GB1356927A - Telephone intercommunication systems - Google Patents

Abstract

1356927 Interconnection systems POST OFFICE 16 Sept 1971 [6 Oct 1970] 47508/70 Heading H4K An interconnection system includes a plurality of telephone instruments having access to a common exchange line but capable of interconnection without passing through the exchange wherein each instrument has a calling circuit including first and second calling indicators for use when the particular instrument being called is disengaged (on hook) and engaged (off hook) respectively. In the part of the four station system shown the call lines 1, 2, 3, 4 interconnect the stations and each station can selectively energize a call line by operation of keys KR1-KR4. The instrument "hook switch" comprises contacts GS1, GS2 which are shown in the operated state for station 1 and cause the intercom lamps L to light. Latching key KE is used to obtain an exchange call and operation of this key will extinguish the intercom lamps. If station 2 is called by depressing key KR2, as shown, a circuit is completed through the call 2 line and the unoperated GS2 contacts to sound buzzer Z in station 2. The current in this circuit flows through R1 in station 1 and drives the emitter of transistor T more negative to extinguish lamp L in station 1. If station 2 was engaged in an exchange call then GS2 would be disconnected from the buzzer circuit and the calling current would be applied to the base of transistor T in station 2 and will cause the associated lamp to be extinguished. The lamp in station 1 will not extinguish but may be dimmed to give an indication that station 2 is engaged. The system described has an additional station connected in parallel with the fourth station. In this case either station can answer or make an intercom call and one station can be used on the intercom whilst the other is making an exchange call.