PLCChap 03 - Lecture notes Chapter 3 PDF

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CNC Programming Concepts...

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Chapter 3: Memory Organization and Addressing

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Memory Organization & Addressing Learning objectives 

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 Technology Overview   



IEC 61131-3 Protocol International Electrotechnical Commission (IEC) V d IIndependent Vendor d d t St Standardized d di d Programming P i L Language for f Industrial Automation Specifies Syntax, Semantics and Display for:     



Ladder Diagram 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 P t 5 - Communication Part C i ti

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Memory Organization & Addressing

IEC 61131-3 Model IEC 61131- 3 Protocol: A mutually agreed upon set of rules, conventions, and agreements for the efficient and orderly design of PLCs

Layered Software Architecture  Each layer hides details of lower layers Common Elements of IEC 61131-3 Architecture:  Configuration (Top layer)  





It describes entire software (program + 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 Provides an interface between a program and and aa I/O ports of of the PLC

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IEC 61131-3 Model C Common Elements El t off IEC 61131 61131-33 A Architecture hit t ((continued) ti d) 

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 Resource Task

Task

Resource Task

Task

Task

Program Func- Function tion Block Block

Program

Program

Program

FB

FB

FB

Function Bl k Block

FB

FB

Lcl Glbl

Lcl Glbl

Local Global Prog. Vars Prog. Vars

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 A Accessible ibl tto allll elements l t contained t i d in i the th configuration fi ti  



Global Config variable can be accessed by all software elements Global Program Variable can be accessed by all function blocks in the program

Directly Represented Variables 

Refers to memory and I/O locations

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A-B ControlLogix Memory Project Task1

Task2

Program 32 Program Program 1 Main Routine Fault Routine

Controller (global) Tags

Config. Task3 Program (local) Tags

Status

Watchdog

Task32

Other R ti Routines

I/O Data

Controller Fault Handler

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System-shared Data Power Up Handler

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Simple Ladder Logic NOT Operation Tasks    

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 be interrupted by a higher-priority periodic task.

Programs   

Consists of Routines • Max. of 32 per task • Executed in order as listed in task

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Simple Ladder Logic NOT Operation NOT Truth Table 

Possible Combinations of the 2 Switches: (22)

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A

B

Light

OFF

OFF

OFF

OFF

ON

OFF

ON

OFF

ON

ON

ON

OFF

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ControlLogix Program Organization

Tasks    

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 be interrupted by a higher-priority periodic task .

Programs 

Consists of Routines Max. of 32 per task



Executed in order as listed in task



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ControlLogix Program Organization Routines  



Encapsulate code in single programming language Main Routine is first one executed  Must call other routines Fault Routine is executed if a program fault is encountered during execution

Controller Tags 

Global Gl b l directly di tl represented t d varibles ibl  Represents global controller memory  I/O data

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ControlLogix Scan  



Control logic (program) continuously scanned. Data is transferred to/from I/O modules any time during the logic scan. If input data must remain constant, make a copy at beginning of scan and use the copy throughout scan

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