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I still have machines running LinuxCNC (named EMC2 back then) on Ubuntu 8.04, and 10.04 and...
It is all dependent on how you look at things, but i would definitely not discard parallel port as it is extremely reliable, extremely cheap be it as a old PC for 20-50$ or add on card for 10-15$, does not need a BOB (although it is recommended for new users as it adds a bit of safety for wiring).
Overall, when you realize you can have a fully functioning and very reliable machine control for under 50$,that will control whatever contraption you can think of by adding some cheap drives and motors found on old photocopiers, the possibilities rise sky high.
So no, parallel port and 32bit are not obsolete, nor will they be any time soon, they are the perfect setup form learning CNC on a very low budget.
Go out, scavenge some motors and belts and linear rods, build something, then build something else even better, then...
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Personally i use Mesa cards for everything and by now i have installed over 70 or 80 or 100 (probably more but i have no clue of the exact number), but still sometimes i build machines using a parallel port, even plasma machines that produce huge amounts of interference and THCAD wired directly to parallel port with no BOB, using HF start plasma sources work perfectly but do require very strict wiring and shielding and grounding.
Visit the "show your stuff" section, find topics started by me, plenty of pictures and videos of machines i have built and/or retrofitted, and some other stuff...

I wanted to share my experience from the last several days in case it helps someone else planning a Mesa Ethernet / QtPlasmaC plasma retrofit.

This is not a complaint about LinuxCNC, far from it. After working through the issues, I am still happy to be moving forward with LinuxCNC and QtPlasmaC because I think the plasma feature set is excellent. But I do think my experience shows that the “PC side” deserves more attention than I initially gave it.

My setup:

- Home-built plasma table conversion from MASSO to LinuxCNC / QtPlasmaC
- Mesa 7I76EU
- THCAD2 planned
- Ethernet Mesa connection
- QtPlasmaC
- Separate camera-assisted project running alongside LinuxCNC during some tests

One of my reasons for moving away from a closed standalone controller was that I wanted to experiment with a camera-assisted tracing/scanning workflow, somewhat similar in concept to SheetCam’s Scanything. I have been working on a small project I call FabScan, which uses a USB camera mounted to the machine to help trace parts/templates and eventually export geometry. That type of experimentation is much easier on an open PC/LinuxCNC system than it would be on a closed standalone controller such as MASSO.

I originally planned to use a small MeLe mini PC that I already had. It booted fine, LinuxCNC installed, QtPlasmaC launched, Mesa connected, and the machine homed and jogged. At first glance it seemed usable.

The problem was intermittent realtime/Mesa communication errors. I saw errors such as:

- Unexpected realtime delay
- hm2 error finishing read
- Watchdog has bit
- Smart Serial communication error / timeout
- Smart Serial Port 0 stopped

The PC had a Realtek RTL8111/8168 Ethernet controller using the r8169 driver. I tried the usual tuning steps:

- Dedicated Mesa Ethernet interface
- Static IP, no gateway, no DNS on the Mesa NIC
- Disabled Ethernet offloads
- Disabled EEE
- Disabled coalescing
- Disabled Wi-Fi during testing
- Disabled power-management options in Linux
- Increased servo period from 1 ms up to 3 ms and later 4 ms
- Switched from r8169 to r8168-dkms

The r8168 driver helped a lot. The system would run longer, but it still eventually produced realtime delay / watchdog / Smart Serial failures. In my case, the MeLe was simply not boring enough to trust as a control PC.

I also found a separate issue that was not the PC’s fault. My generated config had PID loops driving Mesa velocity-mode stepgens, with DEADBAND = 0.0 on the joints. After homing or jogging, the axes would twitch slightly because the PID loop was chasing sub-step position error. Adding deadband fixed that:

X/Y/Y2:
DEADBAND = 0.0008

Z:
DEADBAND = 0.00025

That stopped the axis twitching.

Eventually I replaced the MeLe with a used Dell OptiPlex 7060 Micro with Intel Ethernet. With the Dell, I tested QtPlasmaC, FabScan/camera load, and NoMachine remote desktop while watching Smart Serial. The Dell stayed clean at a 2 ms servo period. Smart Serial stayed at:

fault-count = 0
port_state = 3
run = TRUE

For over an hour, the takeaway for me:

LinuxCNC can run on modest hardware, but for Mesa Ethernet I would not say “any old PC” is good enough without testing. I would now strongly prefer:

- Used business-class Dell/Lenovo/HP mini PC
- Intel wired Ethernet
- Good BIOS power-management controls
- Dedicated Mesa NIC
- Long latency tests, not just short ones
- Real workload testing: QtPlasmaC, camera, remote desktop, Wi-Fi if used, etc.

