I just thought I'd post here because none of my search results definitively answered my question.
Is it possible to use a Macintosh 9-pin mouse on the Apple IIc? Yes! With qualifications.
First, I'll say a given Macintosh 9-pin mouse is not guaranteed to work with the Apple IIc as-is. The input circuitry for the Macintosh and the Apple IIc are different, in spite of the mice using the same pinout. The difference, on the surface, is that the IIc has to be able to read a joystick from the same port, whereas the Macintosh does not. Additionally, through my experiments here, I seem to have inadvertently discovered that the Macintosh expects the quadrature output of the mouse to short to ground when not active, and to be pulled up by resistor when active, whereas the IIc seems to expect the quadrature output to short to +5V when active and may or may not need a pull-down to ground when not active (it actually looks like it may work with the non-active state left 'open', although I didn't fully pursue this avenue). As it happens, some Macintosh mice, as-is, use a comparator for quadrature output, which shorts to ground in the non-active state, and is pulled up via a 3.3k resistor to +5V in the active state. The main reason this doesn't work on the IIc is because that pull-up resistor behaves somewhat like the joystick potentiometer circuit. The IIc can't read the quadrature signal off two of the signal lines (Pins 5 and 8 ) because it's trying to read the resistance of the pull-up!
So, since these differences exist, how do you make a Macintosh mouse work on both the Macintosh and the IIc? Well, the easiest way is just to use a 7400-series logic chip as a buffer! I used a 74LS08 Quad 2-input AND circuit just because I had one handy, but you could use anything with at least four inputs and non-inverted outputs. I installed mine internally to the mouse, and cut all four quadrature traces (pins 4, 5, 8, and 9 -- find the traces on the board using a multimeter) between the 3.3k pull-up resistors and the pins on the cable connector. Then I placed my 7408 upside-down on top of the comparator and soldered both inputs of each AND gate to each output of the comparator directly (pins 1, 2, 13, and 14 of the comparator). Following this, I soldered each AND gate output to the correct cable connector pin on the bottom side of the board. And of course wired VCC and Ground to the proper spots. And it worked! My Macintosh Mouse now works on both my Mac Plus and my Apple IIc!
You could also have the same circuit in an in-line adapter with a female and male DE-9, but I don't think that's as fun.
If anyone's interested in pictures or drawings, I might be inclined to do some later if asked. But hopefully this will give a good idea of what to do to anyone with electronics experience who is interested in this in the mean time.
(Edited to correct some assumptions I made about most Macintosh mice having similar internals. Apparently they don't.)
Is it possible to use a Macintosh 9-pin mouse on the Apple IIc? Yes! With qualifications.
First, I'll say a given Macintosh 9-pin mouse is not guaranteed to work with the Apple IIc as-is. The input circuitry for the Macintosh and the Apple IIc are different, in spite of the mice using the same pinout. The difference, on the surface, is that the IIc has to be able to read a joystick from the same port, whereas the Macintosh does not. Additionally, through my experiments here, I seem to have inadvertently discovered that the Macintosh expects the quadrature output of the mouse to short to ground when not active, and to be pulled up by resistor when active, whereas the IIc seems to expect the quadrature output to short to +5V when active and may or may not need a pull-down to ground when not active (it actually looks like it may work with the non-active state left 'open', although I didn't fully pursue this avenue). As it happens, some Macintosh mice, as-is, use a comparator for quadrature output, which shorts to ground in the non-active state, and is pulled up via a 3.3k resistor to +5V in the active state. The main reason this doesn't work on the IIc is because that pull-up resistor behaves somewhat like the joystick potentiometer circuit. The IIc can't read the quadrature signal off two of the signal lines (Pins 5 and 8 ) because it's trying to read the resistance of the pull-up!
So, since these differences exist, how do you make a Macintosh mouse work on both the Macintosh and the IIc? Well, the easiest way is just to use a 7400-series logic chip as a buffer! I used a 74LS08 Quad 2-input AND circuit just because I had one handy, but you could use anything with at least four inputs and non-inverted outputs. I installed mine internally to the mouse, and cut all four quadrature traces (pins 4, 5, 8, and 9 -- find the traces on the board using a multimeter) between the 3.3k pull-up resistors and the pins on the cable connector. Then I placed my 7408 upside-down on top of the comparator and soldered both inputs of each AND gate to each output of the comparator directly (pins 1, 2, 13, and 14 of the comparator). Following this, I soldered each AND gate output to the correct cable connector pin on the bottom side of the board. And of course wired VCC and Ground to the proper spots. And it worked! My Macintosh Mouse now works on both my Mac Plus and my Apple IIc!
You could also have the same circuit in an in-line adapter with a female and male DE-9, but I don't think that's as fun.
If anyone's interested in pictures or drawings, I might be inclined to do some later if asked. But hopefully this will give a good idea of what to do to anyone with electronics experience who is interested in this in the mean time.
(Edited to correct some assumptions I made about most Macintosh mice having similar internals. Apparently they don't.)
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