USB to RS232 problem

Hello!

I have one com port on board .I use script for reseting AP and i use command
echo "UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU" > /dev/ttyS0.
Everything works fine.

Now i need one more com port. I installed USB to RS232 adapter pl2303 driver.
I attached cable to this adapter and i used command
echo "UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU" > /dev/ttyUSB0

It works but only once . I can reset only once -than i have to reset pc and it works again only once.

What could be wrong - any buffer filled -is there maybe an irq problem, is there any log for USB?

Thanks for help,
Urban