From that pdf
The System4C board uses a 12 KHz charge pump signal from the controlling software to operate the
enable line on all four driver circuits. This signal uses pin1 from the parallel port for its input and must
be setup in your software configuration. When the signal is present the red enable light will illuminate
showing that the board is active.
so is it illuminated?
John
ps, just a thought, Oh, and do the motors lockup?