most PLC's I have encountered run at higher voltages than the arduino, so direct connection would make a wonderful smoke generator (ie 24 volt signal going into a 5 volt digital pin)
Huh? That makes no sense. A PLC controls circuit relaysor transistors from CMOS type devices. All CMOS devices I have encountered are 2-5V. Do you have a PLC circuit diagram that describes this 24v serial communication? I have seen ladder logic setups that require a 12v pulse but like most micro-controllers, it was probably used for high voltage programming (set everything to factory).