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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old Yesterday, 11:32 pm   #601
Timbucus
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Join Date: Mar 2019
Location: Barry, Vale of Glamorgan, Wales, UK.
Posts: 564
Default Re: Mk14 vdu

Well this is probably a good news report and should be repeatable for Sirius.

I played 12 levels of Minefield on the SOC VDU with no problems until I got bored and triggered my explosion; If you hold F down to continuous fire (or pulse it a lot) then when it gets the end of the screen it crashes through memory - it was a bug to do with the slight recoil on fire that I left in as it is a good anti-cheat...

I was only half joking on the Pot but thanks for the warnings. I tried soldering another 4.7K in parallel and that seemed to have little effect - I could still cause FALLMAN (occasionally) and MINEFIELD (always by the end of Level 1) to crash. CHARSET ran fine. Still saw the odd reverse C

Just replaced the effective 2.3K with a 10K - my original fault on CHARSET is back in the same spot and FALLMAN/MINEFIELD crash a lot quicker.

However, my clock may say half ten but, my body says it is bed time and I have work again in the morning...

EDIT: For clarity this is the pullup on NWDS.
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Old Yesterday, 11:35 pm   #602
Phil__G
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Default Re: Mk14 vdu

Quote:
Originally Posted by SiriusHardware View Post
... in other words a byte value of b0001111 or 0x0F is somehow eventually getting written to that display cell.
just a thought, hope you dont mind me chipping in, shouldnt the 'Ↄ' be a 'O' which are the same except for bits 4 & 5 being low? maybe its not 0x0F being written, but bits 4&5 being pulled low? just thinking, these are nibble-wide RAM chips, and bits 0-3 are correct... ? ? ?
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Old Today, 12:03 am   #603
Timbucus
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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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Old Today, 12:05 am   #604
Timbucus
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Default Re: Mk14 vdu

Quote:
Originally Posted by Phil__G View Post
Quote:
Originally Posted by SiriusHardware View Post
... in other words a byte value of b0001111 or 0x0F is somehow eventually getting written to that display cell.
just a thought, hope you dont mind me chipping in, shouldnt the 'Ↄ' be a 'O' which are the same except for bits 4 & 5 being low? maybe its not 0x0F being written, but bits 4&5 being pulled low? just thinking, these are nibble-wide RAM chips, and bits 0-3 are correct... ? ? ?
It should actually be a blank space not anything at all but, good observation.
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Old Today, 12:12 am   #605
Phil__G
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Default Re: Mk14 vdu

Quote:
Originally Posted by SiriusHardware View Post
It should actually be a blank space not anything at all
Ah yes of course, sorry
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Old Today, 12:15 am   #606
Timbucus
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Default Re: Mk14 vdu

Quote:
Originally Posted by Phil__G View Post
Quote:
Originally Posted by SiriusHardware View Post
It should actually be a blank space not anything at all
Ah yes of course, sorry
No problem appreciate the contribution keep em coming - it is a useful observation. I should have mentioned though that the SEGTRIS corruption of the display was also a 'Ↄ' (nice trick Phil_G) in the first two or three 7 segments on the left.
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Old Today, 1:00 am   #607
Slothie
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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.
It is also interesting that d0-d3 are the only data bits pulled high by resistors on the MK14...
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Old Today, 1:10 am   #608
Mark1960
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Default Re: Mk14 vdu

Quote:
Originally Posted by Karen O View Post
Mark1960 has a point in post #583.

Were the VDU to assert NENIN part way through a write cycle to screen memory, then that write cycle may be corrupted, resulting in temporary corrupted data in memory. Corrupted data may then show up in the screen display.

When the SC/MP is allowed to resume, the SC/MP will repeat the cycle, complete successfully and restore correct data to memory.
If the address is not held at the end of an interrupted write cycle it might be corrupting some other address, then when the same bus cycle is repeated later it would not be the corrupted address that is written.

The SCMP outputs will be tristated by NENIN high, so rise time on NWDS might be slow, while the address lines may be driven to a different address by the PIC quite quickly.

For me this is just guesswork as I donít have anything set up to try this.
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