GB984462A - Improved transmission system - Google Patents

Abstract

984, 462. Electric selective signalling systems. INTERNATIONAL BUSINESS MACHINES CORPORATION. May 15, 1962 [May 26, 1961], No. 18698/62. Heading G4H. A communication system comprises a transmission line L, Fig. 1, having a central station S c at one end and a terminal impedance R o at the other end, and a plurality of remote stations S;, Si &c. connectible to the line, and is characterized in that each remote station includes both a line compensation impedance equal to the impedance of that part of the transmission line beyond the remote station and an impedance equal to the line terminal impedance R o , whereby when a remote station is connected to the line and that part of the line beyond the remote station is disconnected, a constant impedance is presented to the central station. In the system described any field station can call the central station but the central station cannot call the field stations. The calling sequence for remote station S i is as follows. The call is initiated by the remote station connecting a higher impedance source A, Fig. 1b, to the line which raises the normally negative potential of the line. This is recognised by the central station which then, Fig. 1c, transmits a sequence of combinations of frequencies, each combination corresponding to a different remote station, until the combination corresponding to the calling remote station is reached at which time the calling station signals to central station thereby identifying itself. Also, at this time, the remote calling station disconnects the transmission line beyond it and connects its transmitter B, Fig. 1d, to the line. Thus the remote station has established communication with the central station and data can now be transferred.