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One big issue with reading the plasma voltage is that the GND connection
will have large (possibly 50V or more) high frequency transients when the
torch fires. This makes is very easy to damage connected devices. This is why
the THCAD is fully isolated and has only a few pF of capacitance between the
plasma side and the control side.

Funny aside: We had to change the name of the THCAD on a invoice for a
shipment to Brazil as anything with the letters THC gets held up in customs...

7i84 should also have 4 analog inputs that can do up to VCC, so yes you can use it to measure 9V, but at low resolution and at low speeds.
Although speed might not be much of an issue for slow cutting with plasma, the resolution is as it will cause big jumps when correcting.
Buy a THCAD.
Or make a THC using a "voltage to frequency converter" chip, plenty of those around and quite cheap, until you figure out you need high voltage isolation and care with wiring and stuff, so it really does not pay off to build one, given the price of THCAD is really low for what you get.

1. Thats correct

2. You need to set the encoder in counter mode

3. Yes, you need to use the encoder velocity signal as the feedback source
for the Z axis position loop, you may also need to offset and scale the feedback
because the THCAD2 has an inherent offset (since it has an about 100 KHz signal
at 0V)

Thank you very much for your reply. I will arrange to purchase the THCAD2. Just a few more questions, please:

( 1 ) - Are the electrical connections between the THCAD2 and 7i77 electronic boards: FOUT+ with QA0, FOUT- with /QA0, +5V with +5V, and GND with GND of TB3 of the 7i77?

( 2 ) – Since the QB0, /QB0, IDX0, and /IDX0 connections of the TB3 connector of the 7i77 will be left without any connection, do I need to update it so that the 7i77 recognizes only the A signal channel?

( 3 ) – Is there an update for my configuration that allows the 7i96s + 7i77 to recognize the THCAD2 as the Z axis encoder?

2 options I can think of would be to add either
a 7I87 (8 channel  sserial analog +-10V input card)
or a THCAD2 (0 to 10V voltage to frequency card)

The THCAD2 frequency would be read by a encoder input

 

I love these pendants
 
But they take some wiring in.
 
I tried some complex hal trickery to get eveything working but gave up and wrote a custom  component
In addition to that, we are using my Lam stepboost component to get higher performance out of the Lam drivers.
And My Ohmic3 Hypersensing component that uses a THCAD which will be a bug upgrade for Steve.

It was pretty hard to wire up a plasma cutter without a table, so I made a couple of errors to make Steve's job a bit harder.
Shh don't tell anybody but I had to get some hellp to set up  my own ohmic3 component!

I forgot to mention the control box uses a safety relay for the estop circuit. Thats the red relay with the row of lights on it.I elected to utilise an external estop reset button,so we just use the QTplasmac estop as an indicator.

To the left of the safety relay are a couple of force guided relays which add some redundancy to the connection back to linuxcnc that tells us that we are awake! Thse have a plastic pin that forces the relays open to prevent any possibility of the contacts being welded shut. These are the only relays used in the build as the torch is turned on by the 7i76e's onboatd spindle SSR.

So that is a brief summary of the build. Next post will show the after.

I built a second plasma controller utilising what I learnt making the spaceship plasma a few years ago.
 
This still uses a Mesa 7i76e and a couple of Thcads for torch voltage and Ohmic sensing
Additionally it has a 433 Mhz Wireless pendant which has a wired in receiver. This is the second one of these I have used and I like them a lot
Steve in Melbourne saw some of my videos and asked me if I could help out with his plasma table which was  a bit tired and in need of a refresh.

 
So a deal was struck.
The table was using a 24 volt power supply to drive the motors which was well underpowered. My controller includes a 60 volt AC toroid I had custom made which powered 5 x Lam Technologies D1076 6 amp drivers.
We replaced the motors with industrial quality ones befitting the Lam drivers from Moon's Industries. I wanted to use NEMA34's but we could not fit them in. We also added 5:1 reduction planetary WUMA gearboxes. These needed some customisation to the output shaft. We had to use an 8mm shaft. The 14mm standard shaft just would not fit into the exisring pinion.

 

to continue

I think I may give up on the hypertherm thc.
I went ahead and ordered a thcad but I gotta figure out the z axis. 
I have the hypertherm dc h bridge z axis or was offered a stepper powered z axis from a candcnc setup.
I have a mesa 7i97t, but I’m not sure if I can control either of these z axis’s with it, since it has analog outputs for servos.
I assume getting the stepper axis going would be easier than the h bridge, but I’m often wrong.
Or maybe I should just buy one.
I also know I’ll need a driver. I have an old gecko 540 I could potentially use, but maybe a single axis driver would be better suited. One that’ll work with 7i97t’s analog signals? Or maybe an h bridge driver makes sense?
What do you guys think? 

I just tried what I think is the top right example without any impact.
I put a jumper between pin 17(thc)and pin 25(thc field +12v) on the thc connection.
then I ran pin 4 (thc) to tb4 pin12(7i97t).
and pin 20(thc field common) to tb4 pin 11.

Is there an easy trick to get arc ok signals without having the arc going?
It was hooked up to a hypertherm hsd 130, but I hooked up a thermal dynamics a60 just to make sure the plasma was actually sending the signal.

Rod,
If I don’t get this figured out soon, I may pick up the z axis you recommended in the other thread. Do you have a link?
Also thanks for sending the corner hold, I haven’t had a chance to look at it, but I appreciate it. 

