G540 A-Axis will only turn in one direction (fairly new to CNC)

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?

Sorry everyone, disregard my previous post.

I still have the problem, but I have confirmed that the fault is in the G540.

I decided to check voltages when the ESS Smoothstepper is connected (even though the behaviour is the same as the PC parallel port).

The direction pin high condition is 4.7-4.8V on all axes including the A axis and the low condition is about 0.1V on all axes.

So, that that tells me that the G540 is at fault.

I'll chase that one down.

Further to the above post, I found the problem - a lack of clearance between a conductor and the ground plane of the motherboard which stopped the direction pin of the A-axis opto-isolator from getting above 0.0 volts - hence one direction only was possible.

Looked like a manufacturing fault, but who knows?