What is PLC ?

1. PLC known as a programmable logic controller.
2. PLC designed for many arrangements of digital and analog Input and Output, with comfortable temperature ranges, Reduce electrical noise and resistance to vibration and impact.
3. PLC is an example of a real time system.

History of PLC :

1. PLC was introduced in late 1960.
2. First programmable logic controller (PLC) was designed and developed by modicon.
3. Earlier, it was a machine with thousands of electronic parts.
4. Before control logic for manufacturing was mainly composed of relays, cam, timers, drum sequencers and dedicated closed-loop controllers.

PLC Block Diagram :

     1. Power Supply

          ✔ To provides the voltage needed to run the primary PLC components.

     2. I/O Modules

          ✔ To provides signal conversion and isolation between the internal logic level signals inside the PLC and the field's high level signal.

     3. Processor

          ✔ To provides intelligence to command and govern the activities of the entire PLC systems.

     4. Programming Device

          ✔ Used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine.

PLC Programming Scan Cycle :

PLC SCAN CYCLE

     1. Self Test

          ✔ Testing of its own hardware and software for faults.

     2. Input Scan

          ✔ If there are no problems PLC will copy all inputs and copy their values into memory.

     3. Logic Scan/Solve

          ✔ Using inputs, the ladder logic program is solved once and outputs are updated.

    4. Output Scan

         ✔ While solving logic the output values are updated only in memory when ladder scan is done, the outputs will be updated using temporary values in memory.

PLC Programming Languages :

1. Ladder Diagram (LD)
2. Functional Block Diagram (FBD)
3. Sequential Function Chart (SFC)
4. Instruction List (IL)
5. Structured Text (ST)

Advantages of PLC :

1. Reliability
2. Flexibility in programming and reprogramming
3. Cost effective for controlling complex systems
4. Small physical size
5. High speed of operation
6. Reduced space
7. Energy saving

Disadvantages of PLC :

1. Limited design and cost option
2. Fixed circuit operation
3. PLC manufacturers offer only close architectures