Connect Arduino to Omron PLC
This is easiest:







simulate this circuit &ndash; Schematic created using CircuitLab

Output high from the Arduino sends a low to the PLC.

Output low from the Arduino send a high to the PLC.

Any small signal NPN transistor than can handle more than 24 volts should work.  



This is almost as easy:







simulate this circuit

You don't have to invert the Arduino output for this, and it doesn't waste current through the pullup like the first circuit did.



Note that the PLC has opto isolated inputs, so it is well protected.  Using another opto isolator from the Arduino would protect it from anything that might happen on the 24Volt and ground lines to the PLC.

The opto isolator in the PLC also has a series resistor, so you shouldn't need a resistor in the line going to the PLC input - doing so would probably cause trouble (not switching or slow switching.)