OK, you seem willing to do some investigation, and I'll help a little. Allen's questions will be thought provoking, but a couple more hints might be in order.
A programming panel has little or no memory, and will only display a couple of rungs at a time, and doesn't allow documentation. What are the implications of that?
As far as connecting a PC to a PLC, the basic requirements are the same as any other communications system. You have to have a carrier for the message (think telephone line) and the two ends have to be electrically compatible (you can't hook a cable TV to your ordinary phone line, can you?) and the message has to be understood by both ends (can it work if you speak Spanish to a listener that knows only Russian?).
The answer to these questions and to your other two posts are contained in Phil's book and his tutorial. You should go through the tutorial first, and come back if you need further explanation of individual items. It will help if you quote the part of the titorial that is confusing you!