Introduction into IEC 61131-8

Guidelines for the application and implementation of programming languages

Scope

This technical report applies to the programming of programmable controller systems using the programming languages defined in IEC 61131-3. It also provides guidelines for the implementation of these languages in programmable controller systems and their programming support environments (PSEs).

IEC 61131-4 should be consulted for other aspects of the application of programmable controller systems.

Overview

The intended audience for this technical report consists of:

· users of programmable controller systems as defined in IEC 61131-3, who must program, configure, install, and maintain programmable controllers as part of industrial process measurement and control systems; and

· implementors of programming languages, as defined in IEC 61131-3, for programmable controller systems. This may include vendors of software and hardware for the preparation and maintenance of programs for these systems, as well as vendors of the programmable controller systems themselves.

IEC 61131-3, in contrast, is mainly oriented toward the implementors of programming languages for programmable controllers. Users who wish a general introduction to these languages and their application should consult any of several generally available textbooks on this subject. Subclause 1.4 of IEC 61131-3 should be consulted by those who wish a "top-down" overview of the contents of IEC 61131-3.

Clause 2 of this technical report provides a general introduction to IEC 61131-3, while clause 3 provides complementary information about the application of some of the programming language elements specified IEC 61131-3. Clause 4 provides information about the intended implementation of some of these programming language elements, while Clause 5 provides general information about requirements for hardware and software for program development and maintenance. Hence, it is expected that users of programmable controllers will find Clauses 2 and 3 of this document most useful, while programming language implementors will find Clauses 4 and 5 more useful, referring to the background material in Clauses 2 and 3 as necessary.

