Modifying parallel port

OK - I've been scouring the internet world for info re limit switches and have found various options to do with interference between spindle motor and limit switches tripping and causing false readings to be transferred to MACH3 software (most of the comments on the sites I've found have been US and thus both MACH3 and US voltage levels may create problems that UK power and linuxcnc don't see).

I would like members opinions on the following:
Shielded or non-shielded cables from the limit switches? - US people recommend shielded but are at odds when it comes to how to ground the shielding (one-end with either control box end or switch end connected ; or both ends with raised concerns about ground loops). Then the discussion goes into whether the voltage supply is below or above 5v - above this and these transients don't seem to affect anything.
Also many suggested putting a capacitor across the limit switch to get rid of these spurious spikes that provide false reading - again no need if linuxcnc members have seen no reason to do this.

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

In the meantime - I will link just the X limit switch pair in series to the designated input X position on the controller board plug identified in previous thread reply. The limit switches will be joined with 2 core shielded cable with the shielding grounded at the controller plug end only. I will also isolate the limit switches from the cnc machine itself. This will enable me to see if there are interference problems with this arrangement before wiring up the whole lot - which in the first instance will be for each axis limit switch pair to go to separate pins - easier to fault find that way and I don't at this time have need of these pins for other duties. I will let you know how I get on once the cable arrives.

you can have all the limits on one pin of course
if using sheilded cables only ground the shield at the breakout board and leave the other end open but isolated
use some heat shrink over the end for example
dont use capacitors .

it';s not manitory to use sheilded cables , but makes a big difference if you can problems later , mostly with VFD'S
if you need to just loop the signals in and out the cables at the breakout board , makes it easier for testing too
it's also good practice to use normaly closed switches and make sure the input pin at your breakout is pulled up to 5v
( most are ) .

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?

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.