kyuusaku
Joined: 26 Jul 2003 Posts: 941 Location: .ma.us
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Posted: Sat Apr 28, 2007 3:35 pm Post subject: |
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Yes, it's possible, I don't know the MGD pin assignment though. It will be difficult to modify an existing cartridge because you must add in a tristate buffer chip and IDC connector.
You can figure out the pins if you open your PCE I/O, it is the same as SFC because both are 8-bit ROM consoles. All you have to do is adapt the Hucard to SFC and add in SRAM. SRAM in early LoROM games is decoded using a 74LS139 chip. I have made this diagram for you to show the decoding:
Remember, "PCE /OE" is "MGD /OE"; For SFC, "ROM /OE" is "MGD /OE" because of the decoder.
To make a LoROM cartridge (like Bung original), connect MGD A0-A14 to SFC A0-A14, then connect MGD A15-A19 to SFC A16-A20; A15 is not used for LoROM.
For HiROM connect MGD A0-A19 to SFC A0-A19 (but you must change SRAM decoder), I don't think there are many good 8M HiROM games anyway.
You can make a DUAL LoROM and HiROM I/O by adding 2x 74LS157 multiplexer chips to select address lines. This way you can use one cartridge with a switch to select either Lo/HiROM mapping.
If you want AUTO Lo/HiROM switch you can use this equation: A22 AND A23 = HiROM. |
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