AN OPTICAL-FIBER RING INTERFACE BUS FOR PRECISE ELECTRICAL MEASUREMENTS

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 sources of these disturbances may be in other parts of the measurement apparatus or in its surroundings. Many commercial digitally controlled instruments act as sources or conduits for these disturbances. The ring described here was developed to eliminate such interactions without sacrificing the advantages of computer control of the measurements. The ring 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 appartus incorporating a SQUID detector.