Generic Features of DCS

Currently, most large-scale process plants such as oil refineries, petrochemical complexes, and various other processing plants are controlled by microcomputer-based DCS (distributed control systems). these systems generally include the following features

• Cathode ray tube (CRT)-based operator consoles and keyboards, which are used by plant operators or engineers to monitor and control the process
• Controllers, multifunction control modules, and programmable logic controllers (PLCs), which provide the basic control computation or operation 
• A communication network, which is used to transfer the information between control modules and operator consoles across the node on the network 
• I/O (Input/Output) modules, which are used to convert the field instrumentation signals from analog to digital and digital to analog form for controller modules and console displays
• Fieldbus communication links, which are used for communication between remote I/O devices and control modules 
• Historical module, which is used for data storage for pertinent process data or control data and for online data retrieval or archiving 
• Computer interface, which is used for communication between the nodes on the DCS network and the supervisory computer
• Software packages for real-time monitoring, control, reporting, graphics, and trending

The generic arrangement for these components is shown in Figure

This arrangement is a natural outcome for large-scale process control because the parallel operations of the control tasks are required for both continuous and batch process control. Therefore, the distributed control is a natural way of satisfying the parallel nature of the process from a geographical point of view. DCS replaces conventional control systems to perform basic control. The supervisory control functions that in the past were performed by the computer systems described above, plus additional functions such as online information evaluation, are usually still performed by a supervisory computer, which is linked to a DCS.