In November of 2003, I chanced to discover that certain optical encoder chips used in inexpensive optical mice produced quadrature output, such as would be generated by the optomechanical motion sensors in old-fashioned optomechanical mice. Since the old Archimedes and Risc PC computers used mice which only gave quadrature output, plus the 3 button states, it occurred to me that one of these mice could be modified to work with the older Acorn machines, using the lead from an old Acorn mouse.

Here's how it was done, but first a couple of notes. The wire colours given here are not universally used: I found a couple of mice with slightly different assignments. Before soldering anything up, use a continuity tester to check the wire/pin assignments against the diagram and table given below, and make your own table if any differ.

The buttons in an Acorn mouse are sensed differently from those in this mouse. When a button is pressed on an Acorn mouse, the signal changes from +5V to zero, this mouse does the opposite. For this reason, it was necessary to cut some circuit board tracks and add some pull-up resistors to make the buttons work properly. These resistors connect the button signal wire to a track carrying +5V. Some wires have been added to connect the other side of the button switches to the 0V rail. See the picture below.

	This is the mouse, after modification.
	This is the mouse viewed from below.
	The inside, showing the top of the circuit board. The optical motion sensor is the big chip in the middle. Note that the legs are staggered. Note the two wires, green & orange (yes, it looks like yellow here, but it's orange, more orange than the actual yellow wire anyway). The original PS/2 lead had 4 wires, but these two connections are the only ones we're interested in: they're the power supply (full connection details given later). See that chip at the top of the board? that's the PS/2 encoder chip. Since taking these pictures I concluded that it's best to remove this from the board altogether - I experienced some spurious movement with it left in place. Plus you can then use some of the holes to connect the wires.
	Now the complicated bit - underneath the board. I've left this picture at full size, so it's rather large (1340k), to show maximum detail. Note the 2 0V wires (black), pull-up resistors (47k) and broken tracks to the button switches. Besides the +5V tracks, I've also cut the tracks connecting switches to the PS/2 encoder chip - not strictly necessary, esp. if the chip is removed altogether, but it doesn't hurt.
	These are the pins of interest on the motion sensor, which need to be connected to the correspondingly named pins of the Acorn mouse plug.
	The Acorn mouse plug (looking into the unplugged plug). 1 Ya (blue) 2 Adjust (grey) 3 Yb (white) 4 Menu (red) 5 0V (orange) 6 Xb (yellow) 7 +5V (green) 8 Xa (black) 9 Select (brown)

I couldn't find a data sheet for the exact chip used here, but there's a whole family of similarly packaged chips from the same maker (Agilent), which have identical pin assignments. Here is the data sheet for the ADNS 2051. You can find this and others at www.agilent.com under semiconductor products/optical navigation.