7i96s Arc Voltage + Encoder Fault

Hello everyone.
I have an error with the reading of arc voltage (-45 V) after one piercing.
Before piercing I was reading 0 V.

My plasma source is AutoCut 200XT from Thermal Dynamics.
I am using a 7i96s mesa card with THCAD-300 also from mesa.
The interface I am using is QTPlasmaC.

The THCAD-300 is mounted inside of the plasma source connected directly to raw Arc Voltage.
The output from THCAD-300 is connected to the encoder from 7i96s (TB2.7 and TB2..

Already switch the THCAD-300 and 7i96s to brand new ones and at start the readings of arc voltage seems to be ok.
But when I put Torch On the Arc Voltage goes to -45 V, after that the value doesn't change. No matter if plasma is on or off.
If I look to halshow I get no readings at all in encoder counts signal.

Looks like for some reason that I cannot understand I am damaging the encoder input on the 7i96s board.
I've made the exactly same connections on Hypertherm Plasma Systems and works fine.


  

I would expect the THCAD300 to connect to the 7I96S TB2 pins:

7  A+
8  A-
9 GND
12 +5V
A+ and A- should be a twisted pair

Its also very important that none of those pins connect to _anything_
inside the plasma source.

Hi PCW,

My connections are in twisted pair cables and are connected as follows:
 

From TB4 (200XT plasma system) to THCAD-300 to TB2 (7i96s)

Edit:
 
 

It sounds like a large voltage spike (>25V) is being coupled into the encoder input.
I guess this could be a wiring error. I would disconnect the encoder connector at the 7I96S
and verify that there is no connection from any THCAD digital side pin to the
plasma source GND

Is the cable shielded? If so the shield should be terminated at 7I96S TB2 pin 9 (GND)

 

Thank you for your reply.

The cable is shielded and twisted pair.

Sorry, what you mean by digital side of THCAD (IN+ IN-) ?

P1 is the digital (and low voltage side)

Is the shield terminated ONLY at the 7I96S end?