An optical-fibre ring interface bus for precise electrical measurements

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Abstract
A ring interface bus has been developed for use in computer-controlled apparatus for making precise electrical measurements. To achieve a low uncertainty in the results of such measurements it is necessary to eliminate unintentional electrical disturbances in the quantities being measured. The ring described allows a single microcomputer to control a number of remote digital interfaces, in a simple manner, as though they were connected directly to its input/output (I/O) bus. The interfaces transfer data to and from measurement and control circuits required by the apparatus. The interfaces are grouped into outstations which, along with the controlling microcomputer, are connected in a ring by means of optical fibres which electrically isolate each outstation whilst providing bidirectional data transfer at 200 kbyte s-1. Care has been taken to eliminate or minimize both line-frequency interference and digital interference produced by an outstation. The ring has been applied successfully both to room-temperature metrology and to a cryogenic apparatus incorporating a SQUID detector.

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