If the Atmel bootloader is like the Philips bootloader (which I believe it is but would have to double-check to be sure), then it already is half-duplex - it receives a character and immediately echoes it back. The problem is at the PC end where the application may send out a burst of characters at once. While the second character is being transmitted from the PC under the COM port hardware control, the echoed characters are starting to come back from the device - i.e. full duplex.

So the problem is that at the PC end the application must send a single character, wait for the echo, send another character, wait for another echo, etc. This is how Flash Magic's half-duplex feature works. How FLIP does it *might* be determined by a COM port sniffer, but I can 99% guarantee it sends out bursts of characters, because sending one at a time is quite slow.

Andy
ESAcademy