A distributed control system (DCS) is the system that keeps a continuous process steady, such as a refinery, a power plant or a chemical plant. It holds each temperature, pressure and flow at its target value. The industry calls this target value the set point.
A DCS differs from a PLC in what it controls. A PLC controls separate steps, such as a machine that stops and starts for each item. A DCS controls a process that never stops, where liquid or gas keeps moving through pipes and vessels. In such a process, a small change at one point affects all the points after it.
Here is a typical case. A reactor in a speciality chemicals plant in Dahej must stay at 80 °C. If it gets hotter, the product is spoiled, and if it gets much hotter, the pressure can be dangerous. A sensor sends the temperature to the DCS every second. Say the reading rises to 82 °C. The DCS compares 82 °C with the set point of 80 °C and opens the cooling-water valve a little more. The temperature falls to 80.3 °C, and the DCS opens the valve a little less. It does this for hundreds of loops at the same time. Such plants are described under speciality chemicals. Without the DCS, operators would have to turn these valves by hand, and no one could keep up with so many loops.
The word distributed means that the work is spread over many controllers placed around the plant, so that if one fails, the rest continue. The DCS sends every reading to SCADA for the operators and to the process historian for the record. Engineers test new settings first in chemical process simulation software, not on the real plant.
A refinery or a power plant cannot stop for a repair, so a DCS is built with spares. Each important controller has a standby twin that runs the same program, and takes over within a fraction of a second if the first one fails. The network between the controllers is doubled too. A DCS is also sold differently from a PLC. A plant can mix PLCs from several makers, but a DCS is bought as one complete system from one maker: controllers, network, operator screens and engineering software. This is why a plant that has chosen a DCS usually buys the same maker's system again for its next unit.
A DCS is rarely replaced; plants keep theirs for decades. Its connections to the rest of the plant are protected by OT security.