What is SCADA ?

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What is PLC ?

PLC (Programmable logic controller)

• A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes  are designed to be operated by engineers with perhaps a limited knowledge of computers and computing languages. PLC (Programmable Logic Controller) is also known as Industrial Computer. It is a major component in the industrial automation sector.

• PLCs are similar to computers but whereas computers are optimised for calculation and display tasks, PLCs are optimised for control tasks and the industrial environment.
• Due to its robust construction, exceptional functional features like PID controllers, sequential control, timers and counters, ease of programming, reliable controlling capabilities and ease of hardware usage
• A PLC system has a basic functional components of processor unit, memory unit, power supply unit, input/output interface section, communications interface and the programming device. Show Below the basic Architecture of PLC System.

• The power supply unit is needed to convert the mains a.c. voltage to the low d.c. voltage necessary for the processor and the circuits in the input and output interface modules.
• The processor unit or central processing unit (CPU) is the unit containing the microprocessor and this interprets the input signals and carries out the control actions, according to the program stored in its memory, communicating the decisions as action signals to the outputs.
• The programming device is used to enter the required program into the memory of the processor. The program is developed in the device and then transferred to the memory unit of the PLC.
• The memory unit is where the program is stored that is to be used for the control actions to be exercised by the microprocessor and data stored from the input for processing and for the output for outputting.
• The input and output sections are where the processor receives information from external devices and communicates information to external devices. 
• The communications interface is used to receive and transmit data on communication networks from or to other remote PLCs. It is concerned with such actions as device verification, data acquisition, synchronisation between user applications and connection management.

How to Work PLC

• The Programmable logic controller work in three steps.

1. Input scan: The state of the input is scanned which is connected externally. The inputs include switches, pushbuttons, and proximity sensors, limit switches, pressure switches. Ideally, they are transformers and not relays.
2. Program scan: The loaded program is executed to carry out the function appropriately.
3. Output scan: The input sources have a control over the output ports to energize or de-energize them. The outputs include solenoids, valves, motors, actuator, and pumps. Depending on the model of PLC, these relays can be transistors, triacs or relays