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Chapter 3: Memory Organization and

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Memory Organization & Addressing

Understand memory organization and addressing for

the following PLCs:

IEC 61131-3 Model

Allen-Bradley ControlLogix

Allen-Bradley PLC-5/SLC-500

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IEC 61131-3 Model

IEC 61131-3 Protocol

International Electrotechnical Commission (IEC)

en or n epen en an ar ze rogramm ng anguage or

Industrial Automation

Specif ies Syntax, Semantics and Display for:

Sequential Function Charts

Function Block Diagram Structured Texts

Instruction Lists

IEC 61131- 3 refers to Part 3 of IEC 61131 suite

Part 1 General Overview

Part 2 Hardware

Part 3 Programming Language

Part 4 User Guidelines

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ar - ommun ca on

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Memory Organization & Addressing

IEC 61131-3 ModelIEC 61131- 3 Protocol:A mutually agreed upon set of rules, conventions, and

Layered Software Architecture

Each layer hides details of lower layers

Common Elements of IEC 61131-3 Architecture:

Configuration (Top layer) It describes entire software ro ram + data for one PLC

Each PLC, within an organization, has a separate configuration

Includes resources, memory addresses for I/O channels, system

capabilities

Resource (within Configuration layer)

Provides support function for the execution of IEC programs

Configuration may consist of one or more resources

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rov es an n er ace e ween a program an a por s o e

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IEC 61131-3 Modelommon emen s o - rc ec ure con nue

Tasks (within Resource layer)

Control the execution of a set of programs and/or function blocks

Programs/function blocks can be executed periodically or can be

event driven

Programs must be assigned to a task in order to be executed

The task must be configured to either execute periodically or upon a

trigger

Resource layer may consist of one or more Tasks

http://www.personal.kent.edu/~asamba/tech43550/iec61131.pdf

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IEC 61131-3 Memory Model

Configuration

Program Program

Task Task Task

Resource

Program Program

Task Task

Resource

Func-tion

Block

Func-tion

Block

Func-tion

FB

FB

FBFB

FB

oc

GlobalProg. Vars

LocalProg. Vars Lcl GlblLcl Glbl Lcl Glbl

Global and directly represented variables

Access Paths

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IEC 61131-3 Memory Model

Variables

Local Variable

Defined within the software element

Accessible only by the software element

Software Element: Function Block, Resource or Configuration

Global Variables

Defined for a configuration ccess e o a e emen s con a ne n e con gura on

Global Config variable can be accessed by all software elements

Global Program Variable can be accessed by all function blocks in the

Directly Represented Variables

Refers to memory and I/O locations

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A-B ControlLogix Memory

Task1 Task2

Project

Program 1Program

local Ta s

Program

Config.

Status

Task3

Routine

Fault Other Watchdog Task32

ou nes

Controller Fault Handler Power Up Handler

(global) Tags I/O Data ystem-s areData

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Simple Ladder LogicN T peration

Configured to control execution of one or more programs

Maximum of 32 tasks

No more than one continuous task

Others are periodic, scheduled at fixed interval. May beinterrupted by a higher-priority periodic task.

Programs

Consists of Routines

Max. of 32 per task

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Executed in order as listed in task

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Simple Ladder LogicNOT Operation

Possible Combinations of

the 2 Switches: (22)A B Light

OFF ON OFF

ON OFF ON

ON ON OFF

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ControlLogix Program Organization

Tasks

on gure o con ro execu on o one or more programs

Maximum of 32 tasks

Others are periodic, scheduled at fixed interval. May be

interru ted b a hi her- riorit eriodic task.

Programs

Max. of 32 per task

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ControlLogix Program Organization

Encapsulate code in single programming language Main Routine is first one executed

Must call other routines

Fault Routine is executed if a program fault isencoun ere ur ng execu on

Controller Tags

o a rec y represen e var es Represents global controller memory

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ControlLogix Scan

.

Data is transferred to/from I/O modules any timedurin the lo ic scan.

If input data must remain constant, make a copy

at be innin of scan and use the cothroughout scan

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