I'm usually confuse on how to tell if a particular device gives an analog input to the PLC or does it need an analog input from the PLC (hence PLC output). I usually get my way around by talking to the vendor. By looking at it based on the drawings e.g. for the kinetrol actuator that I'm working on (need to create the loop diagram), it requires 4-20mA for its positioner signal and angle re-transmit supply (see attachment).
How do you guys figure out which one is the analog output and which one is the input? Or are they all input to the PLC?