IEC TR 61131-8:2003

TECHNICAL IEC
REPORT
TR 61131-8
Second edition
2003-09
Programmable controllers –
Part 8:
Guidelines for the application and implementation
of programming languages
Automates programmables –
Partie 8:
Lignes directrices pour l'application et la mise en oeuvre
des langages de programmation
Reference number
IEC/TR 61131-8:2003(E)
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TECHNICAL IEC
REPORT
TR 61131-8
Second edition
2003-09
Programmable controllers –
Part 8:
Guidelines for the application and implementation
of programming languages
Automates programmables –
Partie 8:
Lignes directrices pour l'application et la mise en oeuvre
des langages de programmation
 IEC 2003  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
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– 2 – TR 61131-8 © IEC:2003(E)

CONTENTS
FOREWORD .6

INTRODUCTION .8

1 General.9

1.1 Scope .9

1.2 Normative references .9

1.3 Abbreviated terms .9
1.4 Overview.10
2 Introduction to IEC 61131-3.10
2.1 General considerations.10
2.2 Overcoming historical limitations.12
2.3 Basic features in IEC 61131-3 .13
2.4 New features in the second edition of IEC 61131-3 .14
2.5 Software engineering considerations .14
2.5.1 Application of software engineering principles .14
2.5.2 Portability .17
3 Application guidelines.17
3.1 Use of data types .17
3.1.1 Type versus variable initialization.18
3.1.2 Use of enumerated and subrange types .18
3.1.3 Use of BCD data.19
3.1.4 Use of REAL data types .21
3.1.5 Use of character string data types.21
3.1.6 Use of time data types .22
3.1.7 Declaration and use of multi-element variables.22
3.1.8 Use of bit-string functions .23
3.1.9 Strongly typed assignment .24
3.2 Data passing .25
3.2.1 Global and external variables.25
3.2.2 In-out (VAR_IN_OUT) variables .26
3.2.3 Formal and non-formal invocations and argument lists .28

3.3 Use of function blocks .30
3.3.1 Function block types and instances .30
3.3.2 Scope of data within function blocks.31
3.3.3 Function block access and invocation .32
3.4 Differences between function block instances and functions.33
3.5 Use of indirectly referenced function block instances .33
3.5.1 Establishing an indirect function block instance reference .34
3.5.2 Access to indirectly referenced function block instances.35
3.5.3 Invocation of indirectly referenced function block instances .35
3.5.4 Recursion of indirectly referenced function block instances .38
3.5.5 Execution control of indirectly referenced function block instances .38
3.5.6 Use of indirectly referenced function block instances in functions .38
3.6 Recursion within programmable controller programming languages.39
3.7 Single and multiple invocation .39

TR 61131-8 © IEC:2003(E) – 3 –

3.8 Language specific features .40

3.8.1 Edge-triggered functionality .40

3.8.2 Use of EN/ENO in functions and function blocks.41

3.8.3 Use of non-IEC 61131-3 languages.42

3.9 Use of SFC elements.42

3.9.1 Action control.42

3.9.2 Boolean actions .44

3.9.3 Non-SFC actions .47

3.9.4 SFC actions.48

3.9.5 SFC function blocks .48
3.9.6 “Indicator” variables .49
3.10 Scheduling, concurrency, and synchronization mechanisms.50
3.10.1 Operating system issues .50
3.10.2 Task scheduling.51
3.10.3 Semaphores .52
3.10.4 Messaging .53
3.10.5 Time stamping .53
3.11 Communication facilities in ISO/IEC 9506/5 and IEC 61131-5 .54
3.11.1 Communication channels .54
3.11.2 Reading and writing variables .54
3.11.3 Communication function blocks .55
3.12 Deprecated programming practices.56
3.12.1 Global variables.56
3.12.2 Jumps in FBD language .56
3.12.3 Multiple invocations of function block instances in FBD .56
3.12.4 Coupling of SFC networks.56
3.12.5 Dynamic modification of task priorities .57
3.12.6 Execution control of function block instances by tasks.57
3.12.7 Incorrect use of WHILE and REPEAT constructs .57
3.13 Use of TRUNC and REAL_TO_INT functions .58
4 Implementation guidelines .58
4.1 Resource allocation.59
4.2 Implementation of data types .59
4.2.1 REAL and LREAL data types.59
4.2.2 Bit strings .59

4.2.3 Character strings .59
4.2.4 Time data types .60
4.2.5 Multi-element variables .60
4.3 Execution of functions and function blocks.60
4.3.1 Functions.60
4.3.2 Function blocks.61
4.4 Implementation of SFCs .62
4.4.1 General considerations .62
4.4.2 SFC evolution .62
4.5 Task scheduling .63
4.5.1 Classification of tasks .63
4.5.2 Task priorities.63

– 4 – TR 61131-8 © IEC:2003(E)

4.6 Error handling .64

4.6.1 Error-handling mechanisms.64

4.6.2 Run-time error-handling procedures .65

4.7 System interface .67

4.8 Compliance .67

4.8.1 Compliance statement .67

4.8.2 Controller instruction sets .68

4.8.3 Compliance testing .68

5 PSE requirements .68

5.1 User interface.68
5.2 Programming of programs, functions and function blocks .69
5.3 Application design and configuration.
...