Yesterday I was off work trying to get a GP appointment... Managed to get a "physician associate" appointment instead and, most importantly antibiotics.
Anyway, that left me with the afternoon spare so, having noticed that the upper "pin 2" of both the Commodore 64 tape port and the Plus/4 user port were both +5V and the SD2IEC virtual floppy drive used the cassette port for power I thought, why not make the adapter fit so the SD2IEC can work on both machines?
So, out with the Dremel... Bwahahahaha!
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The result... it works!
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Found how to load the file browser/launcher application...
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Downloaded a game and put it onto the SD card to test...
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Yay! I can now use more than just the built in office suite and the diag cartridge.
Modern games, which are written with the Plus/4 rather than C16 in mind, actually look quite nice. The sound is nothing like the C64, more the BBC Micro, but it's generally OK and good enough for most games. There's even a port of the C64 version of Elite which seems to play just as well as the Beeb version but with more colours.
I can see why the machine didn't take off though... The C16 killed it. All the original games are targeted at that cheap machine which had too little memory and they are quick, cheap and awful games and none take advantage of the Plus/4's extra memory.