I'm a patient man and my glass is always half full, but my CNC router sure is testing my resolve    

I'm having nuisance limit switch triggers and have decided to use the second parallel port (PCI slot) on my PC to double the number of inputs available to Mach3.  The idea was to use these additional inputs to help identify if there was a pattern to which limit switch is supposed to be getting triggered.

To test the parallel port, I just enabled the port and changed the port settings within Mach3 (via ports and pins) and swapped the printer cable over.  Things were going great until I turned on my spindle (Teknomotor driven by a Delta VFD, speed set manually via a dial on the VFD).  When the speed dial was turned Mach3 registered an immediate emergency stop but even so the speed dial on the VFD controlled my stepper motors    

Anyone had this before?

Can anyone offer any things to try to solve this issue?

Thanks.

Plain and simple answer is the VFD is throwing out a bucket load of electrical noise which is being picked up by the parallel port and also the stepper drives.
You need to try and contain that noise by having all wiring shielded and properly grounded (star point) You also may want to try filters on the VFD and also try and route your cables so that low and high voltages cables do not run together.

Hood

Thanks Hood for the reply and for the suggestions.

The PCI parallel port must be more susceptible to the problem than the port on the motherboard.  The unintended stepper motor behaviour doesn't present itself when the MB port is used.