View Full Version : Help with THC wiring

TCF Engineering
29-03-2017, 08:24 AM
Hello, I have had a plasma table for the last 4 years. I built the mechanical side and bought the electronics. I am totally clueless when it comes to the electronics side, so I bought a Proma THC as it looked the most simple way to do THC

Now it comes to wiring it up and I cant figure it out, so I am after some help


This is the THC I bought, and I have attached a picture of my card which is going to the Z axis. How do I identify which wires are which going to the Z ? Can anyone identify it from the card, or can I test each output pin and determine this ? our motor control card is a Gecko G251-4

Any help would be greatly appreciated


29-03-2017, 11:31 AM
Your easiest option is probably going to be to identify and cut into the appropriate 2 wires in the ribbon cable.
What do all the ribbon cables attach to?

TCF Engineering
29-03-2017, 11:52 AM
Thanks for the reply. The ribbon cable goes to another card where the parralel port cable comes in from the PC.

The green plug at the bottom goes out to the motors, and there are 4 wires coming out of this plug and then going to the Z axis motor. I was hoping to cut into these, but how do I identify them ? Can I put a tester on them and then move the machine ? What am I looking for ?


29-03-2017, 12:05 PM
Don't touch the wires going to the motor!

What you need to identify are the two wires within the ribbon cable, that send the step/dir signals to the stepper driver.
The most reliable way, may be to remove the Z-axis driver, disconnect the ribbon cable from the board, and then using a multimeter set to ohms, identify what pins on the IDC connector (where the ribbon cable plugs in), are connected to pins 9 & 10 for the driver.
The manual for the G251 drive can be found at - http://www.geckodrive.com/g251x-rev-9

However that's assuming the board isn't doing anything other than passing the step/dir signals straight through to the stepper driver. Is there any chance of a photo of the other board the ribbon cable connects to?

TCF Engineering
29-03-2017, 12:15 PM
Good job I asked for help !

Here is a picture of the other board where the ribbon cable originates from


So if I disconnect the ribbon from the Gecko board, then with the meter set to ohms, do I activate pins 9 and 10 in Mach 3 and then see which wire ? which is steps and which is dir ?

Sorry if I sound a bit thick, but I am a bit thick when it comes to wiring and I dont want to cock it up !

29-03-2017, 11:52 PM
You'll need to disconnect the ribbon cable from the gecko board, and also remove the G251 drive itself.
Set the multimeter to Ohms, connect one probe to pin 9, then go through the IDC connector and find what one has zero resistance.
Then repeat for pin 10.

However, that is assuming that single wires are being used in the ribbon cable for step/dir. There is a possibility that differential signals are being used, where a pair of wires are used for each signal.

If you have a multimeter capable of measuring frequency/Hz, the other option is to leave everything connected and powered up, set the Z-axis to cycle up and down continually (aim for about 1 sec up, followed by 1 sec down), and then with one lead connected to 0V, using a sharp probe, probe along the cut end of the ribbon cable, and see what wires are giving a varying reading. There may be some kind of charge pump signal, but that should be a fixed frequency, whereas the Z-axis should be continually varying as the axis accelerates/decelerates.
Once you've established what wire(s) have a reasonable frequency reading, switch the multimeter to volts, repeat probing each wire, looking for one or two wires that are now changing from 0 to 3-5V as the z-axis changes direction. Depending on how quickly your multimeter responds to voltage changes, you may need to slow the up/down cycle to give the multimeter enough time to show the changing voltage.

That should identify the required wires. It would then be a case of cutting into the ribbon cable, and extending the relevant wires to the THC module.

TCF Engineering
30-03-2017, 12:53 PM
Thanks for the info, my friend was just here with me reading and now we understand. 1 more question, what is the best method to cut into the ribbon cable ? Can you get a special kind of connector, or do you literally have to cut into it with a knife and strip the wires back ?

Thanks so much for your help, we would not have figured it out without it

30-03-2017, 01:22 PM
Some kind of connector will most likely exist however I've never seen one.
I'd personally just cut the wire, extend them by soldering on some extensions, then heat shrink over them. Done well it'll look reasonably tidy.

One other thing, is once you've cut the wires, you could try moving the axis before wiring in the THC just to make sure you've cut the correct wires. If you've got the correct wires, the Z should not move, but everything else should still work.