You are experiencing a buffer overrun. The receiver cannot process the incoming characters fast enough. If you cannot implement handshaking (RTS/CTS or XON/XOFF), the only thing to do is to split the string into several smaller ones and transmit them in intervals. In the String Function palette there is a String Subset function that you can use to create smaller strings from a large one. Transmit the first substring, allow time for receiver to catch up, then transmit the next one, and so on...