I'm not sure if it's a typo, but the serial ports on linux are NOT /dev/tty0......they are /dev/ttyS0. Maybe your system has a different naming convention. I'm not surprised you can write to, but not read from /dev/tty0. Alot of devices will let you print to them, even if they don't understand it.

Also try running the command as root, and see if it helps. If it does, then it's a port-permissions problem. On many linux's, you need to be a member of group UUCP to access the serial ports.