Every output card spec sheet I read says something about electrical isolation. One I read recently said "Isolated 1500VDC channel to channel". What does isolation mean and what advantages do isolated output channels have over non-isolated channels?
Isolated means each DC input has its own + and - connection that is electrically separate from the other inputs. To make it clear, see below wiring diagrams of 2 DC input cards for AutomationDirect Productivity3000 Controller.
The above is a 8-point discrete input module, 12-24VDC sinking/sourcing, 8 isolated commons, 1 point per common. This means each channel is its own circuit. Since this is a 12-24VDC card, Channel 1 can be connected to a 24V power source, channel 2 can be connected to a 12V power source, etc. As long as this card sees 12V and above, it's a 1. Now compare with the one below
Above is a 16-point discrete input module, 12-24VDC sinking/sourcing, 4 isolated commons, 4 point per common. That means there are for group of inputs. The groups are separated(isolated) from each other but within a group the inputs are not isolated. This means if you have two inputs, you can connect them to channel 1 and channel 2 as long as they share the same power source. If they are powered from different sources, you must connect them to inputs in different groups such as channel 1 and channel 5. Now suppose channel one is connected to an input with 12VDC but everything else you have is powered by 24VDC. That means channels 2,3, and 4 are wasted since you can't mix voltages per group. You may ask, what would happen if you mix voltage sources in the non-isolated channels? If the commons are all grounded, then probably they may be fine. However most DC supplies are ungrounded or cannot be grounded so you end up with sources at different potentials tied together and you will end up with interference.
So as you can see, isolated channels require 2 terminals per input so a card has a limited number of channels and tends to be more expensive than a non isolated with the same number of channels but gives you more flexibility in your wiring.
Functionally though, having isolated I/O means that if something goes horribly wrong in the field with your I/O wiring and wires get crossed up, you might damage that particular I/O point, but it will not go further into the PLC and fry the whole thing. it's easy to replace one I/O card, it's not easy to replace an entire PLC.