in series with the pot just to be on the safe side.

For information, the reverse 'C' character is formed by segment a (=1) + segment b (=2) +segment c (=4 ) + segment d (=8) , in other words a byte value of b0001111 or 0x0F is somehow eventually getting written to that display cell.
D7-D4 of course share the IC11 input from the keyboard strobe buffer pulled up - I see you point out they that they are 0 but, they will behave differently when the keyboard is scanned. Anyway Sirius may remember some shenanigans with mine - I have currently been running with an 80C95 not 80L95 so I put in the 74LS365 from my JMP (I do not own an 80L95) but, as we both had the same symptom it seemed unneeded I tried anyway. I noticed that the reverse C now appears subjectively more often? Also the system is unusable as my PI cannot type data even with the VDU off or disconnected. I will experiment more with that tomorrow as I have not used these scripts on the JMP - I have an older version for it with different constants.

Putting the C back in restored operation - with the same repeatable char on CHARSET with the 10K NWDS pullup (the old 4.7K must have been close to that with the crack).

While I was panicking that I had broken something I was uploading SEGTRIS so I tried that with the VDU on and OFF - its 7 digit display corrupts sometimes when playing with the VDU on consistent with a corrupt memory write.
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