What is a Programmable Logic Controller (PLC) ?

1. What is a Programmable Logic Controller (PLC) ?

2. Agenda • History • PLC Overview • Hardware • Inputs/Outputs • Operation • Programming Software • Communication • What you need to know when specifying a PLC • Glossary

3. History of the PLC • First Programmable Logic Controllers were designed and developed by Modicon as a relay replacer for GM and Landis. • These controllers eliminated the need for rewiring and adding additional hardware for each new configuration of logic. • The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic. • The first PLC, model 084, was invented by Dick Morley in 1969. • The first commercial successful PLC, the 184, was introduced in 1973 which was designed by Michael Greenberg.

4. Inputs Outputs What is a PLC? The Basic Block CPU A PROGRAMMABLE LOGIC CONTROLLER is a solid state control system that continuously monitors the status of devices connected as inputs. Based upon a user written program, stored in memory, it controls the status of devices connected as outputs.

5. Inputs Outputs Remote Outputs Inputs What is a PLC with Distributed I/O? Getting Complex CPU

6. CPU CPU Outputs Inputs Inputs Outputs Outputs Inputs What is a PLC with Distributed Control? Getting Really Complex Remote

7. MEMORY data program Inside a PLC Communications Port Circuits Circuits Central Processor Unit (CPU) Input Output CR High Voltage High Voltage Isolation Barrier Isolation Barrier Low Voltage or DC Power Supply AC Power Supply

8. Inside a PLC • The CPU contains an “Executive” program that tells the PLC how to: • Execute the control Instructions • User’s Program • Communicate with other devices • Other PLCs, Programming devices, I/O devices, etc. • Perform Housekeeping activities • Diagnostics, etc • This program is stored in “nonvolatile” memory • Meaning that the program will not be lost if power is removed

9. What are Inputs? • Switches and Pushbuttons • Sensing Devices • Limit Switches • Photoelectric Sensors • Proximity Sensors • Condition Sensors • Pressure Switches • Level Switches • Temperature Switches • Vacuum Switches • Float Switches • Encoders

10. What are Outputs? • Valves • Motor Starters • Solenoids • Actuators • Control Relays • Horns & Alarms • Stack Lights • Fans • Counter/Totalizer • Pumps • Printers

11. START Housekeeping Input Scan Output Scan Program Scan PLC Operating Cycle • Four Steps in the PLC Operations • Input Scan • Scan the state of the Inputs • Program Scan • Processes the program logic • Output Scan • Energize/de-energize the outputs • Housekeeping • This step includes communications, Internal Diagnostics, etc. • The steps are continually repeated - processed in a loop

12. Programming the PLC Types of Programming Software Languages • Ladder Logic (LL) • IEC Format • Function Block Diagram (FBD) • Structured Text (ST) • Instruction List (IL) • Ladder Diagram (LD) • Sequential Function Chart (SFC) - also known as “Grafcet”

13. Ladder Logic Inputs Outputs Read / Conditional Instructions Write / Control Instructions ( ) | | Start (Rung #1) ( ) | | |/| | | ( ) | | |/| ( ) | | |/| End (Rung #4) | |

14. IEC - Function Block Diagram (FBD)

15. IEC - Structured Text (ST)

16. IEC - Instruction List (IL)

17. IEC - Ladder Diagram (LD)

18. IEC - Sequential Function Chart (SFC)

19. Communication

20. Communication Basics • Protocols • A set of rules for data exchange (format and timing of data) in a communications system • Stack / profile = selected set of protocols for a communication application • Communication Relationships • Master / Slave - request / response & response only • Peer-to-Peer - client / server, publisher / subscriber • Communication Architectures • Star (point-to-point) • Bus, Ring, LAN (multidrop)

21. Communication Protocols • Ethernet • Modbus Plus • Modbus • S908 • LonWorks • Interbus • Profi-bus • DeviceNet • Uni-Telway • CAN-Open • ASi • Seriplex • FIPIO/FIPway • RIO • HART • DIO • ControlNet • ASCII Protocols are similar to languages - Devices must support the same protocol to understand each other

22. What you need to know when specifying a PLC • Quantity, Type and Location of I/O • Number of Inputs and output points • AC or DC voltage • Analog or Discrete • Concentrated or spread out (distributed) • Communication Requirements • Protocol/Network used • Devices to communicate with (HMI, other PLCs, etc) • Speed of Application • Response time required (throughput) of the system • How fast does the process change

23. What you need to know when specifying a PLC • Control Architecture Philosophy • Centralized Control, Distributed Control or combination • Redundancy - CPUs, Power Supplies, etc • Programming Software • IEC vs. 984 • Installed base / what is currently being used • User Logic • Size and complexity of Program • Feedback control used • etc.

24. Alphabet Soup HMI / MMIHuman Machine Interface / Man Machine Interface I/O Input(s) and/or Output(s) LSB / MSB Least Significant Bit / Most Significant Bit MODICONMOdular DIgital CONtoller PID Proportional Integral Derivative (feedback control) RF Radio Frequency RTU Remote Terminal Unit SCADA Supervisory Control And Data Acquisition TCP/IP Transmission Control Protocol / Internet Protocol