Super Zaxxon Replica

A while back — following the repair of a C64 — I posted about testing the PLA replacement that I had used. In the great video on that topic shared by MindFlareRetro, Eslapion mentions two possible tests for a PLA replacement. One of these I covered in my post back then. The other test involves using a Super Zaxxon game cartridge.

Due to the unique way of bank switching implemented by that module, it seems to be particularly vulnerable to glitches produced by PLA replacements. Less-than-perfect replacements tend to produce crashes or skewed graphics when running that game.

Schematics based on Siem’s design.

I don’t own a Super Zaxxon cartridge and they seem to be hard to find these days. So I thought it might be fun to create my own replica. Doing the research for this project, I came across a post on the Lemon64 board. It turned out I was lucky: Siem Appelman had already examined the original module and documented the essential parts of the schematic. I contacted him and he was willing to share his findings, including his own design for a reduced version that would require a single memory chip, only.

I opted for a W27C512 EEPROM chip because I had plenty of them lying around and they seem to be the cheapest parallel PROM chips around these days. You can get them for less than 0.50€ each on eBay. With Siem’s simplified design, only the lower 32KiB are used and need to be programmed like this:

$0000     lower $1000 bytes (4KiB)      mapped to $8000
$1000     lower $1000 bytes (4KiB)      mapped to $9000
$2000     $2000 bytes, bank 0 (8KiB)    mapped to $A000
$4000     lower $1000 bytes (4KiB)      mapped to $8000
$5000     lower $1000 bytes (4KiB)      mapped to $9000
$6000     $2000 bytes, bank 1 (8KiB)    mapped to $A000

The game switches between the upper banks 0 and 1 by reading the contents of the lower memory either starting at $8000 or $9000.

Rev.2 of the expansion port breakout board.

After rounding out the schematics with general knowledge on C64 cartridges gained from Jani’s excellent post, I first wanted to create a prototype to ensure that the design would be working. For this, I tried to use my expansion port breakout board that I had created for the 74LS279 test circuit. But I soon realized that connecting all the data and address lines to the EPROM using jumper wires wasn’t only a pain in the rather tedious but also very unstable.

First working prototype of the Super Zaxxon cartrdige.

So, I created a new revision of the breadboard adapter that allows for an optional 27C512 or 27C020 EPROM to be placed on the PCB and to be connected directly. Using this new board, I finally managed to create a working prototype!

Actually, the prototype turned out to be working too well. When I connected it to my C64 that is featuring the replacement PLA, I was a little disappointed that I still didn’t see any crashes or broken graphics. It remains to be examined whether this is due to the modified schematics or to an exceptionally well working PLA replacement, although I suspect the former.

AND gate used in the replica’s schematic.

Alternate AND gate.

I did a few more experiments and found that driving the CE/OE line with two diodes and a pull-up resistor instead of the two NAND gates would still work on an unmodified C64 but would crash when using an EPROM based PLA replacement.

Then I lost interest in the subject and instead decided to layout a nice PCB to turn my prototype into a fully functional Super Zaxxon replica. The layout of the components was quite easy but I turned it up a notch by branding the PCB with the Super Zaxxon logo. This time, I ordered the boards from PCBWay again and they did good job producing the somewhat unusual design at a very reasonable price. It would actually be a waste to hide the beauty in a cartridge case. Maybe a transparent one would do.

The finished replica.

The BOM for this replica is quite short. Apart from the PCB and the optional DIP sockets there are only 4 different kinds of components:

1 U2 74LS00 quad 2-input NAND gate eBay
1 U3 74LS74 dual edge-triggered flip-flop eBay
1 U4 W27C512 64k-8bit EEPROM eBay
3 C1-3 100nF capacitor eBay

All in all, this was again a fun project with many things to learn along the way.


8 thoughts on “Super Zaxxon Replica

  1. Hi,

    Will this board be available for purchase from PCBWay like the Pi1541?


  2. I would like to make this project publicly available, too. But I’m worried what Sega might do when they notice. So, I’m afraid the answer currently is “no”.

  3. I have a genuine Super Zaxxon cart and I can confirm this modded design has exactly the same sensitivity to a problematic PLA as the original cart.

    If you want to make this cart consume less power, replace the 74LS74 with a 74HCT74 and the 74LS00 with a 74HCT00.

    If you want to make it incredibly sensitive to any slight glitch from the PLA, replace the 74LS74/HCT74 with a 74HC74. Then it will work fine with only the very finest PLA substitute. It might even have problems with a genuine Commodore PLA.

  4. Thanks for your feedback! I’ll give the 74HC74 a try and I’m curious how that works out. I actually wouldn’t have expected for it to work at all. But I have a few replacement PLAs that I’ll put to the test.

  5. Pingback: Bank Switching Cartridges |

  6. Hi,

    What eeprom burner do you use and/or recommed buying for burning C64 cartridge chips?

  7. I’m using a TL866CS programmer (like this one on eBay) and I would definitely recommend it, it’s very good value for money. I’m not sure if you can still get the original one or only the TL866-II these days. This newer model lacks some higher programming voltage required for certain older EPROMs if I’m not mistaken. But it should do for any kind of EEPROM.

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