How can i design a circuit for tapping digital DC signals from PLCs?
I desire to create a circuit with a MCU which can be attached to a PLC signal in the range 0-24V and log the signals on the MCU. I only need to log on/off state of the PLC. "Off" is defined as 0-5V and "on" as 13-24V. It is of high importance that the PLC and MCU are isolated from each other. I have thought of using either Darlington or linear optocouplers.
My design thoughts are to use a single resistor or a voltage divider to scale the PLC voltage down in the range of the optocoupler in order to not burn the diode. However I doubt that if I do this it is possible that the circuit mesh with the PLC signal? Eg. pull it lower than 24V. I have never worked with PLCs before so I have no clue what the general or standard output current is.
Any thoughts/experience or good advice?