I went deeply into all this a while back, sadly most is now forgotten, I guess I should keep better notes of this stuff
GadgetFreak said:
Actually all those pins are connected on the mitchel boards I have....
Yes they go to unused stuff, there's actually not many pins you actually need (1-6, 9, 13-14) the rest can be used for extra address lines, rom oe outputs, whatever... yes of course it does mean cutting tracks, wires etc. They were just bootlegs I was messing with (1 single-board, 1 "exact-copy" b clone) so not fussed, I wouldn't hack up a genuine board.
For eproms, 27c800(8Mbit), 27c400(4Mbit), 27c2100(2Mbit, rare) are all compatible. I was using 2x 27c800 for the 2MB gfx games (ffight, msword, cawing etc.) 2x 27c160 for 4MB gfx games (ghouls, 3wonders, strider etc.) 2x 27c322 for largest 6/8MB gfx games (sf2, sfz, megaman etc.)
The smaller chips are useful if you want to replicate the original game arrangements, eg. ghouls using 8x 4Mbit chips or whatever. To me using 2x 27c322's for 2MB data is just insane, but each to their own
My modded pal pinout was:
16V8
a22 - 1 I VCC 20 - vcc
a21 - 2 I IO 19 - a13
a20 - 3 I IO 18 - a14
a19 - 4 I IO 17 - a15
a18 - 5 I IO 16 - rom ce
a17 - 6 I IO 15 - rom oe
b.7 - 7 I IO 14 - rom a20
74.5 - 8 I IO 13 - rom a19
b.108 - 9 I IO 12 - rom a18
gnd - 10 GND I 11 - a16
That was just using a single pair of roms, if memory serves, I cut the a18 line to the roms and controlled it from the pal (extra output pin 12)
At this point i'd learnt enough about cps1 that I went on to design my own pcb for a 16 game b-multi, with a old 5v cpld doing the mappers, a PIC auto-writing the b-chip config keys to the c-board (no rom hacking required
) and various old 5v smd flash chips instead of eproms for most game data. It was a complete success, so basically no need mess with single game conversions any more