The distributed control system continuously interacts with the process in the process control application, so as to get the instructions from the operator. It also helps the operator to manually control the valve's variable set points and switches. Human machine interface (HMI), panel and trend display can effectively monitor the industrial process.

The controller is the supervisory controller of all distributed processing controllers. The controller performs the control algorithm and configuration of various devices. The network communication between processing and engineering PC can be realized by simplex or redundant configuration.

Local controller or distributed control unit

It can be close to field devices (sensors and actuators) or connected to specific locations of these field devices through communication links. It receives instructions from the engineer station, such as set points and other parameters, and directly controls the field equipment.

It can sense and control analog and digital input / output through analog and digital I / O modules. These modules can be expanded according to the number of inputs and outputs. It collects information from discrete field devices and sends it to operation and engineering stations.

In the figure above, AC 700f and AC 800f controllers act as communication interfaces between field devices and engineer stations. In most cases, these are local controls of field instruments.

Operation station or HMI

These operating stations are of different types. For example, some operating stations (PCS) are only used for monitoring parameters, some for trend display, and some for data recording and alarm requirements. These can also be configured to have control capabilities.

Communication media and protocols