Saturday, 27 July 2013

Recycled Master-System Cartridge

Recycled Master-System Cartridge

Recently received some freebee Master-System cartridge PCB's from steveBM1 over on Jamma+, a very kind fellow indeed!

Went and fitted a 32 pin chip socket to one of the boards and naively burned a 27C020 (256k / 2Mbit) Eprom with Wonderboy 3 game data (love that game - still remember the cheat password: WE5TONE)

But no boot.. :'(

Later learned that Master-System Roms have custom logic inside to page in the blocks of data for games bigger than 32k.

Apparently you can re-use carts that have a second 'mapper' chip, so for now I thought I might try a 32K game burned onto a 27C256 chip.

But............. The 27C256 has 4 less pins than the 27C020 (it's 28 pin, rather than 32)

From experience with the R-Type conversions, I knew that all the lower 26 pins are the same between the two chip types.  So I burned a 32K Eprom with Teddy Boy and inserted it into the lower end of the socket.
I had to then fit 2 wire bridges to bring the 5v line down to the top two pins of the 28 pin Eprom.

I also had to cut one trace that led from the third pin - to the edge connector (Shown below).

And here is the finished board in full:

And again from below:

And you know what..? It worked! :D

Just have to wait for a RGB SCART lead to turn up, my TV won't tune in well enough to get a good picture on RF.

I'll also have to research this mapper-chip stuff.. might be able to build some logic to do this.