URM06-RS485 Ultrasonic Problem

i have a project using URM06-RS485
http://www.dfrobot.com/wiki/index.php/U ... KU:SEN0152
my arduino serial print just read only 300mm. whats the problem ?
http://www.dfrobot.com/wiki/index.php/U ... KU:SEN0152
my arduino serial print just read only 300mm. whats the problem ?