Just use a resistor! The input pin is limited to vcc so a resistor works quite well and is very cheap and simple - if the output of your PLC is left floating you may have to tie it down with another resistor. I've used a resistor straight into a PIC from 240Vac for sensing the zero crossing to synchronize firing a TRIAC (I think I used some where in the region of 200k). It seems so wrong I know, but it really works well and is simple.