Eaglewood

Still curious about the inner workings of the PLC. 

This statement is too generic to give a constructive response. 

Use words like a lawyer does when setting up a contract:
Use enough words and syntax to avoid as misunderstandings.

Repeating from above:

Eaglewood

Repeating from above:

Do the digital outputs disconnect from the source voltage when the output is OFF, or are they still connected, just set at zero?

Transistor (PNP) outputs are also connected all the time but is more resilient. Connecting another source to the output (for example a switch) is not textbook recommended but can be done. Here one can put a diode on the output to ensure extra protection, which is a better option.

Relay outputs are disconnected. Mixing AC phases and/or DC is something to look out for.