| 摘要 |
<p>In an ISDN network, terminals connected to an S-bus (4) are fed via this S-bus (4) by a d.c. supply arrangement (6, 7, 20). The d.c. supply arrangement comprises a main supply unit (7) and an emergency supply unit (6) which are coupled with opposite polarity to the S-bus (4) via a coupling circuit (20). Consequently, the emergency supply unit (6) needs to supply only those terminals which are also made suitable for voltages with opposite polarity, and which generally present only elementary telecommunications functions. A mechanical change-over switch forming the coupling circuit (20) has, in addition to the widely known drawbacks of mechanical switches, a further drawback that it requires a long supply line if the two supply units (6, 7) are to be positioned at a large relative distance. An electrically arranged coupling circuit (20) does not present these drawbacks and, besides, has additional advantages such as current limiting and a readily realizable monitoring of the value of the output voltage.</p> |
