| 摘要 |
<p>PURPOSE:To assuredly prevent the useless feed in the non-communication state and at the same time to normalize communication when the connection is wrong, by feeding the DC power with normal polarity regardless of the polarity of connection to a terminal device of 2-wire line and a center device. CONSTITUTION:In case a terminal corresponding circuit 30 of a switchboard is set at the originating side, no signal TER is produced from an incoming signal input terminal 33 and the DC current having no inversion of polarity is supplied from a DC power supply 31 to a 2-wire line 2 connected to a terminal device 10. When a current detector 37 detects the feed current, an AND gate 36 is closed since an originating signal ORG is 0 to stop the operation of a polarity inverting swich 32. In case the circuit 30 is set at the incoming side, the switch 32 is operated through an exclusive OR gate 35 and by the signal TER. The feed current, if detected by the detector 37, is prevented by the gate 36 to continue the feed. Then the DC current with the normal polarity is supplied to avoid the useless feed in the no-signal state.</p> |
