Since my last post got everthing working until I turn on the plasma unit. As soon as the plasma lites the SS looses comunication with the pc and I have to restart mach to establish coms again.  I have been able to cut 1 or 2 small parts and at the end of the cut it will loose coms. I have tried all different grounding and even tried powering the pc and control system from my generator to get a separate power source. Same results. Do I need to go back to parallel port coms or will the SS work with a plasma? How do you eliminate noise from a plasma?
Thanks, Jeff Simpkins

Plasma systems can be some of the hardest systems to get/keep working properly. Even the big name high $$$ controls have trouble with the noise generated by plasma power supplies.

The first rule of thumb is good grounding. A ground rod driven by the machine is next to useless as it is being driven in bone dry ground. A single ground rod may also not be enough. A really simple test is to drive an 8' ground rod 4' or so away from your building and connect a 100W incandescent light bulb between the ground rod and an 110V AC neutral. If you have a good ground the bulb will not light. I have seen a few 'extreme' cases over the years where we had to go to great lengths to get a good ground. In one case the company had to drill a 20'~30' deep hole beside the building, fill it with dolomite (sp?) and run a ground rod all the way down. The second case was not as bad and just required three ground rods driven about 4' apart and all tied together. The cutting table and plasma power supply were both bonded to this earth ground. Make sure your wiring is properly earth grounded, remember that DC Common is not the same thing as earth ground.

Once you have a good ground the next step is to properly isolate the controls from the plasma noise. You must use isolation between the plasma power supply and the controls. Be very careful with how torch leads are routed. We used to wrap the leads in a wire mesh shield and then cover that with a leather cover (for wear protection). This helps keep the noise from radiating out of the torch leads and into other things.

Other things to try are to use an external power supply to power the SmoothStepper (or a powered USB hub). Some folks have also reported that bonding the SS 'ground' (DC Common) to the PC case did the trick.

i have had the ss working on a plasma , but because of no thc  function at the time  am using pports. i have the pc, montor, keyboard, gecko drives, and power supply built right on the gantry.never have had any connection problems or shutting down even when trying out the smoothstepper. i have no special grounding techniques. what is the brand of plasma cutter that you are using? if it uses a hi frequency to initiate the arc, it will freeze up the computer. i had a hypertherm powermax 40  that i had tried to use when my main plasma power unit , a hypertherm 1100 was  in for repair. once that thing started the arc the computer would freeze up and have to be powered off. could never get that to work. parallel port or ss.

Tripleblack & Jeff Birt : My plasma unit is a Thermal Dynamics Cutmaster XL80 and I do belive it uses HF to strike. The thing I don't understand is why it might run 2 small parts and then on the 3rd one it quits. Jeff- I checked the ground rod I drove to tie the plasma to and it had less than 1 1/2 ohms to my existing building ground. I bonded them together and to the table in a star manor. I'm in South Carolina, lucky I didn't get wet driving the ground rod. The water table here is about 3'. I 've got stuff coming today to hook it back up on the paralell ports. I will advise of what happens.
Thanks to all ,Jeff Simpkins

something that might help. what are you using for referencing the z axis? are you using the probe for initial height for piercing? with a microswitch? if so i found that before the arc strikes the 2 wires going to the sensing switch need to be BOTH disconnected. i had to created a macro that enabled a relay for both wires before  probing. then after touching off disabling the 2 wires.

Tripleblack: My next step is to set up a touch off probe, right now I'm doing it manually (jog z down until it touches then zero it) Hooked everything back up on PP last night and it works fine. I put a piece of 16 ga expanded in it and cut a 4" square When it finished cutting I found the mouse had quit. Mach still running, everything else ok. Unplugged the mouse, plugged it back in and it came back, tried it again did the same thing. The interupted cut knocks the mouse out every time. Doesn't effect the key board though. I think whatever is doing this is what knocked out the SS. It cuts solid material without any problem. I do have to cut a lot of expanded material and I just want to make shure this isn't hurting anything else. I can cure the mouse problem by going back to a non-usb mouse. That would be a pain to write the g-code to turn the torch off at every opening!! Yes, cutting expanded eats up the torch tips as the electrode is arcing to the tip instead of material. Oh, I was not trying to cut expanded when I was trying the SS, I was cutting 16ga sheet.
Thanks, Jeff Simpkins

I can cure the mouse problem by going back to a non-usb mouse.

No, you are masking the symptom; you still have a noise problem and you will have random PC issues because of it. Fixing the source of the noise issues is the only way to 'cure' it.

Jeff: I have a delta - delta transformer for running 480 volt equipment, wonder if it would help to power the plasma from it? My shop has 240 - 3 phase. Another thing, the gantry rides on cam followers,  and the Z axis unit rides on Thomson linear bearings with a ball screw ,wonder if grounding it would help?
Jeff Simpkins

The cutting table should be tied to the ground rod as well. If you have a HF start you need to shield the torch leads too.