I have Mach3 driving a G540 and have been successfully using it to generate clock parts for the last year or so.

I recently purchased a 4th (rotary) axis and wired it up, only to find that it would only turn in one direction when connected to the A-axis.

The behavior is limited to the physical A-axis, that is, the electronic parts of the G540 that are driven by pins 8 and 9 of the parallel port. Also, note that changing from the parallel port on the PC to a parallel port on an ESS Smoothstepper makes no difference.

After much checking and double-checking, I made a DB25 breakout board to give me access to the individual conductors of the parallel port cable.

There is a difference in the voltage on the 'direction' pins of the first three axes (X,Y,Z) and the A axis.

The 'high' voltage with the DB25 cable NOT connected to the G540 is 3.36V for all four axes.

However, when the DB25 cable is connected to the G540, the direction pin voltage in the high condition is 2.96V on the X, Y and Z axes, but only 2.63V on the A axis.

Does anyone know if this is normal, and is the threshold voltage for the G540 recognising a 'high' signal above 2.63?