Instrumentation surveillance from central point - using single coaxial cable bus and HF frequency shift signalling

摘要

Both the central point and the distant instruments are associated with standardised units using binary h.f. data. Each instrument station uses two distinct frequencies allotted to it for representing a logic '1' and logic '0', e.g. 2,225 KHz and 2,025 KHz respectively, the central station using the frequencies 1,270 and 1,070 KHz respectively. Active filters are used to produce 40 db attenuation neighbouring frequencies. Each instrument has its own address and, on being called, automatically sends out a status word and relevant extra data to the central station. The uniform structure of the interface units, the easy check on performance, the ability to extend the system from any point, and the min. wiring, are some of the claimed advantages.