Patent GB-1401369-A (GB)
Title: Key telephone system
IPC: H04M9/00
Filed: 1973-05-17
Abstract: 1401369 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 17 May 1973 [23 May 1972] 23524/73 Heading H4K In a key telephone system in which each subset has a key for each line to which that sub-set has access, the sub-sets are each connected via a respective speech pair to a switching network giving access to the lines and are individually connected to a central control via data pairs, the central control storing the state of the keys at each sub-set, and the condition of the lines. Exchange of data between the sub-sets and central control uses di-phase coded binary with one cycle of 70 KHz representing a binary 1 and ¢ cycle of 35 KHz representing a binary 0. The central control supervises the sub-sets on a TDM basis. For each group of 5 lines accessible from a sub-set a 1 x 5 crosspoint network is utilized together with an associated group memory in central control and a module in the sub-set. Reed relays or silicon controlled rectifiers may be used as crosspoints. One subset may have access to a maximum of 30 lines in which case six 1 Î 5 crosspoint matrices must be connected to the speech pair of the sub-set, six modules must be supplied in the sub-set circuit and six group memories must be used, each memory comprising 5 memory elements. In operation, a scanner sequentially enables each of the memory elements of the system in turn and within a group of 5 also enables the appropriate station circuit 32. The resultant output from the memory elements triggers the line circuit 40 to which it is connected, the line circuit 40 thus triggered sending data to the transmit control indicating its status. After the status of the five lines of the group has been received, transmit control supplies this information to the station circuits, the station circuit identified by the scanner transmitting this data to the sub-set. The sub-set responds by transmitting data representative of the five corresponding line buttons and the hook switch status to the receive control via the station circuit. A 16 bit signal is used for the sub-set to receive control data message. The status of the five line buttons is coded in 2 out of 4 code, the fifth bit conveys the state of the hook switch, and bits 6, 7, and 8 convey the state of other buttons such as hold recall &c. Those 8 bits are then transmitted as 16 bits in a highly redundant code. On receipt of this data message the five memory elements are rescanned and up dated and if one of the memory elements is active (a connection to one of the lines exists) the corresponding line circuit is triggered by the memory output so that it can receive command codes such as Hold &c. The sub-sets (Figs. 2, 7A, 7B, not shown) have all the features common to most key telephone systems as regards illuminated line buttons, which are lit, flashed or winked according to the line condition. The buttons are non-latching. A calling line may be programmed to send out one of two ringing codes A or B which is conveyed to the sub-sets having access to the line via transmit control and the corresponding station circuits. By arranging the sub-sets to respond to A only, B only, or A or B, only some of the sub-sets may produce a ringing signal by means of a two tone oscillator contained therein, although all sub-sets having access are given a visual indication. A volume control is provided for the audible signal generator. A further feature of the system is that a line may be selected up to 10 seconds before the hand set is lifted. The arrangement may be associated with an intercom system. Conference facilities may be provided but no details are given and in the described embodiment a sub-set has access to only one line at a time. Reference has been directed by the Comptroller to Specification 1,264,821.