12/16KB VIDEOPAC+ GAMES

In order to better understand the paging of 12 and 16KB games with videopac + graphics, and to be able to include those games in my "Videopac micro-SD cart", I have reverse engineered one of these cartridges.

The following games fall into this group:

 - 55+ Neutro Star 

 - 58+ Air Battle 

 - 59+ Helicopter Rescue 

 - 60+ Trans American Rally

Any of those games can serve my purpose as they all use the same type of pagination.

In general they are rare games and they are expensive to acquire, so I have bought the most affordable of all which is the 55+ Neutron Star.

After following the ways with the multimeter I have reached the following diagram:

Basically, this cartridge works as follows:

- The A0-A9 and A11 address lines of the console are connected to the A0-A10 address lines of the ROM chips, resulting in 2K per bank as in standard paging games.

- Any write access to the external RAM (P14 = 0 and P16 = 0) in the range 0800h-0bffh causes the values ​​of D0..D2 of the data bus to be stored in the latch (IC2 in the diagram).

- When P10 = 0 the latch is activated and the stored bank number is selected.

- If P10 = 1 the latch is deactivated and the pull-up resistors select bank 111b.

In the schematic, D0 and D1 are connected to the A11 and A12 address lines of the ROM chips, while D2 is used to select the active chip.

The EPROM chips used in these cartridges are 8KB / 4KB, and are currently hard to find, so I have modified the schematic to allow a single 64KB EPROM to be used much easier to find and also cheaper.

Finally I leave here an image of how the PCB would look once routed.

GERBER FILES