1. Prepared by: Engr. Qurban Ali Memon 11ES (SEC- 1 & 2)
Lecture #1
Introduction to PLC
Prepared by: Engr. Qurban Ali Memon
11ES (SEC- 1 & 2)

2. What is PLC PLC is an abbreviation of Programmable Logic Controller.
The main function of PLC only on and off.
A PLC is used for multi purpose while Microcontroller is used for single purpose.
The first PLCs was been introduced 1968’s and became popular in the 1980’s.
A PLC is basically type of box with number of inputs and number of outputs.
For example: When a switch S is pressed then light turns on.
S Light
PLC or Programmable Controller is a digital type of output electronic device.
PLCs are used in many industries and machines.
The PLC is designed for multiple inputs and output 

3. Box Type
Modular

4. Block diagram of PLC System

5. Parts of PLC system. Power flows when the input device is off
1) Inputs Module: Input devices consist of digital or analog switches.  Initial condition of digital inputs that is either on or off
Power flows when the input device is on.
Power flows when the input device is off
Example: Switches
Pushbuttons
Sensing Devices
Limit Switches
Condition Sensors
Pressure Switches
Level Switches

6. 2) Output Module :
Output devices also consist of digital or analog switches. 
Initial condition of digital outputs either on or off
Examples:  
Valves
Motor Starters
Stack Lights
Fans
Pumps
Printers

7. 3) CPU: The Central Processing Unit (CPU) is the brain of a PLC controller. In CPU we can store programmed. CPU itself is usually one of the micro controllers. 8-bit micro controllers such as 8051, and now these are 16- and 32-bit micro controllers.
4) Programming a PLC controller : PLC controller can be reprogrammed through a computer (usual way), but that each PLC controller can programmed through a computer if you have the software needed for programming.
Simetic s7-300, Logo4 and Melsec Medoc
5) Power supply: Most PLC controllers work either at 24 VDC or 220 VAC

8. 6) Extension lines: Every PLC controller has a limited number of inputs/outputs lines.
1. Simetic s Logo soft4
16 inputs & 16 outputs inputs & 24 outputs
I124.0 to I I1 to I24
Q124.0 to Q Q1 to Q24
3. Melsec Medoc
8 inputs and 6 outputs
X0 to X7 & Y0 to Y5

9. Ladder Diagram
A vertical and horizontal line is connected together that is called ladder diagram.
Ladder logic is the main programming method used for PLCs.
Power always flows from left to right
Output devices are in the ON or off state if power flows through them.

10. Types of Sequential diagram
1) Electrical or Electronic diagram:
Any circuit is connected with a wires that is called Electrical or Electronic diagram
Let's consider In the below circuit, the coil will be energized when there is a closed loop between the + and - terminals of the battery

11. 2. Ladder Diagram
A vertical and horizontal line is connected together that is called ladder diagram..
coil S1 S2
End

12. Basic Instructions of PLC
Load:
The load (LD) instruction is a normally open contact. It produces a HIGH output when its input is HIGH or ‘1’. The symbol for a load instruction is shown below.
A Load (contact) symbol

13. A Load Not (normally closed contact) symbol
2. Load Bar :
The Load Bar instruction is a normally closed contact. It produces a HIGH output when its input is LOW or ‘0’. The symbol for a load bar instruction is shown below.
A Load Not (normally closed contact) symbol

14. An OUT (coil) symbol 3. Out :
The Out instruction is sometimes also called an Output Energize instruction. The output instruction is like a relay coil. Its symbol looks as shown below.
An OUT (coil) symbol

15. An out bar (normally closed coil) symbol
The Out bar instruction is sometimes also called an Out Not instruction.
The Out bar instruction is like a normally closed relay coil. Its symbol looks like that shown below.
An out bar (normally closed coil) symbol

16. Example1: Draw a ladder diagram to turn on the Green indicator light (Q1) when the pushbuttons I1 and I3 are pressed as indicated in the below electrical circuit. Q1 I1 I3

17. Example2: Draw ladder diagrams for the electrical circuits given below:Q3 I1 I3

18. Steps are required to creating a Sequential diagram:
There are some steps required to creating a ladder diagram.
Define the process operation and list the step by step sequence operation.
Define the list of input and output device.
Assign the corresponding PLC numbers to the input and output devices.
Draw the PLC scheme. (Ladder diagram).
Wire the PLC system to simulator and check its operation.
Transfer the program from PC to PLC.
Make the modification as required.

19. Exercise Examples:
Exp1: The motor is to be start, if first input switch is turned ON and second input switch is normally off.
The motor is to be stop, if second input switch is turned ON.
Exp2 : A lamp is to be turn ON. If first input is already ON and either Second or third or both Second and third inputs turn ON.

20. Exp3: A lamp in the corridor can be switched ON by using three switches S1, S2 and S3 as per the condition below:
The lamp will be ON only if any two of the three switches are pressed.
Exp4: To Design a ladder diagram of alarm system for industrial process having four hazard inputs, which could satisfy the condition given below.
i) If any one input is ON, nothing happens.
ii) If any two inputs are ON, a red light goes ON.
iii) If any three inputs are ON, an alarm siren sounds.
iv) If all four inputs are ON, fire department is notified.