What is the difference between PLC and DCS control systems?

01

Different origins.

PLC was born for discrete sequential control. First, a relay control device was used to develop a first generation of programmable controllers. With the development of integrated circuit technology and computer technology, there are now five generations of PLC products.

DCS was born for continuous process control. It is based on microcomputer processing, characterized by decentralized risk control and centralized operation management. It integrates the latest computer technology, communication technology, CRT technology and control technology, namely 4C technology, a new control system. The concept of DCS system is "decentralized control" and "centralized management", and it is an automated control system.

02

The application areas are different.

In terms of process control, PLC is used in aerospace, steel, tobacco, pharmaceuticals, food, metallurgy, mining, water treatment, transportation and other fields.

DCS is mainly used for process control and is applied in automation fields such as power generation, petrochemicals, steel, petrochemicals, and metallurgy. Among them, the main control units of aerospace, thermal power, nuclear power, large petrochemicals, steel, etc. currently need to be controlled by DCS.

03

The environment is different.

PLC is generally used in environments with a small number of points and a large amount of switching.

DCS is used in large-scale continuous control environments at multiple points.

04

Databases and programming languages ​​are different.

PLC has two databases, upper and lower, which are mainly used for sequential control and use logical quantities for judgment and output. The lower computer uses ladder diagram programming, and the upper computer uses SCADA as the control interface of the upper computer data acquisition and monitoring system.

The DCS upper and lower levels have a unified database, the lower level function blocks are programming tools, and the upper level has its own independent visual control system control interface.

05

The control method is different.

When it comes to processing IO signals, PLC manufacturers are accustomed to converting all signals into digital quantities for processing.

When it comes to processing IO signals, DCS manufacturers are accustomed to converting all signals into analog quantities.

06

The redundancy approach is different.

The PLC hardware dual-machine hot standby redundancy mode is a dual-rack, dual-power supply, dual-processor, and dual-network card redundancy mode, which is equivalent to 2 sets of hardware equipment redundancy.

DCS is a dual-machine hot standby redundant architecture consisting of two integrated DPUs.

07

The advantages are different.

PLC small system has cost advantage, so it has advantages in small and medium-sized discrete control work.

DCS is more like a packaged system. The corresponding modules can be used to easily configure the system. From the perspective of horizontal system scalability, DCS has better convenience and protection. DCS cannot be used in SIS/ESD systems.