We have a plasma cutting machine running Mach3 USB. Our limit switches work well until we turn the plasma torch on. The best fix for plasma/ limit switch interference is supposed to be by using an optocoupler. When using only a limit switch, it was connected to DCM and IN1; successfully switching the IN1 pin from 24 volts to 0 volts. I've used a NOYITO 1-Channel PC817 Optocoupler Isolation Module from Amazon. The input turns the sensing LED on and off as it should. So far, I am using the 2nd connection diagram from the top, yielding 24 volts and 14.9 volts to pin IN1. Am I connecting something wrong, or is the optocoupler not what should be used here? Shouldn't the voltage swing from 24 volts to closer to 0 volts on pin IN1?

USB connections for an external motion controller with Mach3 are 'noise prone' and it is unlikely to be truly sucessfull when using an 'electrically noisey' plasma torch.
My opinion is that the problem is not with your limit switch connections but is related to the USB.
The best long term solution I can think of is to swap out your motion controller for one that uses Ethernet connection such as (and just for example) the Warp9TD ESS Smoothstepper. There are other (Ethernet connected) motion controllers available but make sure you choose one of EU or USA manufacture and steer clear of those from China.

Tweakie.

Does your machine work ok with the torch on  until activating the limit switch?  If so then the problem is probably not USB interference.  Could you give more precise information on exactly what happens in what circumstance?   Also give a diagram of your connections?