| 摘要 |
PROBLEM TO BE SOLVED: To provide a digital service unit that connects with a subscriber line simply and accurately without confirming the polarity of a terminal of the subscriber line in the case of connecting a connection terminal of the digital service unit to the terminal of the subscriber line. SOLUTION: In the case of connecting connection terminals 301, 302 of the DSU 300 to terminals of conductor lines L1, L2 of a subscriber line 100, even when they are not connected by correct polarity, a polarity inversion control circuit 309 detects it and a polarity inversion switch 308 is switched in the inside of the DSU 300 so as to switch over the polarity of a couple of power supply lines SL1, SL2 automatically that supply power to a power supply circuit 307 of the DSU 300 from the subscriber line 100. Since correct polarity possible of communication is automatically selected in the inside of the DSU 300 by having only to interconnect the terminals of the both without caring about the polarity of the terminals of the subscriber line 100 and the DSU 300, trouble such as re-connection to correct polarity or manual changeover of the polarity inversion switch due to incorrect connection is avoided. |
