I've just spent the greater part of 2 weeks trouble shooting Mach3 THC issues. I read all of the posts I could find at mach support, Warp9 (ethernet smooth stepper), and through google. I've read manuals for Mach3, ESS, Proma, & stepper drivers. I watched all of the vies I could find on youtube. I had Newfangled Solutions check out my computer remotely with a verdict of all setting correct. Still, THC would move Z axis up when necessary, but not down. I finally got it working by downloading and installing the Vodoo screen set. After loading the new screen, and setting THC Min to -1.000, the THC moves Z up AND down properly while cutting. This is an easy thing to try for anybody else having similar issues.

Look for the Voodoo.zip file here: http://www.machsupport.com/forum/index.php/topic,17065.0.html

I will note a couple of things that are still puzzling:
1. Videos show DRO and/or Z axis movement while Proma THC is in test mode. My system does not do this even though LEDs light up on the diagnostics screen and THC up/down now work properly while cutting.
2. Mach3 seems to ignore ArkOK signal in that it will execute gcode with, or without, the ArkOk signal. I've tried using the ArkOK from the THC and from the plasma cutter. In both cases, the LEDs light up on the diagnostics screen, but Mach3 runs gcode in both states (ArkOK on or off).

Not sure this is going to work for me since Im not running bladerunner