Lecture #1

Prepared by: Engr. Qurban Ali Memon11ES (SEC- 1 & 2)

Introduction to PLC

What is PLCPLC 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 

Box Type Modular

Block diagram of PLC System

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

Parts of PLC system.

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

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 Medoc5) Power supply: Most PLC controllers work

either at 24 VDC or 220 VAC

6) Extension lines: Every PLC controller has a limited number of inputs/outputs lines.

1. Simetic s7-300 2. Logo soft4 16 inputs & 16 outputs 24 inputs & 24

outputs I124.0 to I124.15 I1 to I24 Q124.0 to Q124.15 Q1 to Q243. Melsec Medoc 8 inputs and 6 outputs X0 to X7 & Y0 to Y5

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.

Types of Sequential diagram1) Electrical or Electronic diagram:Any circuit is connected with a wires that is called Electrical or

Electronic diagramLet's consider In the below circuit, the coil will be energized when

there is a closed loop between the + and - terminals of the battery

2. Ladder DiagramA vertical and horizontal line is connected

together that is called ladder diagram ..

S1 S2coil

End

Basic Instructions of PLC

1. 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

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

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

4. Out bar: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

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.

I1 I3 Q1

Example2: Draw ladder diagrams for the electrical circuits given below:

I1

I3

Q3

Steps are required to creating a Sequential diagram:There are some steps required to creating a ladder

diagram.1. Define the process operation and list the step by

step sequence operation.2. Define the list of input and output device.3. Assign the corresponding PLC numbers to the

input and output devices.4. Draw the PLC scheme. (Ladder diagram).5. Wire the PLC system to simulator and check its

operation.6. Transfer the program from PC to PLC. 7. Make the modification as required.

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.

Exercise Examples:

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.