Dear sun919,
Please take note that this forum is meant for users or potential users of TRi brand of PLCs to raise their techical questions and not for Mitshubitshi PLC. Sending or receive ASCII string with M-series PLC is a piece of cake. But we are not sure how that is done in other brands of PLCs. To Using ladder program to do ASCII string processing is really using a wrong tool for the right job.
You could post your questions at the General Discussion area and see if there are folks who are familiar with other brands of PLC. Good Luck.