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Does the machine have a floating head with switch?
Use that, skip ohmic for now, make the machine works properly first without it.
I never use ohmic probing, floating head only. To much trouble for not much gain, and my machines are used by clients, so as simple as possible for daily use.
Rod and others have made huge improvements to ohmic probing as it was useless before due to water ingress inside the torch, so i am thinking of adding it to one of my machines for quite some time, just need time.

I hate feeling ignorant here but where does the 24v power get applied? I'm guessing this is the High Voltage side and is the voltage used for the sense but just want to be certain.
I built this circuit years ago for Mach 4 using two 12v relays and thought it was just a grounding situation where the nozzle touches the metal it completes the circuit and sends the signal.

the low voltage side of the thcad connects to the encoder input. The high voltage is either the ohmic power supply or the plasma cutter depending on application

In either use case I would power the THCAD 5V from the 7I96
(Ohmic sensing with a THCAD requires a separate ISOLATED
24V supply)

If you have a 50:1 divider the THCAD needs no external resistors
Setting it to the 5V range gives you a 250V full scale plasma voltage
which is fine.

Mounting the THCAD in the plasma source is probably better noise wise.
In this case, I would use shielded  twisted pair and make sure that the
shield is grounded to the 7I96 GND and not anywhere  at the plasma
source end.

I have searched with not much luck on info for the THCad2 (lots of info on the others) I'm curious on a few points that I've come across. When using the THCad2 for ohmic sense I've seen a separate power supply should be used, yet I've seen power being pulled directly from the 7i96s from the encoder pins.  What would the recommended setup for that be? And are there certain pin configurations the THCad2 should be set for,  as well as the pins for the encoder on the 7i96s? Also for using the THCad2 for height control I will be using Hypertherm 65 with 50:1 divider. I saw on a diagram a HV resistor is used but that was for 1:1 I believe. Will I need something in place of that running 50:1? And Does the THCad2 need to be mounted by the plasma or can it be located in the main control panel as I've seen conflicting ideas for that?

Decided to show my setup off, still a work in progress the drivers shown in the picture will be replaced with the ones currently on my MACH4 plasma table. Hoping this will be a good decision to make the switch as I have had nothing but issues over the last 3 years in MACH4. I went with the 7i96s and two THCad2 cards. Basically this design follows the Lowrider series at least that's what I'm seeing it called. Using a Beelink s12 Pro with 16gb ram only issue with it so far appears to be the WIFI and Bluetooth arent supported in the 6.1 Kernel I tried the 6.5 kernel and they worked but than had latency issues in Linuxcnc 6.6 kernel seems ok so far but wont know really until I have it all in place and physically running the table. The goal is to get this controller having only 3 cables coming out of it and going to the table, minus the power cord. I currently do have 5 aviation plugs installed as Im still researching if i might have issues with EMI or any other random issue running say a 22/20 shielded cable to the gantry. All Prox switches are run in the same manner using a 22/12 shielded cable. I ran an extra HDMI and USB extension cable to the right side of the panel in case I want to add a larger monitor down the road and the usb to bring in files for the machine to cut. The Beelink is setup in bios to power on when the master switch is turned on so no need to open the panel to access the power button. When I do transfer the electronics over to the table Ill be making a few more cuts to the panel to add a fan in and some extra breather holes for ventilation.I couldnt figure out how to add the images inline with the text so I guess Ill Add them as attachments.

It asumes you have the mesa bit file installed that ends in *pl.bit
This gives you 3 x encoder A's on the A,B and IDX terminals
The example assumes you will use encoder 1 which should be the ENC B terminals
Install the bit file, then review the pins in halshow to get the right pin names for your 7i96s

ok, Im guessing i will need to modify these files to reflect the 7i96s?

Also do these files specify where on the encoder the wires will be? I just glanced over them real quick.

