Nice project - only one thing is missing: it'd be nice if you'd explain how it works! I mean, what is a AMX mouse, and how does it transfer the data to the CPC?
I've had a look at the original AMX Art software. At first glance, it seems to be joystick compatible... so my first question would be:
Is that all about it? It works like a simple joystick? Or did I miss something important? Like a mouse-detection which switches between special-mouse-protocol-mode and normal-joystick-mode?
If it's really simply using the joystick-style protocol. What I've found out is that it seems to check the joystick port once every 300Hz, and seems to treat joystick up/down/left/right signals as mickeys in the corresponding direction(s). So I guess, on vertical movement, the mouse interface generates a LOW pulse of 1/300s duration (per mickey) on UP or DOWN joystick input.
Oh, and how does the scroll wheel work exactly? The document says it connects to "5" (joy2up) and "6" (joy2down). First of, isn't that vice-versa? 6=up, 5=down?
And, concerning scrolling, the terms "up" and "down" are rather confusing... If you move UP-wards in a document, then text scrolls DOWN-wards on the screen. So "Move DOWN" could have opposite meaning as "Scroll DOWN". I guess "turn wheel away from user" and "towards user" would be much more precise.
Finally, a lisz of the buttons would be nice, like
Joy1Fire2 (X) = which button Joy1Fire1 (Z) = which button Joy1Fire3 (-) = which button (middle?)
Whereas, looking at the documentation (pdf file) for the original AMX mouse, left and right buttons seem to have been arranged/used confusingly. There's a strange drawing that looks as if the Right Button is used as Execute Button... ie. much like PC mouse in left-handed mode? Although, the drawing is top-down, so the right button is actually on the left side in the drawing... though I guess that wasn't the way how one should use the mouse, with the buttons & cable pointing towards the user? :-)
Regarding: * Note: The above "1/300s" timing would be the ideal value (the existing AMX Mouse software reads the joystick port inputs at 300HZ rate). The real AMX Mouse interface, and this DIY project may not exactly match that timings. I've removed this sentence because, if you'd gone to the bother of building the circuit, you'd know that the statement is completely untrue, or what are the facts that you base this claim on?