Ascii is usually adequate and most commonly used on most PLC platforms, Particularly with barcode applications. I know Mitshibushi can handle it but not familiar with particulars with that platform.
What PLC's are to be connected, CPU-type?
Do you have any communicationscard installed already?
How many data do you have to exchange?
If both PLC are Mitsu ex.Q-system, you could use CC-Link it is a cheap and easy to setup.