DCCS hierarchy

The main elements of a typical large scale DCCS are shown in Fig. 7.106. There are five principal levels of hierarchy in most large scale distributed computer control systems. 

RDM Reserve data aquisition module. CM control module ROM Reserve control module. PLC Programmable controller. 

This consists principally of the host which is used to supervise several process areas and may well have six or more data highways linked to it. Hosts are powerful computers with their own operator stations and other peripherals. They are an integral element of modern control systems and only very small processes may be controlled and monitored effectively without a host. However, if not configured properly, a host computer can be overloaded quite easily and will then fail to operate satisfactorily. One serious loading problem for a host is the fast scanning rate required for DDC loops (usually once per second and up to three times per 

This is occupied by a minicomputer or a superminicomputer with considerably more power and peripherals than the host. Whilst the host s function is limited to process control and monitoring, the minicomputer at this level may perform such tasks as maintenance scheduling, production control, longterm historical data recording and acquisition, simulation, and optimisation. 

Level five is generally called the hub. This is the highest level in the system hierarchy and is applicable to organisations which coordinate the operations of a number of plants often distributed over a large geographical area. Examples are oil companies, pulp and paper companies, the water and gas industries, etc. 

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