Copyright © 2002 Delmar Thomson Learning Chapter 5 Introduction to PLC Operation

Copyright © 2002 Delmar Thomson Learning

Chapter 5

Introduction to PLC Operation


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.


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.


PLC Block Diagram


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.


Common Industrial Hardware Representing the Binary

Concept


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.


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

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.


Information Represented as Combinations of Bits.


Parts of a 16-Bit Word


Bytes, Nibbles, and Bits


16-Point Module’s I/O Points Represented in a

Word


Physical Input Conditions and the Corresponding Input

Data Word


8-Point Input Module Represented in a Word


24-Point I/O Module Represented in Two Words


Two Words Representing Inputs for a 32-Bit Module


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).


Words Arranged in a Data Table


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.


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.


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.


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.


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.


Output Status File Correlation To Module


Modular PLC and Output and Input Status

Tables


Fixed PLC and Output and Input Status Table

Image courtesy of Allen-Bradley, a Rockwell Automation business


Fixed I/O PLC Interaction With Input Status File


I/O Address Structure


I/O Address Format for SLC 500 Family of PLCs


RSLogix 500 Software Input Status Table Screen

View


RSLogix 500 Software Output Status Table Screen

View


PLC Data Formats

Two 8-bit Unsigned Bytes of Data16-bit Unsigned Integer16-bit Signed IntegerBinary Coded DecimalHexadecimal


Two 8-bit Unsigned Bytes of Data


16-bit Unsigned Integer


16-bit Signed Integer


Binary Coded Decimal


Hexadecimal


PLC Memory Categories

PLC memory is divided into two categories:System memoryApplication memory


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.


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.


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


Ladder Files

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 statusInput statusProcessor

statusBinary or bitsTimers

CountersIntegerFloating pointUser defined


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.


Data File Identification

O: Output Status FileI: Input Status FileS: Processor Status File

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


Data File Identification

B3 Binary or bit fileT4 TimersC5 CountersR6 ControlN7 IntegerF8 Floating Point


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.

Documents

Copyright © 2002 Delmar Thomson Learning Chapter 5 Introduction to PLC Operation