2. Programmable Logic Controller (PLC) Overview1
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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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
© Goodheart-Willcox Co., Inc.

7. PLC Block Diagram
© Goodheart-Willcox Co., Inc.

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.)
© Goodheart-Willcox Co., 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.)
© Goodheart-Willcox Co., Inc.

10. Central Processing Unit (CPU)
Microprocessor.
Unit.
Memory.
Support chips.
© Goodheart-Willcox Co., Inc.

11. Microprocessor Unit (MPU)
Brain of the central processing unit.
Carries out or executes program instructions.
© Goodheart-Willcox Co., Inc.

12. Arithmetic Logic Unit (ALU)
Carries out math and logic instructions.
Registers:
Hold data for the arithmetic logic unit.
© Goodheart-Willcox Co., Inc.

13. Cache Instruction cache: Data cache:
Temporarily holds incoming instructions.
Data cache:
Temporarily holds incoming data.
© Goodheart-Willcox Co., Inc.

14. Bus Bus width: Address bus: Data bus:
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.

15. Instruction Set CISC-based microprocessor RISC-based microprocessor
© Goodheart-Willcox Co., Inc.

16. Memory Devices Two types of internal memory devices for a CPU:
Random access memory (RAM).
Read only memory (ROM).
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

22. 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.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

25. Discrete Input Modules (Cont.)
Octal transceiver
Full-wave rectifier
Optoisolator
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

34. Glossary
Address bus: A group of conductors or pathways that help the CPU find the physical locations of memory.
Address decoder: A circuit that will enable only one support chip at a time. This allows the microprocessor to use the data bus to transfer data. This takes place to and from the enabled chip that the support chip connects to the data bus.
© Goodheart-Willcox Co., Inc.

35. Glossary
Analog-to-digital converter (ADC): A chip that converts a variable, or analog, electrical signal (i.e., voltage or current) to a binary value.
Arithmetic logic unit (ALU): The internal part of a microprocessor that carries out the arithmetic and logic instructions and generates the result.
© Goodheart-Willcox Co., Inc.

36. Glossary
Basic input/output system (BIOS): Holds software called power-on self-test (POST). When a PLC is energized, POST software initializes the PLC system and checks the input/output support devices. The software in the BIOS also serves as a communication interface between PLC hardware and the PLC program.
Bus: A pathway on a circuit board on which information can flow from component to component.
© Goodheart-Willcox Co., Inc.

37. Glossary
Cache: The section of RAM that holds the last 256 kilobytes (kB) or 512 kB of data transferred to and from the microprocessor.
Central processing unit (CPU): Includes the microprocessor, memory, and support chip in a PLC system.
CISC-based microprocessor: A microprocessor that uses several math instructions to carry out complex commands.
© Goodheart-Willcox Co., Inc.

38. Glossary
Control unit: Part of a microprocessor that coordinates and controls all activities within the microprocessor.
Data bus: A group conductors or pathways used to transfer data to and from support devices.
Data cache: An area of memory in a microprocessor that temporarily holds incoming data.
© Goodheart-Willcox Co., Inc.

39. Glossary
Decode unit: Part of a microprocessor that translates instructions into a format that the microprocessor can understand.
Digital-to-analog converter (DAC): A chip that converts binary data to a variable, or analog, electrical signal (i.e., voltage or current).
© Goodheart-Willcox Co., Inc.

40. Glossary
Discrete input ports: Ports on a PLC input module that can receive fixed signals (on or off).
Discrete output ports: Ports on a PLC output module that are either energized (on) or de-energized (off).
Dynamic RAM (DRAM): Holds binary data in the form of charged capacitors. DRAM must be written to (refreshed) every few microseconds.
© Goodheart-Willcox Co., Inc.

41. Glossary
Electrically erasable programmable ROM (EEPROM): Programmed by injecting high voltage levels (e.g., +12 V), and erased by injecting another high voltage level (e.g., -12 V).
Erasable programmable ROM (EPROM): A type of memory that is programmed by injecting a higher than normal voltage level (e.g., 12.5 V or 25 V). An EPROM program is erased when it is placed under ultraviolet light for fifteen minutes.
© Goodheart-Willcox Co., Inc.

42. Glossary Firmware: The software that resides in ROM.
Fixed PLC: A single enclosure that holds the CPU, input port, and output port modules.
Flip-flop: Sequential digital device that generates a different output for every input on the next clock pulse. Usually four transistors are used to build one flip-flop.
© Goodheart-Willcox Co., Inc.

43. Glossary
Full-wave rectifier: A circuit that converts AC (alternating current) voltage to DC (direct current).
Instruction cache: An area of memory in a microprocessor that temporarily holds incoming instructions.
Instruction set: A specific set of instructions or commands for operations that the MPU can carry out.
© Goodheart-Willcox Co., Inc.

44. Glossary
Masked ROM: ROM that is programmed by the manufacturer prior to shipment to the customer.
Microprocessor unit (MPU): The main chip in the PLC system that transfers and receives data from I/O ports and carries out the math and logic operations.
© Goodheart-Willcox Co., Inc.

45. Glossary
Modular PLC: PLC with different racks (slots) reserved to hold the CPU module (must be in slot zero), input port modules (e.g., slot one), and output port modules (e.g., slot two).
Octal latch: A memory buffer made of SRAM, or flip-flop circuitry.
Octal transceiver: A circuit in which eight bits are transferred to and from support devices.
© Goodheart-Willcox Co., Inc.

46. Glossary
Optoisolator: A chip used to optically isolate the small voltage (+5 VDC) digital control section of a PLC system from the higher voltage (24 VDC, 120 VAC, 240 VAC) input/output section of a PLC system.
Programmable logic controller (PLC): A microprocessor-based device that can be used to control industrial systems such as electric motors, conveyors, and robots.
© Goodheart-Willcox Co., Inc.

47. Glossary
Programmable ROM (PROM): Memory programmed once by using the PROM programmer device (called PROM burner).
Random access memory (RAM): Memory that can be written to and read from.
Read only memory (ROM): Memory that can only be read.
© Goodheart-Willcox Co., Inc.

48. Glossary
Registers: An area in the microprocessor that holds data for the arithmetic logic unit.
Relay logic devices: Older control panels that have external inputs, outputs, counters, timers, and other circuits wired on it.
RISC-based microprocessor: A microprocessor that uses only one instruction for a complex command.
© Goodheart-Willcox Co., Inc.

49. Glossary
Software: A program that is carried out (executed) on the PLC system.
Static RAM (SRAM): Memory that uses flip-flops to hold data. Data on SRAM remains the same as long as it is not overwritten by newer data and the power to the memory device is on.
© Goodheart-Willcox Co., Inc.

50. Glossary
Triac: A semiconductor device that allows current to pass through it when it has a small “trigger” current signal applied to its gate.
Variable input ports: Ports on a PLC input module that can receive analog signals, such variable voltage (e.g., 0 V to 10 V) or variable current (e.g., 0 A to 1 A), and convert it to a discrete or binary signal.
© Goodheart-Willcox Co., Inc.

51. Glossary
Variable output ports: Ports on a PLC output module that convert a binary data to an analog signal such as variable voltage (e.g., 0 V to 10 V) or variable current (e.g., 0 A to 1 A).
Zener diode: A unidirectional semiconductor device that usually operates in its reverse-biased region as a voltage regulator.
© Goodheart-Willcox Co., Inc.