Signal Isolator

On the "as built" issue drawing of the circuit will be the names of the individuals who designed and commissioned it. Surely a word in these individuals' ears is the best place to start? After all, the drawing cannot be seen from here.

The clue is in the name...
It isolates the signals from unwanted voltage levels. Something like an opto isolator allows you to have some sort of sensor on (say) a high voltage circuit providing a signal into a 5v logic circuit. the signal being transmitted optically across a gap in the integrated circuit to provide electrical isolation.

Mind, I may be barking up the wrong tree as I have no idea what your appliction is or what an AI card is... artificial intelligence? Alien Invasion? Artificial Insemination?
Ah! Me sir me sir I know... is it analogue input?
Del

PLC's have AI, AO, DI, DO, and some others lesser used cards. Analog Input, Analog Output, Digital or Discrete Input and of course Output. Standard acronyms in the control world.

D'uh... but not everyone is in "The control world"
Grrrr hisss ftzzzzzz

Without a signal isolator, you could end up accidentally feeding in a high voltage into your computer / PLC, which could damage it.

Even an incoming signal level is only, say 3.3V, you don't know what it is referenced to!

For example, the "ground" or zero volt level might be at a different potential between your computer's motherboard and the other device that is sending or receiving a signal.

You could have a 100V difference or more between the main DC ground of your computer and the other device.

Without a signal isolator, you could blow up your computer!

For example, Ethernet interfaces are magnetically isolated. Each wire pair is connected to small transformers on both ends ("magnetics"). This, by the way allows for transmitting power in Ethernet cables (PoE).

In my 25 years of building/installing control panels on process skids, it has been my experience that they are there for something to do during system testing. EVERY one that was requested by a customer, or installed in a customer provided panel, are sitting in a box in my largest desk drawer. We removed them all during final testing of the system, at the customer's request. Now I'm sure there are times they function properly and are required, but so far, I've not encountered that.

The exception would be intrinsically safe barriers which would technically fall into this category as well as the discussed noise filters. Those must remain to keep voltage/current in a circuit below the level needed for arcing in a hazardous atmosphere application.

Very interesting timing. The customer built panel we are testing on a process skid today, much to my surprise, had signal isolators on the analog signal to and from a motor mounted VFD. I totally missed that when going over the drawings to layout our wiring plan. These seem to function fine, and I can see why an engineer threw this in - a VFD is a very noisy item. I know this same panel has worked before without them, but it can't hurt to have them. Wiat - just got word that the analog circuits in question are not working - more on this later after I go do some circuit analysis.

Signal Isolators Information on GlobalSpec

https://www.globalspec.com/learnmore/data_acquisition_signal_conditioning/signal_conditioning/signal_isolators