Serial digital multiplexing of transducer data for intrinsically safe applications
The galvanic isolation of fibre optic transmission offers a distinct advantage in the design of a telemetry system to meet intrinsic safety requirements. So far the usage of fibre optics in instrumentation systems has been largely confined to the implementation of point to point links due to the difficulties encountered in tapping into an optical fibre. Recent developments in the use of intelligence within instrumentation systems, however, have generally tended to concentrate on the concept of multiplexed signal paths. The aim of the work here has been to realise an intrinsically safe fibre optic multi-drop bus utilising an unbroken fibre as the transmission path. The technique employed involves the modulation of light within the fibre using an acoustic wave to vary the characteristics of a multi- mode fibre resulting in a differential phase modulation of the propagating modes. Since the system is unidirectional the use of two-way protocols is precluded and instead the method adopted uses the technique of allowing collisions of data to occur on the basis that they can be detected and the corrupted data can be subsequently ignored.