Alistair,
you need to do more reading.
The Bartels system accepts a serial/PS2 mouse as encoders but you
have to enable it first in the config.dat file.
The detector (the black device in the pic attached) has 2 detectors.
It detects either a rising edge one one detector or a falling edge on
the other.
http://www.iceinspace.com.au/forum/a...se.php?a=54746
Whichever one it received first determines direction of the count.
That's quadrature detection. Read up a bit more on it.
This thread is quite clear and straightforward including the pics.
Mel's site is very easy to digest a step at a time too.
Steve