Not quite... The end of the ROM based PLA for the 64

[eslapion]

Burnt an eprom , just need to build a cartridge. Anybody know of a pcb available or do i have to do my own?

An easier solution is to connect the 74LS279 glitch detector circuit directly to the cartridge port leads or to the PLA socket.

74LS279 PLA glitch detector

PLA_tester_LS279.gif (20.73 KiB) Viewed 687 times

Using the tester
To use this circuit you must make sure you use the original C64 kernal (no JiffyDOS or things like that) and that no cartridge is attached to your cartridge port.

When the C64 is first turned on, it will check for the presence of a cartridge and this will trigger the red LED to turn on. Press the button to turn it off and see what happens. If your PLA is not generating glitches, the LED will stay off indefinitely. If there are glitches, they will trigger the LED back on.

Normally, !ROMH and !ROML will always stay above 3V after the initial check and the set/reset latch will stay inactive. The 74LS279 will detect any glitch which brings the voltage below the threshold voltage for TTL logic circuits, about 1.5V.

[levoman]

Like a lot of people on these forums i like to dabble and get 'hands on' that is why i go for a lot of these projects. It is also one of the reasons that i have a stack of c64 computers and drives and all the bits that go with them. I had a c64 just after they first came out . It has been repaired many times by me and i also have lots of other computers that i do the same with. Every so often i will get one out , or a project will catch my eye and away we go again!

[banman]

Hi levoman,


I would just like to add a comment to eslapion's excellent explanation of his glitch testing circuit. Which I might like to say is very easy to build and very reliable.

When purchasing your 74LS279 flip flop IC, I suggest you get several different quality brands from known suppliers. Such as Mouser or Digikey.
Maybe even you have a local electronic shop that sells these sort of chips.

I got caught out using a particular brand of 74LS279 chip. I wasn't getting the readings I was expecting.
This is due to small manufacturing differences between the brands and even the batches as well as manufacturing plants.

I recommend using Motorola branded chips.

I found 1 batch of Motorola branded 74LS279 chips had a slightly lowered triggering TTL voltage threshold to that of another batch of Motorola branded 74LS279 chips I tested. In fact the LED was coming on intermittently in that particular batch.
The Motorola branded chips were overall the best in terms of having a triggering voltage threshold that would accurately trigger at the desired TTL voltage threshold.
I am sure eslapion can do a better explanation of what is happening here...

I mean to say that the trigger TTL voltage threshold between these two batches was microscopic. However it did greatly impact on my recorded readings.
For me in the end it was a matter of trying out different varieties of 74LS279 chips against a known dodgy PLA (such as the Winbond W27c512 electrically erasable PROM ) in combination with a factory Commodore PLA chip.

Once you have the right 74LS279 chip in you glitch tester you'll have a good bit of kit to test out your PLA's. So easy to use.

I love my tester

By the way I think the eBay seller TheLittleDiode is a Chinese knockoff reseller who trades out of the UK. Having said this, there is nothing wrong with buying knockoffs, just be prepared for the unexpected!

[levoman]

I have a few 'original' (from the 90's) ls279's here and will build a tester , but i am also going to build a cart and will leave the info here for anybody else who wants to.

[banman]

Hi levoman,

Great. I look forward to seeing how you go.


Good luck!

[levoman]

Thanks banman. Just waiting for some boards to arrive and test , tidy it up a bit and then it will be on the PCBWay website shared projects.
I have put a link on the board as well so that you can either use a 256k rom or a 512k and select upper or lower bank.

[banman]

Hi levoman,


I look forward to seeing it on the PCB Way shared.projects site..


What are you planning to call it?

[levoman]

Super Zaxx C64

[banman]

Hi levoman,


I like the name....

I am a bit of a C64 cartridge fan.

[levoman]

Here is the schematic for anybody who wants to have a look.

[eslapion]

Here is the schematic for anybody who wants to have a look.

AFAIK, it's the same as posted here: https://www.hackup.net/2018/10/super-zaxxon-replica/

... except for pin 1. Your design allows for a switch on pin 1 of a 27C512 so your cart could have both Zaxxon and Super Zaxxon in a single cart.

[levoman]

Yes it is the same design just redrawn so that i can understand it better , and to make a pcb. Did Zaxxon use bank switching ? or was it just a small game to fit in one segment? the switching circuit uses A12 (4k) to switch A13 (8k)and A14i (16k) banks in conjunction with _ROML and Phi2
and output on( _ROMH and _ROML ).

[eslapion]

Yes it is the same design just redrawn so that i can understand it better , and to make a pcb. Did Zaxxon use bank switching ? or was it just a small game to fit in one segment? the switching circuit uses A12 (4k) to switch A13 (8k)and A14i (16k) banks in conjunction with _ROML and Phi2
and output on( _ROMH and _ROML ).

Both Zaxxon and Super Zaxxon carts had exactly the same hardware. The content of the ROMs is different.

In the original carts, they both use a 16kB ROM and a 4kB ROM for a total of 20kB. However, the C64 ROML and ROMH areas are designed to address only 8kB. Access to the upper/lower half of the 16kB ROM is selected using the 74_74 flip-flop.

In the version using the 27C256, the 4kB ROM is present twice and 8kB remain unused.

[levoman]

Does that mean that the zaxxon cart creates errors with bad pla substitute?

[eslapion]

Does that mean that the zaxxon cart creates errors with bad pla substitute?

I never had a Zaxxon cart (the real one from Sega) to check that but the design is exactly the same as the Super Zaxxon cart so I would assume it does.

The Super Zaxxon cart was not a frequent find before I mentioned it could test a PLA and now it has become a serious prized rarity with prices skyrocketing on eBay. The Zaxxon cart is even much more difficult to find but the CRT file is easy to find. If somebody wants to make a clone of the cart then all that needs to be done is to extract the ROMs in exactly the same way as with Super Zaxxon.

If you look at this table, you can find CRT type 18 is Zaxxon and Super Zaxxon. https://rr.pokefinder.org/wiki/CRT_ID

According to this, only the chameleon can emulate this type of cartridge so how come people can run it on EasyFlash and the 1541 Ultimate series of carts ? That's because Lord Crass made a special version that can run on these platforms. It's not the real thing so it can't detect PLA problems.