DIFFERENCE BETWEEN PLC, DCS AND SCADA SYSTEMS

Nowadays it is commonly seen that students are often confused between the terminologies of PLC, DCS and SCADA systems and consider them as the alternatives of each other. However all of these systems are developed to cater different needs. Since the PLC was integrated with Analog I/O it crosses the boundary of being just digital and crosses to the realm of DCS in handling Analogs, Bus Systems, Distributed I/O and etc. Also, since the DCS now handles logics of Digital I/O it also crossed the boundary to the realm of PLC.

The detail analysis of both of these systems is discussed below:

PROGRAMMABLE LOGIC CONTROLLERS:

 PLC is a specialized processor/ computer with all the hardware and software to do a specific automation task, say running a plant/factory. This means that all the Field/Real world Inputs are processed by the PLC (using Input interface hardware ) and the same then issues commands to various devices and actuators(via Output interface). This has some Pros and Cons.

Pros:-

1.      As a single processor is working it can easily access all the data / inputs and use it as it requires.

2.      It has a simple architecture and is easier to implement and maintain.

3.      It has reduced hardware and reduced cost.

Cons:-

1.      As all the functions are done by the same processor it increases the loading on the controller. For large systems the processing power required can be too high.

2.      There is no physical segregation for critical application such as protections.

3.      Failure of the controllers (though redundant controllers can be provided) will lead to complete shutdown of the control system. For some applications a true standalone system is required.

4.      If the various areas are far away the cabling cost of inputs and outputs to the PLC can increase too much and for long distance it can be impracticable.

If a standalone system is made for such application then it will be difficult to integrate them in a common SCADA/ HMI/ interface and the spare for maintaining different systems will increase cost.

DISTRIBUTED CONTROL SYSTEM:

DCS can solve these problems. In a DCS the entire logic and hardware is distribute or broken into different microprocessors or Functional groups (FGs) for ease the implementation and provide segregation.

These separate Functional groups (FGs) are linked to the same HMI/SCADA/interface. The entire control of the plant is broken in small groups with each groups having something similar. 

For example:-

The AC/ventilation system can be separated from the production part as it is not related functionally.

If separate FGs need to communicate some signals, the same is done through Hardwire or Network/protocol such as OPC, Modbus, profibus etc. As most of the processing is already done in the other FG the cabling is also reduced.

Protection system and critical systems can be made into a separate FG and this processor having low input and controls can work with low loading and higher speed.

As an engineer working in Control systems, I too find it very confusing when people use the term PLC and DCS interchangeable. This is because DCS are an advanced or a larger implementation of the PLCs. A number of vendors call the same processor as PLC or DCS depending on the execution or architecture used for the application as illustrated.

Now some important aspects of SCADA are discussed below:

SCADA: Supervisory Control and Data Acquisition

1.      DCS is process oriented while SCADA is data acquisition oriented.

2.      DCS is process state driven while SCADA is event driven.

3.      DCS is commonly used to handle operations on a single locale, while SCADA is preferred for applications that are spread over a wide geographic location.

4.      DCS operator stations are always connected to its I/O, while SCADA is expected to operate despite failure of field communications.