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Before this gets out of hands, open the box, take some pictures, we can figure out a lot from just pictures.
Reason being, in most cases you yank the control card and leave the drive/motors as is, use a parallel port or a Mesa board as control and make chips.
You can always start with a parallel port (or two) and later get a Mesa 7i92T that replaces both those ports, with much, much higher step rates/speed.

First you need the exact info on how the H bridges are controlled, there are several types/modes, then based on that see what the Mesa 7i95T should output.

Moved to "PlasmaC" section.
Thank you.

I want to understand how can i change the acceleration in linux cnc in realtime,  I also want to do some work on linux cnc professionally and  i am ready to pay , can anyone suggest me professional working on linux cnc?

I Initially thought it was due to an incorrect .ini file version but I eliminated that as the fault
 
Is this error message generated by QTplasmac?
It is not documented anywhere I could see. In any case it should not continue to appear.
 

I solved it (nicht das gelbe vom Ei but it works )

I call the sendData function
if (isButtonPressed(btnX, touchX, touchY))    
{     
btnX.state = !btnX.state;     
drawButton(btnX);     
sendData('I',8,btnX.state);        
}

So I don't use physical buttons. Later I will share my TFT touch handweel (with your ArduinoConnector) on Github. I will modify your Code with virtual IO's and w/r strings (e.g. scale value - 0.01; 0.1; 1.0). Or, is there another why to send the scaling values to my linuxcnc?

Actully I manipulated the analog command in your python code (c["ain.{}".format(io)] = value/100 #remove /100 - now I send 1; 10; 100 for scaling) for the scaling function and will try it later with the r/w strings.

All good advice! My next machine will use a N100 fanless industrial PC with a 7i96s The PC uses Intel NIC's has a RS485 port builtin for the spindle or Hypertherm plasma cutter. Previously I have used similar machines with a Celeron J1900. 
I have always used a wireless keyboard tucked away for when needed with QTPlasmac and QTDragon. Take care to check the screen resolution  is compatible with the GUI. I had to replace a monitor for QTDragon. 

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Thanks so far for the information.
So first step is to ask the cnc.step guys if there is a config file for linux cnc for their machines.

After that (assuming it is more likely they don't) I have to guesstimate which way is more painless:
-how to get a working LinuxCNC machine, which includes some hands on hardware building and embedded software back and forth
-using the cnc-step machine with windows software to run it with the pain included by that. That would be the first windows PC to be administered by me in about 20 years. Which might be even more pain than the above (at least for me).

What about 100-120mb programs? Are they possible? LinuxCNC seems to crash then loading those.

Just my take on it. There are many ways to run linuxcnc, keep that in mind.

My questions:
* What should I run LinuxCNC on?  I've got a spare RPi 4 sitting in my drawer.  I've got a couple of Supermicro Intel Atom-based ITX server motherboards lying around.  What's the most intelligent way to go?

Atom is probably below the acceptable performance limit, but anything faster than that. There are some new mini-ITX main-boards with N100 integrated. CPU performance is more for the user experience, the later HMIs (or guis) may need a bit faster CPU, while the old Axis works with anything. For the cnc part it is about real-time jitter and there usually are ways to tweak settings to come to low jitter. I go with N100 nowadays, but its just one of many possibilities. Once you have seen the newer guis you will want to use one of those. And need at least some CPU performance. I ran on RPi4, I would think a step up to RPi5 would be nice.

* It seems that a parallel port interface is the "old-school" way of doing things.  Now, it seems that most prefer an Ethernet interface.  Is there a disadvantage of going one way or another?

Stepper performance (steps/seconds), number of IOs, and other types of IO than digital IO is what makes people move away from the parallel port. Plus that main-boards nowadays seldom have a parallel port and need to have one added.

* Since I have to replace the steppers anyway, is it foolish to go with a "traditional" setup rather than Ethercat?  It seems like Ethercat will be considerably more expensive (like twice the price or more) - but is it worth the price?

You won't get anything extra from the milling machine using EtherCAT. It is a different way of doing the same thing.

