Abstract: A communication switching system of PABX uses a number of time division multiplexed send and receive highway pairs each servicing a plurality of peripheral devices or ports providing two-way communication with the outside world by means of associated telephones, trunk line interfaces, or similar apparatus. All highway pairs are multiplexed in accordance with the same time division format and the maximum number of two-way peripheral devices serviced by each highway pair is dependent on the number of time slots of the format assigned for audio communication. Each peripheral device transmits on a given audio time slot of its send highway. Switching to establish communication paths between peripheral devices is achieved in part by receiving means in each peripheral device responsive to command signals transmitted on command time slots on the associated receive highway instructing each peripheral device on which audio time slots of its receive highway to receive.

Abstract: A decoder is disclosed for use in a communication system wherein bits of information are conveyed by the transmission, on a communication line, of code tones of distinctive frequencies each corresponding to a respective bit of information. The decoder divides an incoming signal into repetitive intervals having durations dependent on the period of the signal and provides digital measurements of the length of the interval by counting clock pulses during each interval. The digital measurements are then processed to test for persistence of the signal at a stable frequency for a predetermined time and for matching of said stabilized frequency to a recognized code tone frequency. More specifically, such decoder is disclosed as part of a larger decoder for use in, for example, a telephone communication system wherein dial numbers or other characters are communicated by way of simultaneously transmitted code tone pairs.