Hi Todd
Norbert is correct about the Mesa card. You can use many variations of THC control.
With the Mesa THCAD I use two components. The first one look at the voltage that comes from the THCAD and compares it to the THC Voltage setting on the screen. It will output a UP/DOWN signal that is coupled to the THCUD component ( already in the distro's). This component will control the Z axis to follow the material at the correct height. This method requires you to make use of Gcode to control the torch actions.
There are many settings on the Plasma screen but not all are needed for every system. We have included many parameters in order to allow users to create a very comprehensive control system. Many systems use a hardware THC and then it only makes use of Gcode and a simple up/down signal setup. You could also do the whole THC control in a component if you use just the THCAD. It al depends on how much effort you want to put in.
I have been busy with a very comprehensive THC controller in software for some time (to many concurrent projects

) and I hope to finish it soon in the new year as my own plasma machine will be done by then. I will make use of all the settings on the plasma screen as I am only using a voltage input similar to the THCAD. The rest is done in the component without using Gcode. The M3 is remapped in order to allow me to control the sequence of events of the plasma.
Let me know if you need help on setting up the system. I suggest that you read up on the wiki about plasma procedures. I have put a lot of information on there that could be useful in understanding the process.
Plasma wiki