commodore-multi-player Wiki
Multi-player projects for Vic-20 / C-64 / C-128 / Amiga
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muehlenf
The so-called (by me) 8-player adapter plugs into the user-port of a Vic-20 (preferrably, see pictures below), C-64 or C-128, and with a different connector into the parallel port of an Amiga.
It has its own power supply and LEDs showing the active controller inputs, so you can also test 8 Joysticks (or equivalent controllers) without a computer.
Hardware

First Game (VIC-20)

(more to come soon)