In this article, we will understand the basic difference between PLC and SCADA systems with examples in industrial automation.
We know a computer which consists of CPU, Monitor, Keyboard, and Mouse. In industrial automation, we know that PLC and SCADA are used for controlling a system. Well, both are different from each other.
In fact, you can say both are different elements first of all and so, is bound to have comparisons in terms of understanding.
If you compare it with computer components, then a PLC is a CPU and a SCADA is the monitor. Suppose you have a system to control a conveyor system. You have a PLC to write the logic for controlling the conveyor. You have a SCADA to display the conveyor system graphics in it.
So, the main difference is that a PLC is used for writing the control logic whereas a SCADA is used for displaying the graphics and providing set parameter access to the operator so that he can operate and control the variables written in the PLC.
A PLC cannot show the graphics and a SCADA cannot be used for control logic.
PLC and SCADA Example
Let us take this example further. You have three variables named button, conveyor, and speed defined in the PLC. The logic is written as – Toggle the button which turns on or off the conveyor and when on, the set speed should move in the speed variable for varying the speed.
Now, you will map all these three variables in SCADA and show its graphics. When the PLC and SCADA are communicating, and the operator toggles the button from the SCADA screen, the button variable written in the logic will be activated accordingly and turn on or off the conveyor output.
The operator will enter the speed in the SCADA screen and the speed variable in the PLC will be affected accordingly to vary the speed of the conveyor.
PLC is the hardware part where inputs and outputs are connected to it and logic is written inside using languages like a ladder, functional block diagram, structured text, instruction list, etc.
In SCADA, you have to design screens where you have to map the variables of the PLC in it. Using communication protocol, the PLC and SCADA communicate with each other and the control part then works tandem.
The main purpose of SCADA is that it records and manages data and reports, shows trends, shows alarms and basically, shows all the functions happening inside a PLC on its screens.
This helps the operator in troubleshooting and identifying what is happening inside the PLC.
PLC versus SCADA
One point must be noted that a PLC can work in a system without SCADA. There are small applications where the operator does not require graphics as he just has only start or stop control in it.
But, a SCADA cannot work without the PLC. What is the sense when there is no controller and with what will SCADA communicate with then; to show graphics or record data.
So, PLC and SCADA differ in control philosophy and its main purpose of working.
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