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Chapter 5. Introduction to PLC Operation. Objectives. Explain what binary information is and how it is used in PLCs. Describe how computer and PLC data are represented. Examine PLC memory and how it is used. Explain how data gets into a PLC.

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Chapter 5

Chapter 5

Introduction to PLC Operation


Objectives
Objectives

  • Explain what binary information is and how it is used in PLCs.

  • Describe how computer and PLC data are represented.

  • Examine PLC memory and how it is used.

  • Explain how data gets into a PLC.

  • Identify what addresses are and how they are used in PLCs.


The plc is a digital computer
The PLC Is a Digital Computer

  • The PLC is a computer similar to a desktop or notebook computer.

  • A PLC is an industrially hardened computer.



Binary concept
Binary Concept

  • Binary is based on two states, on or off.

  • Two-state devices are described as either discrete or digital devices.

  • Discrete or digital devices are simply either on or off.



Binary data representation
Binary Data Representation

  • We communicate to others using groups of letters arranged into words.

  • The PLC uses groups of bits called words.

  • Different bit patterns represent different information.


Chapter 5
Bits

  • Unlike English, computers have only two characters available, 1 or 0.

  • Each 1 or 0 is called a binary digit, or bit.

  • Binary is base or radix 2.

  • A single bit is the smallest unit of computer data.


Plc words
PLC Words

  • One measure of a computer’s capabilities is the length of the data words it can operate on.

  • Current PLCs use 16-bit words.

  • Newer PLCs use 32-bit words.

  • SLC 500 family PLCs are 16-bit computers.





16 point module s i o points represented in a word
16-Point Module’s I/O Points Represented in a Word



8 point input module represented in a word
8-Point Input Module WordRepresented in a Word


24 point i o module represented in two words
24-Point I/O Module WordRepresented in Two Words


Two words representing inputs for a 32 bit module
Two Words Representing WordInputs for a 32-Bit Module


Data table format
Data Table Format Word

  • Words are 16 bits.

  • Bits 0 through bit 15.

  • First word or bit is always 0.

  • SLC 500 data tables can contain up to 256 words (0 to 255).



Input data file
Input Data File Word

  • Each input screw terminal has one memory location to store on or off status.

  • Input data is stored in the input data file, also called the input status file.

  • Input status file holds input status information, which is used to solve ladder program.


Input data file cont d
Input Data File (cont’d.) Word

  • Identified as an I type data file.

  • Only one input status file allowed per project.

  • Only has words created for actual modules in system.


Output data file
Output Data File Word

  • Each output screw terminal has one memory location to store on or off status.

  • Output data is stored in the output data file, also called the output status file.


Output data file cont d
Output Data File (cont’d.) Word

  • Output status file holds output status information to update outputs.

  • The result of solving the ladder program.

  • Output data sent to modules during output update portion of scan.


Output data file cont d1
Output Data File (cont’d.) Word

  • Identified as an O type data file.

  • Only one output status file allowed per project.

  • Only has words created for actual modules in system.


Output status file correlation to module
Output Status File WordCorrelation To Module


Modular plc and output and input status tables
Modular PLC and WordOutput and Input Status Tables


Fixed plc and output and input status table
Fixed PLC and Output Wordand Input Status Table

Image courtesy of Allen-Bradley, a Rockwell Automation business


Fixed i o plc interaction with input status file
Fixed I/O PLC Interaction WordWith Input Status File



I o address format for slc 500 family of plcs
I/O Address Format for WordSLC 500 Family of PLCs


Rslogix 500 software input status table screen view
RSLogix 500 Software WordInput Status Table Screen View


Rslogix 500 software output status table screen view
RSLogix 500 Software WordOutput Status Table Screen View


Plc data formats
PLC Data Formats Word

  • Two 8-bit Unsigned Bytes of Data

  • 16-bit Unsigned Integer

  • 16-bit Signed Integer

  • Binary Coded Decimal

  • Hexadecimal


Two 8 bit unsigned bytes of data
Two 8-bit WordUnsigned Bytes of Data






Plc memory categories
PLC Memory Categories Word

  • PLC memory is divided into two categories:

    • System memory

    • Application memory


System memory
System Memory Word

  • Differentiates a PLC from another type of computer device.

  • Gives PLC its personality.

  • Programmed into PLC at factory.

  • Also called its operating system.


Slc 500 operating system
SLC 500 Operating System Word

  • SLC 5/03, 5/04 and 5/05 processors have field upgradable operating systems.

    • Add new features.

    • Add new instructions.

    • Fix problems.


Application memory
Application Memory Word

  • Stores user program.

  • Stores data associated with user program:

    • Input status file

    • Output status file

    • Timers and counters

    • Numerical data such as recipes

    • Results of math operations


Ladder files
Ladder Files Word

  • Ladder files contain ladder programs.

  • Ladder file 2 must be main ladder program.

  • Ladder files 3 through 255 are subroutines.


Slc 500 data files

Output status Word

Input status

Processor status

Binary or bits

Timers

Counters

Integer

Floating point

User defined

SLC 500 Data Files


Default data files
Default Data Files Word

  • Data files O through 8 are created by the processor with new project.

  • Floating point file is available on SLC 500 modular processors 5/03, 5/04, and 5/05.

  • 5/03 processor must have operating system OS 301 and above.


Data file identification
Data File Identification Word

  • O: Output Status File

  • I: Input Status File

  • S: Processor Status File

  • Cannot create additional O, I, or S data files.


Data file identification1
Data File Identification Word

  • B3 Binary or bit file

  • T4 Timers

  • C5 Counters

  • R6 Control

  • N7 Integer

  • F8 Floating Point


User configurable files
User Configurable Files Word

  • Data files greater than file 8 up to file 255 can be created by the user.

  • These are user-defined files.

  • B, T, C, N, F file types.

  • Each file can contain up to 255 elements with adequate processor memory.