Patent GB-1466921-A (GB)
Title: Digital communication system
IPC: G06F13/00
Filed: 1974-03-25
Abstract: 1466921 Selective signalling INTERACTIVE SYSTEMS Inc 25 March 1974 [29 March 1973] 13115/74 Heading G4H In a digital communication system, a central station and a plurality of out-stations are all connected by a communication channel, each out-station having an associated digital address by means of which the central station can address it to receive a word message from the central station, counters in one or more outstations and in the central station being changed in synchronism in dependence on the number of words sent from the central station and detected by the out-stations, the out-station counters controlling word message transfer to the central station. In a first embodiment, a central station including a processor can send over a coaxial cable to a plurality of terminals, a "start of heading" code, one or more address words, a "start of text" code, and one or more data/control words for the terminal or terminals addressed by the address words. Each word has ten bits, including a zero start bit and a stop bit. Each word sent advances a counter in the central station and in each terminal. An "idle" word can also be sent which merely advances the counters. Each complete cycle of the counters, a "reset" word is sent, preceded by two "idle" words, to reset and thus synchronize the counters. The two "idle" words ensure bit-within-word synchronization (so the "reset" word can be properly recognized) since each consists of a zero followed by nine ones and bit-within-word resynchronization will occur at the zero of the second word at the latest. At the addressed terminal(s), the data/control words are fed to a CRT display, printer or audio output, or to control the tuner of a TV set to select a channel in the case of the whole system being a cable TV system. An address store in each terminal is used in recognition of the terminal's address when received from the central station, and is also compared with the counter, equality causing a word to be sent from a register to the central station. This register is loaded via a buffer store (or, in a modification, directly) from a keyboard or a transducer measuring parts produced by a machine associated with the terminal, or the input may be a request for display of a particular industrial production schedule which the central station may then send for display or print-out at the terminal, or the input may be a request for a particular TV channel (see above) which is also stored at the central station for billing purposes. The system uses modulated carriers for transmission in full duplex, or half duplex could be used with a gap for reply after each word sent from the central station. The central station may perform an error check (e.g. parity) on received words, error causing a request for retransmission, or correction using the rest of the message. The counter at the central station enables it to identify the terminal from which a given word received comes. In a second embodiment, the central station can send over a primary coaxial cable to a plurality of satellite stations, each satellite station including a primary counter advanced by each word sent, similarly to the counters in the terminals in the first embodiment. When the count equals the satellite station's address, a secondary coaxial cable respective to the satellite station and linked to a respective set of terminals is connected to the primary coaxial cable to permit one of the terminals to send to the central station. A unidirectional amplifier connected between the primary and secondary cables permits transmission the other way at any time. A secondary counter in the satellite station is advanced each time a "reset" word is sent to the primary counter, and the content of this counter is supplied to the terminals to be used in conjunction with counts in the terminals to select one of them to communicate with the central station. The central station has two counters, to identify the satellite station and its terminal respectively. The satellite station address registers may be modifiable from the central station. In a third embodiment, the central station addresses satellite stations in turn. When a satellite station is addressed, it loads the number of its terminals into a counter which is decremented by each word received from the central station. The counter controls a switchboard to connect all the terminals to the central station in turn, after which the central station addresses the next satellite station. The central station includes a similar (terminal) counter. As a modification of the third embodiment, the terminals may be polled by the satellite station, each including a counter compared with an address register.