Communication was sniffed using wireshark's usb capture, with windows application running in virtualbox.

To program sets serial port speed to 921600. Unfortunately speeds above 115200 aren't handled correctly by pl2303 driver (as of 2.6.39). You need to apply included patch to the kernel to make it work (will be present in linux 3.1).

Apparently sometimes this patch isn't needed at all.

Program doesn't (yet) check crc of received data blocks, so it's safer to download data twice, and check if they are identical.