Modifying parallel port

idealy you don't want to ground any low voltage signals to the machine ,
you'll be far better , using screened cables and bringing them all back to one common point the parallel port
if need be , pick off 5v from the pc power supplys 4 pin molex connector as used for hard drives etc , via a fuse holder
or use a breakout board for the purpose which has opto's built in this will assist in protecting the pc , in the event of any failure .

Thanks cncbasher - good idea. I've ordered a breakout board and will do it that way.

So the breakout board has arrived - a 5axis MACH3 Interface board BL-MACH-V1.1 - and so I began to think how I could use it. I can see how you would attach all the stepper motors, spindle, limit switches, etc. to it. But then I got stuck - surely there is something already like this inside my current cnc machine controller box. So I took the lid off that and apart from a huge toroidal transformer, there are 2 interface boards - one that appears to mainly control the spindle motor speed and the other to which the stepper motors are connected. This board is labelled JP-382A and it has 3 molex-type connectors on of which a 2-pin one is connected to the STOP button and the other two are empty - and from left-hand side are - a 4-pin molex labelled PWM-out and then another 4-pin Molex labelled X Y Z G (at least I think it says G).

So it seems as if I can bring the limit switches input wire to this molex with the limit switch earth going to the G. But do I still need to supply the 5v or is that being supplied by the board? Can't seem to find a user manual for the JP-382A board to check but there is other information on other forums that might help once I've sifted through it all. But any comments by the linuxcnc forum would be welcome.

Sorry - after a quick internet search - the connectors are not Molex but JST-XH (or JST-EH - not sure what the difference is even now).

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 ) .