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8bitforever
Account closed
Registered: Oct 2009
Posts: 48 |
Black (Blank) screen problem and the PLA EPROM solution
I have the black (blank) screen problem on my C64C.
Many circuits have been replaced and I'm now sure it is the PLA chip (906114-01) that is the problem.
What's your experience of the Ray Carlsen PLA Chip replacement ?
http://www.users.on.net/~clockmeister/other/C64PLA-27C512/
http://personalpages.tds.net/~rcarlsen/tdsweb/EPROMPLA/eprompla..
http://vic20.de/html/eprom_pla_8296_und_c64.html
Is it working without problems ?
What's your real experience on this ?
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8bitforever
Account closed
Registered: Oct 2009
Posts: 48 |
Thanks for the video, very informative !
That was very interesting information that Andropolis would work with a 150ns EPROM PLA on Assy 250407.
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: I could not resist to upload a small Video to YouTube:
http://www.youtube.com/watch?v=OHevwgKo9YI
This shows my C64 with an Eprom-PLA (M27C512-15XFI) - notice the interesting color-effects.
The same Eprom works fine in another C64 (same Assy 250407), so it's a bit random what you get..
For a serious (demo-)machine, try to get an original PLA.
It obviously have problems coping with the color ram fetches after about 20 chars on each line. Some vic banks more prone to error it seems aswell. Not sure this is about the speed of the chip, looks more like a logic error in the truth table to me. |
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ready.
Registered: Feb 2003
Posts: 441 |
the problem with EPROMs used as PLAs is that EPROMs are not true logic. There's no error in the truth table logic. EPROMs can garantee their outputs according to the truth table implemented only after a settling time, not in between. This is ok when an EPROM is used as a ROM, in a bus system where you have proper delays after which data is considered valid. But EPROMs cannot garantee the intermediate state and some outputs may flicker.
While a true logic chip garantees the outputs even during transitions. If an EPROM can work as a true logic is just a matter of luck.
I suggest yuo check this:
http://www.jschoenfeld.com/products/SuperPLA_e.htm
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Sabbi
Registered: Jan 2002
Posts: 8 |
Quoting JackAsserlooks more like a logic error in the truth table to me.
No logic error - same Eprom in another C64 (same assy) works fine. I also got different Results with other Eproms (also with same Brand/Type). I've ended my Eprom-PLA-Experiments with the conclusion that everything is just random, and that some machines seem more error-tolerant than others.
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: Quoting JackAsserlooks more like a logic error in the truth table to me.
No logic error - same Eprom in another C64 (same assy) works fine. I also got different Results with other Eproms (also with same Brand/Type). I've ended my Eprom-PLA-Experiments with the conclusion that everything is just random, and that some machines seem more error-tolerant than others.
I think Ready's explanation says it all. |
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8bitforever
Account closed
Registered: Oct 2009
Posts: 48 |
Ok, so this is the conclusion:
The best solution is probably the none EPROM based SuperPLA (true logic).
http://www.retro-donald.de/pages/superpla-multi.php
But the EPROM solution at the right speed can be close to perfect in some C64:s.
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