* Everyone seems to agree that Mesa boards are the "gold standard."  How do Pico Systems' boards compare?  Jon Elson lives like 20 miles from my house.  Does anyone have experience with his customer service?  Is it foolish to consider anything other than a Mesa?

From what I can see, and I may be wrong, the Pico Systems' board connect to the parallel port? If you go with a parallel port solution that can work. Mesa's board on the other hand typically use EtherNet, SPI, PCIE, or PCI (and maybe more) to connect to the computer. This gives speed and reliability.

* If I go with a Mesa setup, which direction should I go?  If I use the RPi, I could go with the 7C80 or 7C81 - I could also run an Ethernet setup.  If I use the server motherboard, I could go with a parallel port setup or an Ethernet setup.  Is there an advantage of going one way or another, here?

I've only used Ethernet setup and see few reasons to do anything else. If you are only ever to use Raspberry one can think of the 7C line. I would like to keep all options open as far as possible.

* If I go with a given Mesa board, what else will I need to buy?  It seems that some of the Mesa cards (like the 7C80 and the 7I95T) will hook directly to the stepper drives, while others require a separate daughter card (BoB)?

Send Mesa a mail with those questions, they are usually very happy to help. For a normal stepper motor system I would look at 7i96, which connects to the stepper driver with the step/dir/enable signals.

* What hardware (if any) would I need in order to use the encoders on the steppers as a DRO?  Will any of my hardware selection above affect my ability to have this feature?

For Mesa to answer, I think you are looking at a 7i85. From what I understand, encoder counting is done by the FPGA, so the card must either do it itself on the FPGA board, or have a connector that allows a daughter board to connect the encoder inputs. The 7i96+7i85 does that.

* Will a cheap Amazon/Aliexpress stepper motor/drive/power supply set work, or will I need to buy the stepper drives separately?  If I need a particular stepper drive, should I go with a Gecko or something else?

Yes, will work fine, just size it to suit the stepper motors. I have good experience with such stepper drives.

* If I run a touchscreen and a pendant, is there any reason why I will need a keyboard hooked up to this machine (once it is configured)?

Depends on the gui and how well touch screen input is integrated. For example, I run the qtdragon gui and there the keyboard input is well integrated. Except when I need to edit the G-code file, then I bring in the keyboard. Normally I have it tucked away though.

The linuxcnc support is the big question.
It is neat to buy a ready-made box but you still need to have a place for stepper motor drivers so it is only half the solution.

Gents,

I'm slowly preparing items and knowledge to make my plasma cutter based on Hypertherm Powermax 45 XP (just received CPC port upgrade kit). I have a specific conditions - at work for 4 weeks and away from home/garrage and i could only take limited set of toys with me:
- Lenovo T470 with Linuxcnc installed
- Mesa 7i95T
- 2 x THCAD (for THC and ohmic)
- some limit switches
- 3 x H-bridge and 3 x small PM DC servo motors
- bunch of wires
- old laptop power supply to power those servos.

That's all i have to play with LinuxCNC and learn configuration, etc. Ideally i'd like to make those servos live on my bench.
I couldn't take "real" servos and drivers due to weight limitation - my only option was to take those small servos and h-bridge modules. 
If i understand things correctly, out-of-the-box mesa cards are preloaded with firmware containing Step/dir generators and can't be used to control -bridge based servos it this state. So i need to build a specific firmware to replace step gens with pwm generators, right?
I'm stepping here into very dark area for me - creating custom bit file.
I heard that custom bit file are now easier to create with the new FPGA chips on Mesa cards - is it true?
I case of my failure - would it be possible to get some help from you to create custom bitfile? The only source i found is talla83 videos on yt.

Thank you for inputs and have a great weekend.

Thanks guys. I ended up taking T470 with touch screen. Added Linuxcnc along with already existing Win and 7i96 works with no issues (on the table for now). Just received 7i95T today to have some fun but this requires some deeper tweaking - for separate topic. Thanks again - computer topic solved for me.