Modifying parallel port

No eBay APP ID and/or Cert ID defined in Kunena configuration

Micromet wrote: Now - none of these interference concerns may bother linuxcnc software and operation.

Interference is a hardware thing, and if the Mach3 config has problems with interference, so too will LinuxCNC.

However, it is possible to mask the problem to some extent in the HAL layer.
linuxcnc.org/docs/2.7/html/man/man9/debounce.9.html

VFDs tend to transmit a lot of noise back down the input power lines to other parts of the system, it is a good idea to use a power supply filter to limit this (it helped a lot with my system).

They are not very expensive () though you do need to make sure you get one with a low leakage current if powering the system through an RCD.

Excuse my ignorance - but are VFD's Variable Frequency Drives? And if so - where are they on my cnc system and what are they controlling?

Micromet wrote: where are they on my cnc system and what are they controlling?

If you don't know, then you probably don't have one. I jumped to the conclusion that the spindle was VFD controlled and that was the source of your signal noise problem.

A variable frequency drive or VFD is used to control the speed of an AC (usually an induction) motor and on a CNC machine they are often used to run the spindle.