Patent GB-1264821-A (GB)
IPC: H04M9/00
Filed: 1969-05-01
Abstract: 1,264,821. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. 1 May, 1969 [2 May, 1968], No. 22198/69. Heading H4K.  In a key telephone system in which each of a group of telephone sets can connect to each of a number of line circuits of a group, each set having a push-button and a lamp for each line circuit to which it can be connected, the sets are periodically sampled to detect the operative states of the keys and in response to such detection individual line circuits are selectively interconnected with predetermined ones of said sets and lamp signals are periodically directed to the individual sets. The telephone sets 10-1 to 10-n in the apparatus shown have either 6 or 30 push-buttons of the above type and a hold button allowing them to connect to up to 6 or 30 line circuits LC1 to LCm respectively. Thus any line circuit may have an appearance at a plurality of the sets 10-1 to 10-n and operates corresponding lamps thereat causing them to flash when that line is ringing, winking when the line is on hold or continuously on if the line is busy. These signals are forwarded over a crossconnecting network 40 to a common control where they are stored for transmission to the appropriate telephone sets. To reduce the number of wires connecting each telephone set to the common control, each station set includes a shift register of each group of six keys, the common control has corresponding shift registers and during each cycle of the common control coded signals in each pair of shift registers are exchanged over data wires DC1 to DCn, the bits of each signal being transmitted in serial form. The coded signals include the above referred to lamp signals, ringing signals and signals from the telephone sets such as the identity a of depressed key, a hold signal if a hold key is depressed, on hook signals and other feature signals (if other feature keys are provided). If one or more signal (or signals) requires transmission from a single telephone set a preference gate within that set selects the signal of highest priority. The selected signal is encoded by a diode matrix and entered in the telephone set shift register. During the next cycle of the common control this code is read out in serial form and after transmission entered in the corresponding shift register at the central control, ringing or lamp signals for that set being simultaneously transmitted in the opposite direction. Each cycle of the common control is divided into a number of time slots. During the first two time slots the shift registers are reset and the codes to be transmitted entered therein and during each remaining time slot the codes of one pair of registers are exchanged. Each shift register in the common control includes modulus two adders in its input and between the first and second stages and the output of the third stage is fed back to provide the second input to each adder. In this manner as a received code is shifted into a register it is recorded and if no error occurs the three bits in these stages when the whole code has been received are all zero. If this is verified by a check circuit the bit in the fourth stage is tested and if this is a " 1 " bit the remaining bits specify a line key that has been depressed. These remaining bits are stored in the central control and used to operate a relay tree in a module of the switching network. The module is individual to a shift register and thus to a telephone set and connects the speech wires such as TR1 of that set to a set of speech wiresindividual to the telephone set and individual to the line circuit of the designated line. Thus via the crossconnecting network the telephone set is connected to the selected line circuit either to originate a call or to answer a call incoming to that line. An "A" wire is connected through with the speech wires. If the common control receives a hold signal the speech loop is maintained and earth on the "A" wire is removed. The line circuit responds and connects the incoming line (from the P.B.X. or central office) to a hold bridge.