The Programmable Logic Controller Basic Principle of a

The Programmable Logic Controller

Basic Principle of a PLC Program Input Data Processing Output Data

Components of a PLC • • A PLC is made up of two basic sections • Central Processing Unit (CPU) • Input/Output (I/O) Interface CPU consists of three main parts • Processor • Memory • Power Supply

PLC System Components PLC - Program Input Module Sensors Central Processing Unit Output Module Actuators

Functions of a CPU • • • Reads input data from various sensing devices Executes the stored user program from memory Sends appropriate output commands to control devices

Types of Memory

Input and Output System • • Provides the physical connection between the outside world and the CPU Discrete inputs/outputs are the most commonly used

Input Devices • • Selector switches Push buttons Limit switches Circuit breakers Level switches Relay contacts Motor starter contacts

Input Identification • • Input Word IW 0 - IW 2 Input Bit I 0. 0, I 0. 1, …, I 0. 7 Input Bit I 1. 0, I 1. 1, … I 1. 7 Input Bit I 2. 0, I 2. 1, …, I 2. 7

Output Devices • • Alarms Control Relays Solenoids Lights Horns Valves Motor Starters

Output Identification • • • Output Word OW 0 - OW 1 Output Bit O 0. 0, O 0. 1, …, O 0. 7 Output Bit O 1. 0, O 1. 1, … O 1. 7

Flag Identification • • Flags are Internal Relays Flag Word FW 0 - FW 255 Flag Bit F 0. 0, F 0. 1, …, F 0. 15 Flag Bit F 255. 0, F 255. 1, … F 255. 15

Timer Identification • • Timers are Internal PLC Timers Timer T 0 - T 15

System Unit of a PLC Programming Device PLC Acknowledgements (Sensors) Input Modules (Input Cards) Control Elements (Switches) Sensoric Software Program Memory Central Control Unit (Single or Multibit Processor) Processoric Output Modules (Output Cards) Final Control Elements (Relays, Solenoids, Indicators) Actoric

Selecting a PLC • • Number of Inputs and Outputs Types of Software Languages Types of Operator Interface Mathematical Calculations Analog Input and Output Communication with other PLCs and Computers Ease of Maintenance Ease of Programming