I am not posting this to criticize LinuxCNC, again, far from it. QtPlasmaC is exactly why I am doing this conversion. I just think new users coming from MASSO, Mach, GRBL, etc. would benefit from clearer up-front guidance that the control PC needs to be “boring,” not just powerful enough.

In my experience, horsepower was not the issue so much as the details. The NIC, driver, BIOS/power-management behavior, and long-duration realtime stability mattered a lot. I wish I had understood those details better up front, so I am posting this in case it helps the next person avoid a few days of chasing intermittent gremlins.

stoobydale wrote:

Wow that just got expensive. This is a portable table, the cut dimensions are adjustable at the design stage and I am probably going to make mine 1000x800. The designer suggested a Nema 23 stepper kit with DM542 drivers and a Mach 3 controller from Aliexpress at about $235. I was hoping to use something like this probably with just a different break out board. Is this not possible? This is for hobby use and I am in QLD Australia if that makes any difference. This is also my first build, hopefully learn something and able to modify or make a MK2 at some stage.

I'm also  in Queensland also (Brisbane). You can find my email address on my @MrRodW YouTube channel. You can use a parallel port breakout board.I bought 2 off Amazon for $20. I might have a THCAD which you can use with the encoder component to read torch voltage for THC. If you went with Mach3, you'd probably need to get a Proma THC or similar so it will get expensive quick too. The DM542's should be fine. Very hard to kill. Geckos are way overrated.

For plasma and starting with LinuxCNC with stepper motors, I would get a Mesa 7i96s and a Mesa THCAD2 to read the torch voltage. Also get a Meanwell MDR-60-24 24 volt power supply for field power and a Meanwell MDR-10-5 5 volt power supply for logic. When you order your Mesa gear also get some of their DIN rail mounts and a couple of COMMX2 bus bars. That will set you up for a running start.

Hi everyone,
I'm currently building my plasma CNC setup with LinuxCNC and need some help with wiring. Maybe someone here has the same plasma cutter.
Hardware:
- Plasma cutter: Stahlwerk CUT 60 P Digital
- Motion control: Mesa 7i96S
- Voltage sensing: THCAD-2

My main question:

I'm trying to understand where to find the ARC starting signal on my plasma cutter. The signal that tells LinuxCNC: "The arc is burning, movement can begin."

In many tutorials or forums, I read about a dedicated ARC OK output on the plasma.
(See Attachment: the plasma cutter signal control has 4 pins 1, 2, 3 and 4)

On the Stahlwerk CUT 60 P Digital, I only find:
1. THC interface
2. The control socket (where the hand torch trigger normally plugs in) (pin 3 and 4)
3. I can´t find pins 1 and 2 or find anything to how to get the arc signal otherwise 

My specific questions:

1. Does the 2-pin THC output not only provide the voltage for the THCAD-2 but also a signal that tells me whether the arc is stable?

2. Do I need to get ARC OK somewhere else? I read in another forum that some plasma units don't have a dedicated ARC OK and you have to use a current sensor (e.g., Hall sensor) to detect whether current is flowing. Could this be the case with Stahlwerk?

3. What about the control socket? When I unplug the hand torch from the control socket, I have the contacts that previously connected the trigger. Does the Stahlwerk send any signal back through this socket that I could use as ARC starting signal? Or is it really just a passive input for the trigger?

4. Generally what should I do?

What I already understand:

- The THCAD-2 connects to the THC interface (the 2 pins) and then to the encoder input of the Mesa 7i96S. That's for THC voltage measurement.
- Torch start/stop ill get it directly of the cable
- But what about ARC starting signal output? How does LinuxCNC know the arc is actually there and can safely start moving?

How would you solve this with the Stahlwerk CUT 60 P Digital?** Is there a hidden ARC OK output I'm missing? Or are you all working with a current sensor?

Any help is appreciated – photos or pinouts would be great if anyone has them!

Thanks in advance

I just ordered two Mesa 7I76EUs, two Mesa THCAD2s, two Hp elitedesk 800 G1 USDT with i5 2.9ghz 16gb ram 512gb ssd, and two cat 8 ethernet cables. So I really want to keep trying until I have a working system. This is my third attempt at trying to achieve a working plasma table and so far I have only got to use my machine for five minutes at a time at most. I really appreciate the amazing support from you all

Hi Greg,

I have a spare THCAD board that was donated to me by a fellow Linuxcnc forum member that I can send to you if you still need it. Its out in my storage unit in Dubbo so it might be a while before I can send it to you.

You can contact me on my email This email address is being protected from spambots. You need JavaScript enabled to view it.

In what way do you see 2x50k as a better way than 1x100k?
You get voltages in the 50-60V range to THCAD.

With one 100k, and recognize that ground is really close to ground on the 5V rail,
you only get a few volts in after the voltage divider.

I thought about that on my card.
 

