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

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

7. Inside a PLC Central Processor Unit (CPU) MEMORY Circuits Circuits
Communications
Port
Circuits
Circuits
Central
Processor
Unit
(CPU)
Input
Output CR
High
Voltage
High
Voltage
MEMORY
program
data
Isolation
Barrier
Isolation
Barrier
Primary components of virtually all PLC’s, no matter what size are:
Input Circuitry
Output Circuitry
Central Processor
Memory
Communications Port(s)
Power Supply
PLC’s are industrially hardened devices capable of operating in extreme environments
Wide Temp extremes 0-60º C
High humidity
High Vibration
“Dirty” Electrical Power
High “Noise” (electrical) environments
Low Voltage or
AC Power Supply
DC 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. 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
START
Input
Scan
Program
Housekeeping
Output

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 Communication Relationships
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 / MMI Human Machine Interface / Man Machine Interface
I/O Input(s) and/or Output(s)
LSB / MSB Least Significant Bit / Most Significant Bit
MODICON MOdular 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