Distributed control system including a compact easily-extensible and serviceable field controller

The invention relates generally to the field of digital data processing systems, and more specifically to distributed monitoring and control systems which may be used in, for example, process control arrangements for controlling large industrial operations such as manufacturing plants or chemical processing plants, environmental monitoring control arrangements for controlling heating, air conditioning, ventilation, illumination, and other controllable environmental factors in industrial, commercial and home environments. The invention particularly provides a controller (called herein a "field controller") which provides a compact, computationally-powerful package which is convenient to install and service in a wide variety of environments. A field controller for use in a distributed control system including an area controller and at least one field controller. The field controller manages at least one controlled device in an industrial process operation. The field controller comprises a processor module segment through which it can control a selected number of devices, and it may also include one or more expansion module segments to enable it to control a larger number of controlled devices. The processor module segment includes a processor module and at least one local interface module for interfacing to a controlled device, and the expansion module segment includes interface modules for interfacing to other controlled devices. In the processor module segment, the processor module and said local interface module are interconnected by a bus segment, which is also connected to an upstream off-module connector. Each expansion module segment includes at least one interface module, and also includes a downstream off-module connector and an upstream off-module connector, which are interconnected by a bus segment. The downstream off-module connector of each expansion module segment is adapted to mate with the upstream off-module connector of the processor module segment and of other expansion module segment, so as to facilitate the interconnection of the processor module segment and a sequence of expansion module segments by establishing a unitary multi-drop bus comprising the processor module's bus segment and the bus segments of expansion module in the sequence. The precessor module controls each controlled device through the respective local interface module or expansion interface module connected thereto. The processor module segment and each expansion modul