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Most engineers have heard the words PLC (Programmable Logic Controllers) and DCS (Distributed Control Systems) but most cannot tell you the difference and why they have a particular technology at their facility.

In this article, we would be going through the differences (and similarities) between the two systems.

The traditional distinction was that PLCs were used for systems with mostly digital control while DCS systems were used in processes with mostly analog controls and PID loops. This stems from the fact that PLCs were created to replace traditional relay control systems while DCS systems were created to replace dedicated PID controllers used in the process industry. This is no longer the case as we are beginning to see more PLC-based systems used for complex analog control while DCS systems can now handle a moderate level of digital control.

The difference between the two systems can be broken down along the following lines

1. Architecture

If you look at a DCS control system, the first thing that strikes you is that you would have multiple controllers integrated via a network into different servers that are then connected to a set of operator workstations. The ABB 800xA DCS system for example has connectivity servers and object servers to store control information related to the process.

Typical DCS Architecture

PLC control systems on the other hand can function without servers or operator workstations. All that is necessary is the acquisition and control of signals according to the program. The data for the process is typically stored in data blocks within the PLC memory.

Typical PLC Control System Architecture

2. Communication

The controllers in a DCS system act as nodes of a larger control system. The variables in the controllers are stored in an integrated database. This means that the variables in one controller are available to another controller by default unless otherwise specified in the logic.

PLCs generally act as autonomous systems and do not make their variables available to other PLCs by default. In recent times more and more PLCs support different communication protocols like Modbus, Ethernet/IP, PROFINET, and a host of others. This means that PLCs can be programmed to share information with each other in a way that mimics the controllers in a DCS.

3. Scope

DCS systems are usually deployed to control entire plants and are designed to process tens to hundreds of thousands of I/O points. PLC systems on the other hand are generally used to control a single process.

Hybrid systems are often the case where PLCs are used to control specific processes while the DCS system performs the overall supervisory function.

A simple example would be a food and beverage plant with dedicated PLCs controlling individual machines or process areas. A DCS can be integrated to perform an overall supervisory function for all the process areas. This gives the plant owner the advanced alarming and supervisory features of the DCS while retaining the modularity of the individual machines. Some DCS manufacturers are beginning to offer mini-DCS systems which provide the functionality of a DCS while applying it to a single machine/process. The Compact HMI offering from ABB is a perfect example of this.

4. Speed of Program Execution

Since PLCs were traditionally used for digital controls, short cycle times (usually 10ms – 150ms) are required for timely control of the automation process.

DCS systems, originally created for analog controls would perform with longer cycle times (usually 1s-2s) which would be unsuitable for PLC-based control systems

This is another area where the lines are getting blurred as DCS vendors are beginning to offer controllers that support short cycle times.

Some manufacturers even offer the same hardware for some of their PLC and DCS control systems. The AC800M controller by ABB can be used both as a controller in the ABB 800xA DCS system or as a standalone PLC. The S7 400 PLC from Siemens is another example of a controller which can work as a standalone PLC or a controller in the Siemens PCS7 DCS.

5. Effect and Frequency of Configuration Changes on the control system

DCS control systems are designed to run continuously as downtimes are extremely costly. This means that the user must be able to perform configuration changes without disturbing the entire process. The decentralization of the controllers ensures that changes in the configuration of a controller program only affect the process area controlled by that controller. The parameters in a DCS system are constantly changed to adapt to the needs of the process. DCS systems are designed to accommodate these configuration changes with minimal disturbance to the process.

PLC systems on the other hand are usually configured once and expected to run with minimum interference by the user. Changes in program configuration usually mean that the controller will be stopped while the program is loaded thereby disturbing the process. Newer PLC systems accommodate online edits for minor changes to plc configuration without stopping the controller.

6. Programming and Configuration Styles

PLCs can be programmed by any of the IEC 61131-3 programming languages. The bias is often towards either Ladder Logic (LD), Function Block Diagrams (FBD), and increasingly Structured Text (ST). This usually means that the programmer usually has to implement control functions from scratch.

Typical PLC Program

DCS systems are often geared towards the use of function blocks or control modules for the implementation of different control functions. These different control modules are then organized into Sequential Function Charts which is easier for implementing complex control features.

Example of DCS Control Program

DCS systems often have a lot of advanced functionality built into them that are not built into traditional PLC systems. The ABB 80xA system has an e-mail feature built into it that can be simply be configured by the user through a few clicks within the DCS environment.

E-mail notifications are not often built into a PLC though some manufacturers like Siemens are beginning to include Special function blocks for E-mails.

The take-away is that the emphasis on PLC systems is programming while DCS is configuration using the existing modules.

The line between PLC and DCS systems are becoming blurred with PLCs now offering features only previously found in DCS systems and DCS system performing functions once thought to be reserved for PLCs. Choosing the right technology for your process would require a careful evaluation of your process needs, the features to be implemented, and a host of other factors. Selection of the right control strategy would minimize cost while providing optimum monitoring and control capabilities for the control system.

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