The Commodore C64 Replacement PLA has Failed (PLA20V8)

In this video I investigate a Commodore C64 that has just gone bad sitting on the shelf for a while.
Chapters:
0:00 Intro
0:42 The GAL PLA
1:45 Reading the old GALs
4:45 Programming new GALs
7:12 Boot test
9:40 Cassette load test
11:43 Programming Right GAL
13:07 Boot test 2
13:28 Cassette load test 2
14:55 Conclusion
Links:
The eBay C64 Is Behaving Nicely! (but I have bad luck with GAL PLA's) - • The eBay C64 Is Behavi...
PLA20V8 The Commodore 64 PLA that you can build yourself - www.freepascal.org/~daniel/c6...
Can you replace your C64 PLA for under $3? (Adrian's Digital Basement) - • Can you replace your C...
TL866II Plus Programmer - www.amazon.co.uk/gp/product/B...
GAL20V8B Chips from eBay - www.ebay.co.uk/itm/3929924722...

[@michallukaszek]

Hint for your diff program: use of "ignore whitespace changes" option would make it only concentrate on actual line contents. When you read the chips back and saved the content, it looks like it added all the extra line breaks.

[@ohtrobinson1]

Stumbled across this channel, really glad I did! Very interesting watch.

[@danielmantione]

The GAL20V8A and GAL20V8B are functionally the same, however, under the hood, they are different chips. This means that while the official specifications are similar, the real-world behaviour is slitghtly different. I think the most likely explanation for your problem is timing: The C64 RAM timing is on the edge, so a small difference can result in a non-working system. Another explanation is indeed a broken chip. You could try to flash the alt2 version, there is no reason why it should be restricted to 15ns GALs, which will result in slightly more relaxed RAM timing for your C64. I have experimented with GAL20VA chips myself and was able to build working PLAs, however my failures were higher than for the GAL20V8B. I haven't seen your problem before.

[@DavePoo2]

Hi Daniël, first can I say that I think this PLA project is awesome, it's what's keeping my breadbin C64's alive, so thankyou. If I flash the "A" chip then it will start working again, but then somehow loses it's ability to work over time. It was already busted when it got to me from the eBay seller, and flashing it made it work again. I initially suspected that the seller had done something wrong, but then that same chip lost it's ability to work over time after being sat on a shelf in the C64. I suspect this is just a bad chip as I wouldn't expect it's ability to function to change over time, I think the right hand GAL chip was probably OK. I later tested that same cassette game on a different C64 and it seems like either the tape or the datasette might be in need of service as it failed to load on a different machine.

[@francoisleveille409]

"timing" being specifically the latency on the CASRAM output.

[@danielmantione]

@@francoisleveille409 I don't know if it is CASRAM here: bad CASRAM latency would cause corrupted memory contents, but that doesn't seem to be the case here, since the machine is running well behind the disco show. I am more thining into the direction of the response to AEC, if the CPU bus drivers are not completely switched off when VIC-II has the bus, it can for sure cause a disco show like this.

[@francoisleveille409]

@@danielmantione No. Bad CASRAM latency means the multiplexers aren't done shifting from rows to columns when the access to RAM starts. All access (read or write) will be affected. The PLA receives the CAS signals but the DRAM receive the CASRAM signal coming out from the PLA. The PLA decides if the access goes to RAM, ROM, IO or something else on the cartridge port. If the access is to RAM, the CASRAM signal is delayed by about 25 ns to give some time for the two 74LS257 to switch from the 8 lower address lines to the upper ones.

[@danielmantione]

@@francoisleveille409 I am aware what CASRAM latency is (and it is properly handled inside the PLA20V8). However, CASRAM is no valid explanation, because as you say, CASRAM would affect everything, read and write.What we see in the video is that only the VIC-II has a problem, the CPU is running just fine, which would be impossible in case of a CASRAM failure.

[@ChristmasEve777]

Very nice!! Thanks for such an informative video about fixing a problem with the C64 and chip programming! I wish I knew enough about electronics and C64 hardware to fix mine. One time I went to turn it on and I believe the screen was just a solid color and it didn't match the dark blue background or light blue border color. I didn't discard it. I'm holding onto it, hoping to fix it one day.

[@DavePoo2]

It can be fixed. And these days it's 99% chance the PLA has failed, as they were manufactured by Commodore with "problems" that is causing them to fail over time.

[@ChristmasEve777]

@@DavePoo2 Thank you!! Since the symptoms were so different than yours, I gave up hope. Now that you say 99% chance it's the PLA, I really need to try this :)

[@DavePoo2]

@@ChristmasEve777 Well, do be aware that there are other problems too in these machines. But honestly, every breadbin C64 I have, had a dead PLA in it.

