Patent GB-1492542-A (GB)
Title: Telephone intercommunication system incorporating dialling arrangement
IPC: H04M9/00
Filed: 1974-04-03
Abstract: 1492542 Intercommunication system: key telephones GTE INTERNATIONAL Inc 17 Jan 1975 [3 April 1974] 14879/74 Heading H4K In an intercom system where each substation has direct, key controlled access to one or more exchange lines, the latter are each provided with an integrated circuit which is effective to convert digits "dialled" by a subscriber into a format i.e. loop disconnect or V.F., compatible with the exchange connected to the line. Outgoing exchange line call. A subscriber lifts his handset, thus closing switch hook contacts ON-1, ON-2, and actuates an exchange line key, e.g. K.T. Consequently transistor Tl, in the exchange line circuit EXC, conducts. The increased current flow through diode D2 and lead PU permits the subscriber's relay LT to come up whence T1 is turned-off due to the current flow over the H lead and relay P then energizes. A light emitting diode LED in every substation is energized due to the removal of an inhibition on gate GS. The SB portion of a subscriber's key set is enabled via make contact LT4 while the portion SA (whose contacts SA1-SA10 are operated by the same keys as for contacts SB1-SB10) is inhibited via break contact LT3. The subscriber presses the keys corresponding to the wanted party's number, each digit being converted into n out of 4 code by an associated diode encoder and transmitted over a common multiple W1-Z1, AT7-AT10, MA-MC to a set of relays A-D. The operated combination of relays repeats the code to an out-pulser or VF sender DPC. As shown, the latter is an integrated circuit which converts the code into operating signals for an impulsing relay DP and an offnormal relay DON. The multiple AT7-AT10 forms part of an intercom address bus AT1-AT10 which runs through all the substations. The use of repeating relays A-C prevents transients on the multiple from affecting the integrated circuit. Incoming exchange line call.-Ringing on the exchange line actuates one or more call bells BL and any substation accesses the line as he would for an outgoing call (note, contacts DB are normally operated if fault, below). Hold.-A subscriber actuates his key KL whence P falls and PP in the exchange line circuit come up. When the key is restored relay LT falls back. A holding loop is put on the exchange line via R7, PP1. A pulsating signal is applied from a multivibrator MFU to the LED in each station. Any subscriber can now access the exchange line by actuating his key KT. Intercom or interim call.-By lifting his handset a subscriber is immediately connected to a common channel circuit LTC which may be used for conference calls. He selects another substation by actuating the relevant key thereby earthing via contacts SA one of the leads AT1-AT10 of the internal signalling bus. Each lead is strapped selectively in each station (field STF) to a local buzzer BZ which is energized so long as the earth is maintained by the calling subscriber's key being depressed. Internal signalling current on leads AT7-AT10 is limited by resistor R6 such as not to cause or to interfere with the operation of external signalling relays A-D. More than one exchange line may be provided in which case an additional select and hold key KT, KL is necessary. The external signalling for the extra line is effected over another set of terminals AT1-AT6. Fault.-Normally a local power supply is used and relay DB is operative. If DB falls due to power failure, one subscriber is still able to use the exchange line for incoming or outgoing calls, operating power being derived from the exchange line (see the dotted connections for circuit details of this fall-back subscriber). The exchange line circuitry is preferably accommodated in a plinth unit for the fall-back subscriber's subset and direct connections from his diode encoder to the outpulsing circuit are thus possible. Each exchange line is associated with a different fall-back subscriber.