What is the output of lsusb with the prolific adapter plugged in?
The single problem I know from some prolific based adapters is that they sometimes loose the first byte when setting up communication for the first time, then they work ok.
It it not advised to send/receive data by using "cp or cat /dev/ttyUSB0"
ls usb is
Bus 001 Device 003: ID 0e0f:0002
Bus 001 Device 002: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port
Bus 001 Device 001: ID 0000:0000
dmesg -c is
erial Driver core
[ 55.187356] /build/buildd/linux-2.6.24/drivers/usb/serial/usb-serial.c: USB Serial support registered for pl2303
[ 55.187825] pl2303 1-1:1.0: pl2303 converter detected
[ 55.189142] usb 1-1: pl2303 converter now attached to ttyUSB0
[ 55.189232] usbcore: registered new interface driver pl2303
[ 55.189262] /build/buildd/linux-2.6.24/drivers/usb/serial/pl2303.c: Prolific PL2303 USB to serial adaptor driver
[ 62.333859] eth0: no IPv6 routers present
with ft232m from FTDI chip all ok but i have lot pl2303 serial cables