[@danielmantione]

If you get a solid colour my first guess would indeed be a PLA. PLAs made in 1983 and 1984 frequently fail, later ones are much more reliable. But you should also check your board for Micron MT4264 RAM chips and MOS 77xx logic chips. Those are also frequent failures. If you have such a "suspect" chip on your board and are unable to do any further diagnosis, taking a gamble and replacing the suspect chip often results in success.

[@ChristmasEve777]

@@danielmantione Thank you both so much! I really appreciate your advice. I'm going to go ahead and try all this stuff soon, starting with PLAs. I won't order anything else yet until I know (if) it doesn't end up fixing the problem. And I'll go from there :)

[@walterbeer2386]

I would be interested in seeing if you could successfully re-program the original GAL chips and if they would work. I've successfully done that.

[@DavePoo2]

Yes, I did that when they went wrong when I repaired it originally, but after the computer just sitting on the shelf for some time, they just failed again. So you possibly could use them if you are willing to put up with reprogramming them all the time. I decided it was time to get rid of them as it appears they can't seem to hold their programming.

[@flyguille]

@@DavePoo2 Well, programming it is just storing a little charge in the cells that controls how to connect the logic, so if it was unused , not turned on, maybe those chips didn-t retained the charge.

[@M0UAW_IO83]

@@DavePoo2 The programming algorithms used to 'burn' devices were often proprietary and needed to be licenced from the device manufacturer so the difference between a 'brand' programmer and the cheap programmers was the programming algorithm was only an approximation of the official one or something that had been found to work most of the time if you hold your tongue at the right angle and Saturn is retrograde in Capricorn, the rest of the time it's marginal, at best, which leads to a device that passes verification but fails in use either immediately or some time later. The TL866 hardware also has issues with programming voltages, it struggles to get them right and hold them but it could also just be that the PLA is so timing critical that the variant of GAL used is enough to breach some constraint or other (which is pretty poor device choice)

[@HoldandModify]

That’s not broken. That’s a disco dance show! :)

[@DavePoo2]

Now I wish I had put some music to it!

[@HoldandModify]

@@DavePoo2 it would take so much coding to make that kind of “glitch” happen. You got it for free. :)

[@danielmantione]

Considering the fact that the computer is running normally despite so much flickering, I think the most likely explanation of what is happening is that the PLA is deselecting a chip select line too slow, making one of the chips still send data to the bus when AEC gives the bus to the VIC-II. The VIC-II then reads data intended for the CPU.

[@timetorelaxfocus9642]

It's encrypted.

[@francoisleveille409]

I have been making and selling PLAnkton since 2015. The new implementation is PLAnkton EV. Much more reliable. 3 year guarantee.

[@francoisleveille409]

Sidenote: A good test remains the Super Zaxxon CARTRIDGE - not the software. It is dependent on the PLA's signal being clean.

[@DavePoo2]

@@francoisleveille409 kzbin.info/www/bejne/fXzcoJuVnKyHj68 Super Zaxxon works fine

[@networkg]

The PLAnkton EV is more reliable than the PLAnkton, it is not more reliable than a properly assembled PLA20V8. I have had mine going for 2 years without an issue, and you can't beat the price of less than $6 for the kit.

[@francoisleveille409]

@@networkg Reliable ? I have hundreds of customers who use PLAnktons since 2015. I guarantee them for 3 years. Xilinx XC95xx CPLDs use lower voltages and should be MORE reliable than any antiquated Programmable Logic Array. They are not reliable if you misuse them. IMHO, your 'reliability' claims are baseless. Technically, Xilinx CPLDs have no theoretical lifespan limits. So far more than 4500 units sold and counting. If my products had any reliability issues, I would be out of business.

[@networkg]

@@francoisleveille409 I did not make reliability claims, you did, you said your proprietary product is more reliable than the open source PLA20V8. I do not see the evidence for that, although I am willing to be convinced. I suspect both chips are fairly reliable given their commercial applications. But an anecdotal failure is not proof. If you meant that you have a better assembled product, I am sure you are correct. You have a warranty and a financial interest in good quality assurance that a home DIY'er does not. In my opinion, you and Daniel are to be commended for producing such excellent designs. These two replacements are the only ones I would ever recommend to anyone. However, I see them as equivalent in every practical aspect, you just see yours as more reliable, I disagree.

[@salvocristaldi1689]

@francoisleveille409 - Your arguments to elevate your product, with some aspect of arrogance, have made you an unpleasant person in my eyes. Your Plankon could possibly be the best PLA replacement, but having read your answers in this video, I would never buy your product.