Analog Signal & Digital Signal Explained

Hey
I was wondering if you guys could explain to me how the signal flows on a analog & digital signal.

for example:
Explain the wiring of a 2-wire, 24v, 4-20mA transmitter that measures pressure that goes back into a AI Compactlogix 1769-IF8 PLC card.
-Can you explain loop power & how the signal comes from the transmitter back into the input card?
-Does the Analog Out signal work in the same manner? moving a valve position


Digital signal:
DI vs DO
-Explain the wiring on how a DI & a DO signal travels back into a 1769-IQ16 input card
and a 1769-OV16 output card

thanks for helping

Loop Powered.....

https://www.google.com/search?q=exp...er&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

See "Wiring" at link below

http://www.plcs.net/contents.shtml

More input/output info...

http://accautomation.ca/how-plc-inputs-work/

http://accautomation.ca/how-plc-outputs-work/

Thank you!