After tinkering for some time, and lots of trial and error I believe I have finally built a brain that will do my THC using 2 analog inputs and one discrete input through ModIO.
I have tested it as much as I can until I get the right control cable for my plasma, and will post a 'how to' once I am happy with it but thought I'd share and see if I can get some input in case there are any glaring mistakes or omissions. Perhaps someone else can test it too.
The voltage divider in the plasma cutter gives a 0 - 7V signal (which I need to limit to 5V) on input 1. (For testing I am using a pot off the 5V on the ModIO)
This is compared to the voltage from a pot off the 5V on the ModIO as input 2. This will be the height setting.
Torch speed is compared to set feedrate using a formula with the percentage from the anti-dive DRO and fed through the AND gate for anti-dive on tight corners or slow machines.
TO USE THC YOU MUST USE THE ARC GOOD INPUT IN MACH 3!! If I'd known this it would have saved me hours of needless hair tearing and head scratchin.
Also make sure the THC speed is set LOW or your Z axis is likely to stall, probably more on Torch Up causing downwards creep. Mine works well at 10.
The Torch Up and Torch Down terminators may be back to front, but I don't know until I hook it up fully.

All feedback welcome, and I'd really appreciate if someone can tell me the best way to limit the 0 - 7V to 5.3V max without killing the signal, and perhaps how to get a smoother signal from the ModIO or smooth it out within the brain without sacrificing too much sensitivity.

Cheers.

Nice Brain that you did, look forward to seeing it work!!

about the voltage problem......  I am NOT a deep electronics guy, so someone else can chime in here......... but......

Could you put a "Scaling" resistor on that 0-7v output from the thc control?  where the R value would give you 5 v at 7 v....  Peters Analog ins have a high bit resolution, so you would be getting values even at the low range.....

Another more expensive option would maybe buy or build a Analog input Opto isolator of some type, where you could have 7 v on one side and 5 v on the other.

scott

Thanks very much.
Since it is my first brain it means a lot to have a comment like that from the brain Guru.
Peter has been in touch with a solution for the voltage/input issue. The high res on the input is a bit of a double edged sword, but better to have too much rather than too little I recon. I played with a timer just before the terminator for a while to tame it down a little but ditched it and lowered the resolution on the inputs instead.
Hopefully I will post some video when it is finished.
Cheers.

how do i open your file?

To limit your voltage, build a simple voltage divider.  Most plasma machines have adjustable voltage divider.  Before going to too much trouble, make sure yours is not adjustable.  TD and Hypertherm are adjustable.  Older Hypertherms were preset at 50:1.  50:1 will give you what your looking for.  If your divider is not adjustable, I'd just build a 50:1 divider and connect it to your raw tip voltage.

What plasma do you have?

How are you driving the THCup , THCdown  , Arc Good signals into MACH3 ?

LPT inputs or through Modio communication ?

(;-) TP