I thought I noticed for the 7i96s the THCAD should go into a speciffic location? Believe it was the diagram for the Low Rider 3.

pncconf does not support ohmic sensing with a thcad.
I have attached a later component and a stand alone hal file to set it up
You can add the hal file into your ini file
You will need to install the component with
sudo halcompile --install ohmic3.comp

I Decided to add a second THCad2 to perform ohmic sensing. In PNCconf what is the choice supposed to be on the encoder? seem only option available for plasma is Arc voltage, and under the other options I do not see one for OHMIC. I tried setting it to arc voltage but that doesn't seem right as i only get one option to configure which board is being used.

Not sure what that was but reinstalled Linux Mint Debian Edition and didnt have that issue anymore. made it to finalizing our control box now, Just waiting on our 2nd THCad2 to arrive. decided to use 2 cards one for OHMIC and One for Height Control. That will be our next step hoping it wont be a big step lol..

Uthayne-

I've attached a zip file with the relevant config files. I'm using QtPlasmaC so the .prefs file is important as well. Since QtPlasmaC puts some limits on what are acceptable for the Arc Voltage Offset and the Arc Voltage Scale (it basically expects a MESA THCAD of some sort), for scaling I used a combo of encoder scaling in custom.hal:

# The QtPlasmaC GUI does not allow voltage offsets higher than 5 digits, so scale this down here
setp    hm2_7i92.0.encoder.00.scale 100

and the offset and scale in the laser_cutter.prefs file:

[PLASMA_PARAMETERS]
Arc Voltage Offset = 22900
Arc Voltage Scale = 0.10

This leaves me with a ~50-100 range for "arc voltage" in the QtPlasmaC GUI and is sensitive enough to hold height well, at least for the values in the table that Taloot published a while back (0.5-1.5mm). 

You might also be interested in the fake "ohmic" probing and fake "arc OK" signals I implemented in the custom.hal file:

Do you have ohmic sensing installed and working?
Does the machine use 2 separate stpper motors, 2 drives and 2 home switches/sensors on Y axis?
Does the machine have a floating head/torch holder with it's own switch/sensor?
Also, never copy stuff at random, usually does not work properly, leave the step/dir timings at default for all drives, test properly, then decide if anything needs changing. Especially not from Gecko drives, some use 500ns, some 4000ns, some older ones 15000ns.
-22V is OK but also might not be, it is ok as when no THCAD is connected it does show negative values, but what exact value depends on scaling and offset and divider set on the board.

I finally believe I have linuxcnc setup, just a few questions regarding driver and motor settings, Is there a way to determine they are running optimally based on all the timing settings. I'm currently testing with some cheapo drivers and motors. I grabbed settings for the Gecko 203v drivers from MESA Config and brought them into PNCconf since the MESA config doesnt have the option for QTplasmac afaik and used those to get the motors spinning.

Also I'm not getting the option to manually joy Y axis as its not lit up or highlighted when machine is enabled in Qtplasmac. also appears I don't have a few other items highlighted as usable.

I chose ohmic probe and Float for torch I noticed there is also an option for Enable for both in the config. are those required?
As well THCad2 is not plugged in but is setup and qtplasmac is showing a -22 volts, is that normal or should it read 0?

Im sure ill have a few more questions as I fish this build out. Im using the 7i96s and running linuxcnc 2.9

Man you're precious. Thank you a lot for all these usefull informations. Just bought the Mesa 7I96S and the THCAD-10. Was a bit of a pain to find in europe but finally found it on eusurplus (funny how my google search always end up on one of your answer on another subject). I'm glad I can buy cheap drivers because the Mesa cost more than what I was expecting with taxes and shipping. I've read the plasmaC documentation. I'll take a picture of the inside of the plasma cutter when I have time. I'm still hesitating if I buy the Stamos or try to fiddle with the Stahlwerk I already have. I would learn more thing using the stahlwerk, but i'm afraid to break something, or worse, injure myself with these high voltages.

Thank you again, I'm a bit lost in this project and it's reassuring having someone to help a bit ! Yet, I'm exited. This plasma table will be really usefull in my work (and will save me countless hours with the grinder), and understanding how to do CNC stuff by myself is very rewarding.