Im broke and over budget so I’d like to get what I have working, but who knows if that’ll happen.

Alternatively, maybe I can just reuse the z axis I have with a thcad and something like a mesa 7i30 since the z motor is an h bridge. But I’m not even sure what an h bridge is.
This thing is turning into a can of worms.
 

I hesitate to even ask about anything that’s not a mesa thcad, but I’m going to anyway.
Im trying to get an arc ok signal from a hypertherm thc to a mesa 7i97t.
The thc uses a 25 pin d connector. Arc ok(motion according to the thc manual) uses pin 17 + and pin 4 -.
I originally connected pin17+ to my power supply common and pin4- to tb4 pin4 on the 7197t. Tb4 pin6 is connected to v24+ on the power supply. 
After a lot of unsuccessful testing, I realized I mixed up my positive and negative.
So I swapped pin17 and pin4, but that didn’t help.
I also tried throwing a relay in, but that didn’t help either.
Now I’m wondering if it’s possible I damaged something with my incorrect wiring.
I know you guys probably aren’t familiar with the thc, so I attached what I think is the relevant part of the manual.
It a hypertherm sensor phc. I’m planning on hitting up hypertherm tomorrow to see what they say.
​​​​​​​

Its getting exciting for you.
 2 M Ohm resistor will give you a full scale  reading of 210 volts. (per the THCAD manual p 6)
The Thcad full scale is 10 volts so you have a 21:1 divider.

Hi everyone, and hi @tommylight ! I'm here for an update after a while using the table.
I'm very glad I built this machine, it saves me a ton of time. It already paid for itself as I had an order of 50m of guardrail with a lot of sheet metal. It would have drove me crazy cutting it with the grinder.

I did some changes though. Upgraded the X axis motor from a nema 17 (65Ncm) to a nema 23 (1.26Nm) as it was sometimes missing steps due to a lack of torque. I also removed all the cable to the limit switches (exept the float ofc) as there where not really necessary and added a lot of cable which could get stuck during movement. I can easily put them back if needed though.

I'm really impressed on how clean it cuts thin sheet metal when you got the setting right, even with my cheap chinese plasma cutter. I have almost no post grinding to do and the slag just go away easily with a scrapper. I feel, again, I'm saving so much time and health using less the grinder.

I have a bit of time ahead of me so I started to set up the THC (I wasn't using it until today). Of course i doesnt work and I have no idea why.

First I'm not sure what I should enter when asked for the voltage divider. I'm using a THCAD-2 with two 1MO resistor on both + and - cable. I've entered THCAD-2 with W1 UP when asked during the configuration. By default there is a 20 ratio voltage divider. I would say I have no voltage divider and should enter 1 but I'm not so sure...

Anyway, I'm pretty sure it isn't what makes it not work. When cutting, even though I have enabled the THC (both in QTplasmaC and in sheetcam) the "active" indicator on QTplasmaC never turn on. I'm not sure where to start to know what's wrong (the physical led on the THCAD-2 board is turned on though).

I'd like to again thank the linuxCNC community for helping me to build this plasma cutter. It tremendously help my small buisness to be profitable.

Well, I guess I’m stuck and didn’t explain my issue.
I set up a config in axis. X and y axis move appropriately when jogged, and the dro displays accordingly.
My 4 limit/home switches trigger in halshow. The limits will trigger machine power button.
My 2 inputs trigger digital inputs in halshow.
My 2 outputs get triggered when commanded in halshow.
And I don’t know where to go from here. I’m stuck. If I bought a new lifter, I’d be stuck in the same spot.

I obviously need to link the I/o’s to functions/buttons/gcode, but I don’t know how.
Im reading the manual and trying sample axis configurations, but I just can’t get anywhere.
Like input 1 is the breakaway. But I can’t figure out how to tell that to linuxcnc. 
I thought maybe I can just copy the limit switches, but I don’t even see how they trigger the machine off button in any of the hal or ini files.
And if I can’t figure out the breakaway, I don’t have much hope for cycle start or thc hold.

Is it rude to ask for a plasma specific axis configuration?

I appreciate the input, I just wish knew enough to ask the right questions.

Rodc, I assumed you suggested the lifter for functionality, but maybe you’re actually telling me to use a lifter that integrates with qtplasmac. I did consider using Linuxcnc to control the z motor and read arc volts with a mesa thcad instead of the standalone hypertherm phc control, but I thought that just sounded like more work, more money, and more potential pit falls. But in retrospect maybe I should’ve done that just to make it easy to use qtplasmac. And a mesa card that works with pncconf instead of mesact 

Von 7i96S.
Kein externes strom, das macht probleme mit potential und interference.
Das sind nur 4 drahte fur die THCAD von Mesa 7i96S, dan 2 fur torch on und 2 mehr wen die ARCOK von plasma nutzt.
Ein gute CAT5 oder CAT6 LAN cable mit schirm (shielded) ist OK.

I have a plasma with 6 steppers as well

It's working great with qtplasmac, 7i96s, 2thcads one for the one for ohmic, parallel port breakout for the other steppers board running on a ryzen 1600

Couldn't recommend it enough. Use 2.9.2 if you want the rotary to work 2.10 wasn't working with the sheetcam post as of a few months ago.