p2os error- setup failed

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I want to use p2os to control the Amigobot. I am now using Ubuntu 14.04 with ROS Indigo. The Amigobot is connected to my laptop via USB port. I have successfully installed the p2os-purdue. But when I try rosrun p2os_driver p2os_driver. I have the following error:

fan@Fan--PC:~/test_ws$ rosrun p2os_driver p2os_driver _port:=/dev/ttyUSB0
[ INFO] [1438630934.277844305]: using serial port: [/dev/ttyUSB0]
[ INFO] [1438630934.314938066]: P2OS connection opening serial port /dev/ttyUSB0
...
[ INFO] [1438630935.716930912]: SYNC0
[ INFO] [1438630935.917268096]: turning off NONBLOCK mode...
[ INFO] [1438630936.117623204]: SYNC1
[ INFO] [1438630936.518107007]: SYNC2
[ INFO] [1438630937.118764772]: Done.
   Connected to SUNY_509, a Pioneer Amigo-sh
[ERROR] [1438630939.522397962]: p2os setup failed...

I have read the similar problem here. It seems that I have same problem with sealguy77. But after I try sudo chmod 777 -R /dev/ttyUSB0 , I still get the same error: [ERROR] [1438631499.971642893]: p2os setup failed...