| 摘要 |
1343002 Party line systems STANDARD TELEPHONES & CABLES Ltd 18 May 1972 [21 May 1971] 23438/72 Heading H4K A communication system including a plurality of stations connected to a common communication path has a calling arrangement wherein the calling signals comprise a total number of pulse transitions divided into groups to give a unique station code so that a decoding arrangement in the station responds to the code whereby a timing circuit is prevented from operating on receipt of the respective code by being switched into a long delay mode coincident on the receipt of inter-group pauses. In the system described the calling signal consists of seventeen pulse transitions divided into three groups by 1 second code pauses and prefaced by a 1 second start pause with a terminating 2 second ring pause followed by a single switch off pulse edge. The decoding arrangement includes an electronic, solid state counter which totals the pulse transitions so that a response is obtained for a group call where the first group of transitions is sent followed after the 1 second code pause by the remainder of the total or an all station call when all seventeen pulse transitions are sent. In the circuit arrangement an on circuit timer S is used to operate the switch 6 on receipt of a valid start pause and this provides a power supply to the decoding circuitry. The input pulses are attenuated to a suitable level and filtered in amplifier 4 and reshaped in inverter 9 so that they can be accurately counted in the counter arrangement 10, 11. The count is passed to a decoder which consists of gates connected to a strapping field in which the particular station's code groups are set up so that an output is obtained whenever a valid code group is received. This output is used to change the time delay of off circuit timer 15 from a short to long time delay thus keeping the circuit alive during the code pause. The timer 15 is arranged to be reset whenever a pulse transition is received and thus if a code pause is received at the wrong time, i.e. another station is being called, will time out and disable the switch 6 thus shutting down the station. When the whole code is received oscillator 7 is operated during the ring pause to provide an audible alarm at the station and a ring back tone to the control station. A station can make a call to the control station by short circuiting the idle line using button 18. |