Only what is in red has 100V (well, -100V) and thereabout, the rest is 5V max.
I made some distance between torch voltage and copper as you can see in the read.

I always excuse myself by saying I am not an electric engineer, so I can be wrong.

Regarding filter, I have a digital lowpass filter with 10 Hz pole frequency in the controller. No hardware filter.
Linuxcnc has a "lowpass" hal filter component so it can be done there also.

I used the Herocut equivalent to the Lotos. Worked fine.I live in a place where we only can have 16A, absolutely max 20A, 230V at home.
I couldn't use all the 55A, the fuse blew all the time. I replaced it with a Herocut HC8000, three phase 400V 16A. I can now max out
the plasma cutter. But the compressor need to run off the petrol generator, it's too much with that one on one phase too.

 

I have these:
www.ti.com/product/LM331
but i never bothered to use them as the time is worth more than the price of THCAD.
Easy to use and can be plugged to parallel port with a single optocoupler.

20:1 eg 200 volt full scale is fine and gives a bit more accuracy but I have used 30:1 (300 volt full scale) on one plasma cutter which is also fine..
The thcads are rated for 500 volt over voltage and you will only exceed 200 volts when piercing when the THC is not active. If using halscope you will see that the voltage will flat line at the full scale before dropping back to the 120-130 volt cut voltage before the THC is enabled.

Hello All,

New to the forum and first of all would like to thank you for the valuable information you guys post, it has helped so much me moving ahead with my build. id appreciate your help with my issue where it cannot activate the thc even when all the settings (that i know of) have been properly adjusted. first my setup:
-LinuxCNC QTPlasmac table
-Rpi5 with image LinuxCNC 2.9.8 Raspberry Pi 4/5 OS based on Debian Trixie
-Mesa 7176s over ethernet
-THCAD2 + 3M resistor array enclosed near the plasma source, F+/F- routed to mesaboard encoder input A with shielded microphone cable grounded at control side, also used a more simple twisted pair for testing.
-Used Pnconf for setting thcad2 trimming and ratio
Tested for correct voltage measuring for example, hooked a power supply from 39 to 56 volts and Main screen voltage readings coincide very accurately to multimeter reading.
-Flaring 65 plasma source with no cnc interface. (HF piercing) voltage probing connected to torch (-) and clamp (+) leads.
-For testing im using a straight line of 100mm at full speed so thc is not affected by speed reduction  (tested from 400 to 1800 mm/min)
-THC is enabled
-Mode 0 is used so arc ok is derived from voltage readings
-setup plasmac.arc_lost_delay =.3 as was having some issues for getting it to start cutting due to initial spike/valley.

I took the follwing HALSCOPE trace where:

 

1)torch on triggers,
2)probe voltage goes to 260 for piercing, then falls, then recovers to around 94v
3)Arc OK signal goes to true
4)Pierce delay elapses
5)Movement starts
6)THC active never goes up and of course  no correction

as Far as I can see voltage is "stable" once cut starts and there is no variation that could lead to deactivation as the tresholds are LOW=39 HIGH 990 (for testing purposes), samples 10 and threshold voltage 10.

has anyone had this issue or any lead on what to look at?

Hi Rod, Thanks for the feedback. Unfortunately it does not have THCAD5, and this is what I am using. So calculator does not allow me to double check. It does have THCAD10 and THCAD300. So I have set to 10, and the Voltage scale is 1/2. So hopefully this makes sense. The setting I have are now: THCAD2: Low (i.e. Down) 5V, Frequency divider: 1/32. 0V 95.4, 5V 896.0. Gives me the following as set by pnconf: Voltage Scale 0.005996, Voltage Offset 2981.25. Plasma machine: Unimig Razorcut 45 CNC port is set to 30:1 ratio. I set the Plasma to 30:1 ration as a precaution. Do people think this is necessary, or would 20:1 be fine? It is a 45A plasma cutter.

You can still use the calculator. The only difference with a THCAD2 and THCAD10/THCAD5 is that it rolls the 2 earlier devices into one and gives you a way to select the full scale range of 10v (THCAD10) or 5 v (THCAD5). So just use the mode that suits your use. Normally you would use a 10 volt full scale for torch voltage

Hi Rod, Thanks. Found the section. Looks like no actual settings defined for the newer and current THCAD2. I guess I can muck my way through it. Looks like the calculator doesn't have it factored either. Thanks for the pointer.

aikiaviator wrote:

I read somewhere about a need to set the config of the THCAD2 into the settings of the Software based on a frequency... Cant remember where I saw that but the config setting are my main concern at the moment. 
 

The thcad is a voltage to frequency converter. It has individual calibration frequencies at 0 volts and max scale volts (10v or 5v) on a sticker on the board. you enter this data into the QTplasmac interface. Read the docs and  it is well explained.