| 摘要 |
The invention relates to a duplex communication system with DTMF tone calling and electronic key sending of the called station, the system comprising a calling station and a plurality of called stations, galvanically coupled to two communication lines (L1 and L2), the called station comprising a calling keyboard with 12 keys, a tone generator (2) and call unit (5) activated by a call activation unit (8) which receives an activation signal from the calling station through one of the communication lines, each called unit containing a DTMF tone receiver (9) which takes over the DTMF calling signal from one of the communication lines and transmits it to the key decoder (10) controlling an optical and acoustical call unit (12) through which an activation signal is transmitted to a call activation unit (15) which, when the subscriber accepts the call, furnishes signals for the activation of the calling units (5 and 17) in the called and calling stations.
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