Brian Curry wrote an extensive text on how to identify ABS I fault causes. He was able to see the pulses using an analog meter. An ABS Fault Code Test Box was later described by Andries van den Broek.
The fault codes can be made visible with no more than LED + resistor.
Use a low-current LED (think 2 mA) in series with a 1200 Ohm resistor between the centre number 2 pin of the diagnosis connector (under side panel, bike right hand side, plugged into blue 'blank' connector).