Using an optocoupler to tap into digital PLC signals
Impedance can be easily tested with large load or pullup and compute V/I change.  For this "Logic level" with 5:1  Voltage divider using 1~10M . Darlington solution is ok with Series R to limit IR input current and shunt R to turn off faster. Then loading is only 1~20uA . So this logic translator could have specs Zin=100k~10M and be either a 5:1 CMOS Logic Schmitt trigger to drive Opto or transistor or Op Amp... U decide.

I would use 1M:250K to 'HC14 to opto (with CMOS logic out) or better confirm PCL impedance and then improve choice if latency or transition speed is an issue.

or use tiny SO8 Schmitt inverter (dual) https://www.digikey.com/product-detail/en/fairchild-on-semiconductor/NC7WZ14P6X/NC7WZ14P6XTR-ND/965444  $0.50 (1) with 50mA drive to twisted pair with CM choke and no opto







simulate this circuit &ndash; Schematic created using CircuitLab