copyright © 2002 delmar thomson learning chapter 5 introduction to plc operation
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Copyright © 2002 Delmar Thomson Learning
Chapter 5
Introduction to PLC Operation
Copyright © 2002 Delmar Thomson Learning
ObjectivesExplain 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.
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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.
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PLC Block Diagram
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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.
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Common Industrial Hardware Representing the Binary
Concept
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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.
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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.
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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.
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Information Represented as Combinations of Bits.
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Parts of a 16-Bit Word
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Bytes, Nibbles, and Bits
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16-Point Module’s I/O Points Represented in a
Word
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Physical Input Conditions and the Corresponding Input
Data Word
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8-Point Input Module Represented in a Word
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24-Point I/O Module Represented in Two Words
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Two Words Representing Inputs for a 32-Bit Module
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Data Table Format
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).
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Words Arranged in a Data Table
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Input Data File
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.
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Input Data File (cont’d.)
Identified as an I type data file.Only one input status file allowed
per project.Only has words created for actual
modules in system.
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Output Data File
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.
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Output Data File (cont’d.)
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.
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Output Data File (cont’d.)
Identified as an O type data file.Only one output status file allowed
per project.Only has words created for actual
modules in system.
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Output Status File Correlation To Module
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Modular PLC and Output and Input Status
Tables
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Fixed PLC and Output and Input Status Table
Image courtesy of Allen-Bradley, a Rockwell Automation business
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Fixed I/O PLC Interaction With Input Status File
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I/O Address Structure
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I/O Address Format for SLC 500 Family of PLCs
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RSLogix 500 Software Input Status Table Screen
View
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RSLogix 500 Software Output Status Table Screen
View
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PLC Data Formats
Two 8-bit Unsigned Bytes of Data16-bit Unsigned Integer16-bit Signed IntegerBinary Coded DecimalHexadecimal
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Two 8-bit Unsigned Bytes of Data
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16-bit Unsigned Integer
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16-bit Signed Integer
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Binary Coded Decimal
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Hexadecimal
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PLC Memory Categories
PLC memory is divided into two categories:System memoryApplication memory
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System Memory
Differentiates a PLC from another type of computer device.
Gives PLC its personality.Programmed into PLC at factory.Also called its operating system.
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SLC 500 Operating System
SLC 5/03, 5/04 and 5/05 processors have field upgradable operating systems.Add new features.Add new instructions.Fix problems.
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Application Memory
Stores user program.Stores data associated with user
program:Input status fileOutput status fileTimers and countersNumerical data such as recipesResults of math operations
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Ladder Files
Ladder files contain ladder programs.
Ladder file 2 must be main ladder program.
Ladder files 3 through 255 are subroutines.
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SLC 500 Data Files
Output statusInput statusProcessor
statusBinary or bitsTimers
CountersIntegerFloating pointUser defined
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Default Data Files
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.
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Data File Identification
O: Output Status FileI: Input Status FileS: Processor Status File
Cannot create additional O, I, or S data files.
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Data File Identification
B3 Binary or bit fileT4 TimersC5 CountersR6 ControlN7 IntegerF8 Floating Point
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User Configurable Files
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.
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