Chapter 1 Programmable Logic Controller PLC Overview Objectives

Chapter 1 Programmable Logic Controller (PLC) Overview

Objectives • Describe the invention and the development history of programmable logic controller (PLC) systems. • Discuss the benefits of PLCs over electromechanical relay logic systems. • Distinguish between fixed and modular PLC devices. © Goodheart-Willcox Co. , Inc.

Objectives • Discuss different types of Intel and Motorola microprocessors. • Explain different types of random access memory (RAM) devices and their use in PLC systems. • Explain different types of read only memory (ROM) devices. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Objectives • Explain the use of the address decoder circuit in a PLC system. • List the major components of a PLC power supply. • Describe Allen-Bradley programmable controller devices. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Programmable Logic Controller (PLC) • All devices have: – Discrete input ports – Discrete output ports • Larger, more advanced PLCs have: – Variable input ports – Variable output ports © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

PLC Block Diagram © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Fixed PLC • If all units are in one fixed enclosure. • Number of input ports cannot be expanded. • Number of output ports cannot be expanded. (Energy Concepts, Inc. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Modular PLC • If each unit is placed in different racks. • Input modules can be added if needed. • Output modules can be added on modular PLC systems. (Energy Concepts, Inc. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Central Processing Unit (CPU) • • Microprocessor. Unit. Memory. Support chips. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Microprocessor Unit (MPU) • Brain of the central processing unit. • Carries out or executes program instructions. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Arithmetic Logic Unit (ALU) • Carries out math and logic instructions. • Registers: – Hold data for the arithmetic logic unit. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Cache • Instruction cache: – Temporarily holds incoming instructions. • Data cache: – Temporarily holds incoming data. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Bus • Bus width: – How many conductors comprise the address bus and data bus. • Address bus: – Helps the CPU to find physical locations in memory. • Data bus: – Transfers data between CPU components, such as between the I/O and RAM. – The wider the data bus, the more data that can be transferred at one time. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Instruction Set • CISC-based microprocessor • RISC-based microprocessor © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Memory Devices • Two types of internal memory devices for a CPU: – Random access memory (RAM). – Read only memory (ROM). © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Random Access Memory (RAM) • Static RAM (SRAM): – Stores data bits internal flip-flop. • Dynamic RAM (DRAM). – Holds binary data in the form of charged capacitors. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Random Access Memory (RAM) (Cont. ) • Data on the DRAM: – Must be refreshed every few microseconds due to the discharge of the capacitors. • Data on the SRAM: – Remains unchanged until either a new data is written over the existing data, or power to the chip is turned off. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Read Only Memory (ROM) • Masked ROM, or preprogrammed ROM: – Usually programmed by the manufacturer at the factory. – Customer must order several thousand chips from a factory. • Programmable ROM (PROM): – Programmed or “burned” only once by the programmer. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Read Only Memory (ROM) (Cont. ) • Erasable programmable ROM (EPROM), or ultraviolet-erasable programmable ROM (UVEPROM): – Programmed by the programmer and can be erased by placing it under ultraviolet light. • Electrically erasable programmable ROM (EEPROM), or flash ROM: – Programmable ROM that is programmed and erased by injecting an electrical current through it. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Basic Input/Output System (BIOS) • On power up, the firmware, or the software in ROM: – Carries out several instructions that check the input, output, and other peripheral devices connected to the CPU. – Serves as a communication interface between PLC hardware and the PLC program. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Address Decoder Circuit • MPUs use an address decoder to enable: – – ROM. RAM. I/O port devices. Support chips. • When the MPU communicates with a chip, only that chip is connected to the address, data, and control lines of the CPU. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Peripheral Chips • Also called support chips. • Used to enable the microprocessor to communicate with input/output devices. • Supports the MPU with a different task. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Discrete Input Modules • Terminals receive input signals from switches or other input devices. • Converts the input signal to a digital signal prior to sending it to the CPU. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Discrete Input Modules (Cont. ) • Octal transceiver • Full-wave rectifier • Optoisolator © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Discrete Input Modules (Cont. ) • Zener diode: – Semiconductor device that can maintain a steady voltage level. – When phototransistor becomes saturated, ensures that a steady +5 V is applied to the CPU circuit board. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Discrete Output Modules • Octal latch: – Output bit values remain the same until new input bits write over them. – Two octal latches are needed with a 16 output module. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Discrete Output Modules (Cont. ) • Triac: – Solid state relay. – Can be used for switching the output voltage on and off in order to connect the load to AC voltage. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Analog Input/Output Modules • Analog-to-digital converter (ADC): – Converts analog input signals to digital signals. • Digital-to-analog converter (DAC): – Converts digital output signals back to analog signals. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Allen-Bradley Programmable Controllers • Programmable logic controllers (PLCs) are used to implement specific instructions: – – – – Input/output (I/O) port control. Timing. Counting. Data manipulation. Reporting. Arithmetic. Logic. Communication. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Allen-Bradley Programmable Controllers (Cont. ) • Safety programmable controller: – Used for the monitoring and implementing the safety instructions in case of faults in a control system. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Allen-Bradley Programmable Controllers (Cont. ) • Programmable automation controller (PAC): – A personal computer (PC) with a PLC architecture. – Can be used to perform PLC instructions in addition to controlling electric drives and more sophisticated motion and batch control systems. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Allen-Bradley Programmable Logic Controllers (Cont. ) • Four major types of PLCs are categorized according to: – Input/output (I/O) size. – Processor. – Speed. – Memory size. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.