spcomputing:
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).
He is not talking about a PLC's communication bus but rather it's field input and output loops. Typically one buys a PLC with the type of input and output boards required for the installation and these can work at various voltages (24vdc and 120vac are two common types used for industrial use). One needs to be very careful if attempting to read a PLC's field input channel with an arduino of an existing signal wired to a PLC input channel. It's probably possible to do it safely but not something for an inexperianced person or if all the detailed technical documentation is not avalible.
Lefty