Patent ID: 8090079

Claim:
A loop polarity auto-reversing system of a U interface disposed in a line terminal connected to a digital service unit via the U interface, the loop polarity auto-reversing system comprising: a current detection module for detecting whether there is loop current at the U interface and detecting variations of the loop current at the U interface; a loop polarity determining module for determining whether a loop polarity of the U interface is correct if there is loop current, the current detection module comprising: a first timer for recording a system time when the current detection module detects the variation of the loop current at the U interface; a second judging sub-module for judging whether a time difference of two continuous variations of the loop current is in an open interval (A−x, A+x) ms according to the system time recorded by the first timer, wherein the value A is a preset value, and the value x is a system error value; a counter for recording times that the time difference is in the open interval (A−x, A+x) ms; and a third judging sub-module for judging whether the times recorded by the counter is greater than a preset threshold value, resetting the counter to 0, and starting the loop polarity reversing module if the times of the counter is greater than the preset threshold value; and a loop polarity reversing module for reversing the loop polarity of the U interface if the loop polarity is incorrect.