Patent GB-1317984-A (GB)
Title: Key telephone systems
IPC: H04M9/00
Filed: 1970-07-24
Abstract: 1317984 Key telephone systems WESTERN ELECTRIC CO Inc 24 July 1970 [25 July 1969] 35887/70 Heading H4K A key telephone system has each of a plurality of lines, from a PBX or exchange, connected to a line module and each substation connected by speech pair and two data pairs to a station module. The line modules and station modules are interconnected through a crosspoint array where each station module can be connected to a plurality of line modules up to the capacity of the substation i.e. the number of line keys provided. In operation each line module will be sequentially scanned every 50 ms. by enabling pulses generated by timing control 50. During the period that a line module 503 is enabled data is exchanged between each line module and the station modules permanently connected with that line module through the respective crosspoint data connections. Nine data time slots are provided during each enabling period, the first five being for the transmission of line data to the station modules and the last three being for sending substation data stored in the station modules to the line module. The sixth slot is for sending a crosspoint enabling pulse to make the necessary speech connection. The remaining period of the scan after all the line modules have been enabled is used to transfer data between the substation and station module. The speech connection is made on the single tip conductor through a PNPN device which is gated on by a pulse signal on both data transmission lines, Fig. 4 (not shown). Each substation comprises a plurality of keys corresponding to the number of lines with an additional hold key, each key is provided with an indicator lamp. Power is simplexed on to the data transmission from the station module which data transmission may employ bipolar return-to-zero pulses to provide clock pulses. The data may be transmitted in a two out of five code. The incomping data from the station module is fed to a shift register which operatesthe correct key indication. Outgoing data emanates the key operated. In the station module substation data is held in flip-flops in a register in the station module and at each scan is compared with data already stored in the station memory. If this comparison shows a difference, action is taken to seize or release a crosspoint or to transmit the charge to the line module. Information is also stored in the register for transmission to the substation to provide the lamp information. In the line module a nine state counter defines the nine data time slots. During the first slot a line advance pulse is transmitted to the station modules associated with that line module so all modules are dealing with the same line and the timing control in each station module is synchronized with the timing control output 50. State of line information is sent by providing a corresponding signal on one of the next four time slots which signal is determined from the information returned from the station modules on the previous scan. This signal is stored in the station module register for eventual transmission to the substation. The sixth pulse provides a partial enablement of the PNPN crosspoints on that line which is turned into a full enablement by a signal transmitted during the same time slot from a particular station module on the other data line. The second enabling signal is sent if an off-hook condition is stored in the station memory and the respective line key is operated as indicated by a line key memory in the module. Enablement of the crosspoint connects the tip leads of the speech path. The last three time slots transmit idle, hold or off-hook information from the station modules to the line modules to set the respective flipflops. The order of transmission is necessary to ensure that on off-hook condition gains the priority it needs. The crosspoint connection is released by comparing the state stored in the station memory with the state stored in the register in a comparison logic circuit which applies a signal to interrupt the hold path of the PNPN crosspoint. Additional lines and substations can be readily added by providing the necessary line, station and crosspoint modules.