There is a whole lot of techno-babble involving ground currents and the like but, in the fewest, simplest terms, you DEFINITELY want an isolated one, especially if you are "electronically challenged"..

I had some BTR interfaces that weren't isolated and they worked - until the fateful day when something in the wiring in the building went wrong and "ground" at the PC wasn't the same as "ground" at the CNC controller I was using.

Result: a current surge developed in the GROUND line between the devices. The $200 PC survived but the NC controllerS (yes multiple ones), got fried.

As they are legacy devices and made by Fanuc, it cost a small fortune of at least 10 times that of the PC to repair them. I"d have gladly toasted a 3-4 PC's but life isn't that easy at times.

Guys will write in with all sorts of qualifiers and this's and that's on how NOT to need an optoisolated interface but, from my experience, BUY AN INTERFACE THAT IS ISOLATED.

I wish mine had been!!!!