"Please tell me how to let the 52 understand that ":"?"

When it reads 3AH or 58 decimal - the ASCII code for ":".

"Any symbols that can be sent between them should between 0 to 255."

Not required. All you need to be concerned is between 0 to 127..once again as per the ASCII coding. Suggest spend some time on this ASCII stuff - the net abounds with them.

"So how do the 52 and PC distinguish which is which? "

Where is the problem ?

Raghu