Not quite... The end of the ROM based PLA for the 64
Posted: Thu Dec 08, 2011 8:02 am
A few years ago, people started to use EPROMs to substitute the PLA chip in the 64 that often fails. This technique can also be used to replace faulty PLA chips in older PET computers.
The solution I have used is posted online here:
http://www.vic20.de/html/eprom_pla_8296_und_c64.html
However, there is a problem. The vast majority of modern EPROM chips made after 2000 are much faster than those used in the early 80s but in fact this is what allows the use of a memory chip as a substitute for a logic chip.
The problem is, these faster chips, when used with the relatively sluggish signals of a C64 or PET have a tendency to have an unstable output (as a result of input incertitude for a few hundreds of ns) and this can cause problems or even, in the long run, damage the machine in which it is used.
There is, however ONE specific brand of EPROM I have used over the last few years as a PLA substitute that does NOT cause this problem. This specific chip, the M27C512-90B6 made by ST is now OUT OF PRODUCTION.
This chip worked well because it does not exhibit the output instability of virtually all other brands of other 27CXXX series of (E)PROMs. With a rating of 90ns, this chip is not only considered slow by modern technology standards, it is also very close to the original rating of 82S100 chips of 80ns. It also has a rather high output impedance which reduces the risk of damage to other chips.
I have about 50 of them left in stock and then its over.
I normally sell my substitute PLAs for 10$ each but I can also sell the chips preprogrammed as is (you have to make the adapter) if anyone wants them, for 3$ each. I also offer a quantity discount of 8.50$ for complete subs and 2.60$ for chips with no adapters on quantities of 10 or more.
Over the years I sold about 200 of these PLA substitutes, used in both the C64 and PET computers and only had ONE european customer indicate to me that he had a compatibility problem in ONE specific machine.
For those in need of a replacement PLA, I suggest you get them before its too late.
PLEASE NOTE:
There is a european hacker who claims the solution I offer causes bus contentions on the 64.
In the coming days, I will post screen captures from a professionnal digital oscilloscope which not only prove my solution causes no such problems but that even original Commodore PLAs do cause bus contentions for brief periods of time.
The solution I have used is posted online here:
http://www.vic20.de/html/eprom_pla_8296_und_c64.html
However, there is a problem. The vast majority of modern EPROM chips made after 2000 are much faster than those used in the early 80s but in fact this is what allows the use of a memory chip as a substitute for a logic chip.
The problem is, these faster chips, when used with the relatively sluggish signals of a C64 or PET have a tendency to have an unstable output (as a result of input incertitude for a few hundreds of ns) and this can cause problems or even, in the long run, damage the machine in which it is used.
There is, however ONE specific brand of EPROM I have used over the last few years as a PLA substitute that does NOT cause this problem. This specific chip, the M27C512-90B6 made by ST is now OUT OF PRODUCTION.
This chip worked well because it does not exhibit the output instability of virtually all other brands of other 27CXXX series of (E)PROMs. With a rating of 90ns, this chip is not only considered slow by modern technology standards, it is also very close to the original rating of 82S100 chips of 80ns. It also has a rather high output impedance which reduces the risk of damage to other chips.
I have about 50 of them left in stock and then its over.
I normally sell my substitute PLAs for 10$ each but I can also sell the chips preprogrammed as is (you have to make the adapter) if anyone wants them, for 3$ each. I also offer a quantity discount of 8.50$ for complete subs and 2.60$ for chips with no adapters on quantities of 10 or more.
Over the years I sold about 200 of these PLA substitutes, used in both the C64 and PET computers and only had ONE european customer indicate to me that he had a compatibility problem in ONE specific machine.
For those in need of a replacement PLA, I suggest you get them before its too late.
PLEASE NOTE:
There is a european hacker who claims the solution I offer causes bus contentions on the 64.
In the coming days, I will post screen captures from a professionnal digital oscilloscope which not only prove my solution causes no such problems but that even original Commodore PLAs do cause bus contentions for brief periods of time.