Time-critical communication systems

This article is concerned with shared-medium communication systems, primarily private systems for process-control plant and/or discrete parts manufacture. The particular systems in question are those two layers at the bottom of the CIM hierarchy, namely the `field level´ and `station level´. These systems are often dubbed `real time´, or `time critical´ systems. The distinguishing features between the constraints of physically dynamic systems and the OSI solutions being extensively promoted in office automation are discussed. The primary difference is that, in control systems, considerations for timeliness are crucial while other considerations such as efficiency and throughput are secondary. This `temporal factor´ is probably generic across many fields of engineering, from aircraft flight systems to more prosaic matters such as processing urban effluent. The article draws on the contribution being made in two standardisation efforts, namely `time critical communication architecture´ (TCCA) in ISO and `Field Bus´ in IEC