Programmable Logic Controller (PLC) Overview

2. 1 Programmable Logic Controller (PLC) Overview

3. 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.

4. 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.

5. 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.

6. Programmable Logic Controller (PLC) • All devices have: • Discrete input ports • Discrete output ports • Larger, more advanced PLCs have: • Variable input ports • Variable output ports

7. PLC Block Diagram

8. 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.)

9. 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.)

10. Central Processing Unit (CPU) • Microprocessor. • Unit. • Memory. • Support chips.

11. Microprocessor Unit (MPU) • Brain of the central processing unit. • Carries out or executes program instructions.

12. Arithmetic Logic Unit (ALU) • Carries out math and logic instructions. • Registers: • Hold data for the arithmetic logic unit.

13. Cache • Instruction cache: • Temporarily holds incoming instructions. • Data cache: • Temporarily holds incoming data.

14. 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.

15. Instruction Set • CISC-based microprocessor • RISC-based microprocessor

16. Memory Devices • Two types of internal memory devices for a CPU: • Random access memory (RAM). • Read only memory (ROM).

17. Random Access Memory (RAM) • Static RAM (SRAM): • Stores data bits in its internal flip-flop. • Dynamic RAM (DRAM). • Holds binary data in the form of charged capacitors.

18. 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.

19. 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.

20. 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.

21. 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.

22. Address Decoder Circuit • MPUs use an address decoderto 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.

23. Peripheral Chips • Also called support chips. • Used to enable the microprocessor to communicate with input/output devices. • Supports the MPU with a different task.

24. 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.

25. Discrete Input Modules (Cont.) • Octal transceiver • Full-wave rectifier • Optoisolator

26. 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.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. Allen-Bradley Programmable Logic Controllers (Cont.) • Four major types of PLCs are categorized according to: • Input/output (I/O) size. • Processor. • Speed. • Memory size.