rosrun sicktoolbox_wrapper sicklms error

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When i run the command "rosrun sicktoolbox_wrapper sicklms _port:=/dev/ttyS0" on the Pioneer Powerbot,the terminal shows the error,and i had tried a lot of solutions on the internet,but those did not work.So hope your advice! note:ROS electric ;Ubuntu 11.10

root@ubuntu:~# rosrun sicktoolbox_wrapper sicklms _port:=/dev/ttyS0
    *** Attempting to initialize the Sick LMS...
    Attempting to open device @ /dev/ttyS0
        Device opened!
    Attempting to start buffer monitor...
        Buffer monitor started!
    Attempting to set requested baud rate...
A Timeout Occurred!  2 tries remaining
A Timeout Occurred!  1 tries remaining
A Timeout Occurred - SickLIDAR::_sendMessageAndGetReply: Attempted max number of tries w/o success!
    Failed to set requested baud rate...
    Attempting to detect LMS baud rate...
        Checking 19200bps...
A Timeout Occurred!  2 tries remaining
A Timeout Occurred!  1 tries remaining
A Timeout Occurred - SickLIDAR::_sendMessageAndGetReply: Attempted max number of tries w/o success!
        Checking 38400bps...
A Timeout Occurred!  2 tries remaining
A Timeout Occurred!  1 tries remaining
A Timeout Occurred - SickLIDAR::_sendMessageAndGetReply: Attempted max number of tries w/o success!
        Checking 500Kbps...
A Timeout Occurred!  2 tries remaining
A Timeout Occurred!  1 tries remaining
A Timeout Occurred - SickLIDAR::_sendMessageAndGetReply: Attempted max number of tries w/o success!
ERROR: I/O exception - SickLMS::Initialize: failed to detect baud rate!
[ERROR] [1346919668.935157216]: Initialize failed! are you using the correct device path?