Improvements in circuit arrangements for connecting an intrinsically safe telephone station to an exchange which is not intrinsically safe

摘要

906,182. Automatic exchange systems. SIEMENS & HALSKE A.G. Dec. 7, 1960 [Dec. 8, 1959], No. 42113/60. Class 40 (4). A circuit arrangement for connecting a " safe " sub-station to an " unsafe " exchange includes a transformer LU, one winding of which is connected through a first circuit to the substation St and the other winding through a second circuit to the exchange Z, wherein the first circuit includes a switch operable by calling pulses from the exchange for connecting a " safe " call generator RG to the sub-station and the second circuit includes a switch operable by dialling pulses from the sub-station for relaying the pulses to the exchange. The substation St is connected over " safe " wires aSP1/SP, bSP1/SP to a pulse converter U containing the transformer LU and this is connected to an automatic exchange Z over line a, b. When the subscriber lifts his hand-set contact HU make, and polarized relay IR pulls up connecting the exchange line a, b to the windings W1, W2 of transformer LU. Dialling pulses generated at contacts nsi serve to energize and de-energize relay IR and are thus repeated to the line a, b. For an incoming call, the pulses of ringing current operate relay AR which connects a "safe" ringing generator RG to the safe wires aSP1/SP, bSP1/SP and switches on RG at contact ar3. The generated audio-frequency pulses then operate microphone M in the usual way.