RE: [Fwd: Diagnostic cartridges]
Date: 2002-01-30 10:14:14

>The cartridge logic necessary for this could be pretty simple: First, it
>boot up the C64 in Ultimax mode (/EXROM=H, /GAME=L), and after a write to
>I/O area (either I/O1 or I/O2) change this to normal EXROM mode (/EXROM=L,
/GAME=H). This can be done with a simple LS74 flipflop I guess (/RES and
>as set/reset inputs, /EXROM and /GAME as outputs Q and /Q). The /ROML and
>/ROMH signals must be combined to select the single EPROM, to make the mode
>switch possible from inside the ROM, and this can be done with two diodes.
>like this much better than modifying a Supergames cartridge.

I used the SuperGames cart as it takes 27xxx EPROMS and had the 74175 latch
(and have spares). The upper ROM only has RESET/IRQ/NMI vectors, all
directed to $9FE0

The patch at $9FE0 (in the low ROM), just pokes the latch to change memory
config, then JMP(8000).

I had to split the pad between GAME/EXROM as these are joined by default,
and add a wire to connect to the 74175.

I also added LEDs (via a 7404) for some additional diagnostics: RED for +5V,
YELLOW for RESET high (released), GREEN for ROML (verify that cart is being
started/accessed, PLA works), dual colour for Phi2 CLOCK (so as to verify it
actually toggles).

Nothing published /written up about it yet....

- Nick


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