Mk6 Logic boards question

[Don Indallas]

I have a Buffalo game for my mk6.  If I install the game chips on an xp board, the U3 and U2 system chips go into U70 and U83 sockets respectively.  If I put the game chips on a standard board, it won't boot unless the U3 and U2 chips are in the extra system board.  Is this normal?

[Heihachi_73]

I'd say it's normal, the non-XP boards were a bit weird with that sort of thing. IIRC the original boards were designed for 4-chip system sets (four 1MB EPROMs at U70/U83 and U71/U84 instead of two 2MB EPROMs), which is why the gaps between the system and game EPROM sockets are different between original and XP boards, it probably isn't expecting 2MB chips in U70/U83 so it needs the System EPROM board to map the data correctly so it can boot.

I don't think the USA platform ever had 4-chip systems (they most likely only occurred in very early Australian machines e.g. earlier than 2002) so the system board is probably required at all times on a non-XP board.

[Don Indallas]

Thanks for the information about the boards.  I had tried moving the system chips to the U70 and U83 sockets on th standard board, and was surprised to see it didn't work.  Now I can understand why.  Thanks again!

[Heihachi_73]

It would be interesting to see if it would boot if if the data was split into four 1MB sections each and rearranged on the board.

Looking at MAME it seems that the data is loaded completely differently between original and XP boards:

Original boards (non-XP) actually load the data in U84 and U71 first, and then continue to U83 and U70 for the the next part of the data, with everything being interleaved across the four chips. The data is stored in two bytes between chips e.g. the first two bytes are read from U84, then the first two bytes of U71, then U83, then U70, and then back to U84 for the second lot of two bytes etc.

XP boards only need U83 and U70 as they have twice the capacity but are interleaved between only the two chips, with U84 and U71 being reserved for the 5th and 6th game chips (4MB each).

For it to theoretically work you would have to dump the data of U3 and U2, de-interleave it, and then interleave it a different way across four 1MB lots so it could load on a non-XP board.

After all that I can see why the System EPROM board really is needed!