Patent GB-1510462-A (GB)
Title: Data transmission systems
IPC: H04L12/433
Filed: 1975-04-15
Abstract: 1510462 Data transmission; loop systems HAWKER SIDDELEY DYNAMICS ENG Ltd 15 April 1975 [4 April 1974] 14967/74 Heading H4P [Also in Division G4] A digital data handling system has a plurality of highway access units each including a shift register, the shift registers being connected in series with each other to form a closed loop, each access unit having an address register for storing its own address and a comparator for comparing the address with address bits in the shift register. Each access unit can send a message to any other access unit over the loop which is unidirectional, the message consisting of a priority code, destination address, source address, parameter number and parameter value (data). An access unit receiving a message accepts it (and removes it) or passes it on, according as the destination address is its address or not. The access units may be linked by a second loop with its own shift registers running in the opposite direction (in case of breakdown), the shorter route normally being taken. In a modification, each section of each loop may be switched to take the place of the corresponding section of the other loop, independently of the other sections. Each loop may use a shielded twisted balanced pair of wires, light guide, ultrasonic path, electromagnetic radiation or any combination of these. An access unit may replace a message received (to be transmitted further) by another message of its own of higher priority, the replaced message being sent in the next available blank space. Three priority levels are used: normal, that for messages with time significance, and emergency. An access unit can test for a loop discontinuity by sending a message addressed to itself. It can also test the source address of each message received: if it finds its own address, this message was not received by the intended destination. A separate "monitor" access unit may be provided for testing the system and diagnosing faults, e.g. by requesting particular access units to send a response, to test the loop or transmit and receive circuits in the access units or A/D converters (see below). The monitor unit may check (for one access unit address at a time) that no particular message passes the monitor unit more than once. The monitor unit may command other access units regarding which loops they use, to avoid faulty sections, and (on request from an access unit) to permit uninterrupted bulk data transfer to another access unit from the requesting unit over loop sections dedicated to this transfer for as long as it takes (the data being sent alone). Each transmission channel of each access unit may be separately powered and isolated from the other, and have a by-pass connected in if a relay ceases to be energized by the power supply (due to power failure). Each access unit communicates with an associated sub-system via A/D and D/A converters, multiplexer and de-multiplexer and buffer storage, as appropriate, a portable access unit which can store and display all messages passing through it and can send test messages, can be provided together with shortable jack-points in the loops to enable it to be connected in where desired. The system may be applied to a conventional ship's autopilot, using five access units for gyro compass, electronics, steering motor, bridge controls and monitoring the system, respectively, this being described in detail.