TABLE OF CONTENT IEC 61131-8

1.	General	7
1.1	Scope	7
1.2	Normative references	7
1.3	Overview	7
2.	Introduction to IEC 61131-3	8
2.1	General considerations	8
2.2	Overcoming historical limitations	10
2.3	Basic features in IEC 61131-3	11
2.4	New features in the second edition of IEC 61131-3	12
2.5	Software engineering considerations	12
2.5.1	Application of software engineering principles	12
2.5.1.1	Encapsulation and hiding	12
2.5.1.2	Explicit representation of state	13
2.5.1.3	Mapping to the application domain	13
2.5.1.4	Mapping of design to implementation	13
2.5.1.5	Structured programming	14
2.5.1.6	Software reuse	14
2.5.2	Portability	15
2.5.2.1	Inter-language portability	15
2.5.2.2	Inter-system portability	15
3.	Application guidelines	16
3.1	Use of data types	16
3.1.1	Type vs. variable initialization	16
3.1.2	Use of enumerated and subrange types	17
3.1.3	Use of BCD data	17
3.1.4	Use of REAL data types	19
3.1.5	Use of character string data types	19
3.1.6	Use of time data types	20
3.1.7	Use of multi-element variables	21
3.1.8	Use of bit string data types	21
3.2	Data passing	21
3.2.1	Global and external variables	22
3.2.2	In-out (VAR_IN_OUT) variables	23
3.2.3	Formal and non-formal invocations and argument lists	25
3.3	Use of function blocks	29
3.3.1	Function block types and instances	29
3.3.2	Scope of data within function blocks	30
3.3.3	Function block access and invocation	31
3.4	Differences between function block instances and functions	32
3.5	Use of indirectly referenced function block instances	32
3.5.1	Establishing an indirect function block instance reference	33
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
3.8	Language specific features	40
3.8.1	Edge triggered functionality	40
3.8.1.1	Edge triggering in LD language	40
3.8.1.2	Use of edge triggered function blocks	41
3.8.2	Use of EN/ENO in functions and function blocks	42
3.8.3	Use of non-IEC 61131-3 languages	43
3.9	Use of SFC elements	43
3.9.1	Action control	44
3.9.2	Boolean actions	45
3.9.3	Non-SFC actions	50
3.9.4	SFC actions	51
3.9.5	SFC function blocks	53
3.9.6	"Indicator" variables	53
3.10	Scheduling, concurrency, and synchronization mechanisms	54
3.10.1	Operating system issues	54
3.10.2	Task scheduling	56
3.10.2.1	Performance effects	57
3.10.2.2	Concurrency effects	57
3.10.3	Semaphores	57
3.10.3.1	General	57
3.10.3.2	Deadlocks	58
3.10.4	Messaging	58
3.10.4.1	Global storage	59
3.10.4.2	Mailboxes and queues	59
3.10.5	Time stamping	59
3.11	Communication facilities in ISO/IEC 9506/5 and IEC 61131-5	59
3.11.1	Communication channels	60
3.11.2	Reading and writing variables	60
3.11.3	Communication function blocks	60
3.12	Recommended programming practices	61
3.12.1	Global variables	62
3.12.2	Jumps in Function Block Diagram (FBD) language	62
3.12.3	Multiple invocations of function block instances in FBD	62
3.12.4	Coupling of Sequential Function Chart (SFC) networks	62
3.12.5	Dynamic modification of task priorities	63
3.12.6	Execution control of function block instances by tasks	63
3.12.7	Use of RTC (Real Time Clock) Function Blocks	63
4.	Implementation guidelines	64
4.1	Resource allocation	64
4.2	Implementation of data types	64
4.2.1	REAL and LREAL data types	64
4.2.2	Character strings	64
4.2.3	Time data types	65
4.2.4	Multi-element variables	65
4.3	Execution of functions and function blocks	65
4.3.1	Functions	66
4.3.2	Function blocks	66
4.4	Implementation of Sequential Function Charts (SFCs)	67
4.5	Task scheduling	67
4.5.1	Classification of tasks	68
4.5.2	Task priorities	68
4.6	Error handling	69
4.6.1	Error handling mechanisms	69
4.6.2	Run-time error handling procedures	71
4.6.2.1	Reporting of errors	71
4.6.2.2	System-defined error handling procedures	71
4.6.2.3	User-defined error handling procedures	73
4.7	System interface	73
4.8	Compliance	73
4.8.1	Compliance statement	73
4.8.2	Controller instruction sets	73
4.8.3	Compliance testing	74
4.9	Compatibility with IEC 617-12, 617-13, and 848	74
5.	Programming support environment (PSE) requirements	74
5.1	User interface	74
5.2	Programming of programs, functions and function blocks	75
5.3	Application design and configuration	76
5.4	Separate compilation	76
5.5	Separation of interface and body	77
5.5.1	Invocation of a function from a programming unit	77
5.5.2	Declaration and invocation of a function block instance	78
5.6	Linking of configuration elements with programs	79
5.7	Library management	82
5.8	Analysis tools	83
5.8.1	Simulation and debugging	83
5.8.2	Performance estimation	83
5.8.3	Feedback loop analysis	83
5.8.4	SFC analysis	83
5.9	Documentation requirements	86
5.10	Security of data and programs	87
5.11	On-line facilities	87
Annex A	Changes to IEC 61131-3 2nd Edition	88
A.1	Reasons for the 2nd edition of part 3	88
A.2	Corrigendum	88
A .3	Amendment	89
A.3.1	Numeric literals (2.2.1) – typed literals	90
A.3.2	Elementary data types (2.3.1) - double-byte strings	90
A.3.3	Derived data types (2.3.3) - enumerated data types	90
A.3.4	Single element variables (2.4.1.1) - 'wild-card' direct addresses	91
A.3.5	Declaration (2.4.3) - Temporary variables	91
A.3.6	Type assignment (2.4.3.1) - RETAIN and NON_RETAIN Variable attributes	92
A.3.7 of	Function ( 2.5.1) – Use EN/ENO	92
A.3.8	Declaration (2.5.1.3) - Function invocation with VAR_IN_OUT	92
A.3.9	Type conversion functions (2.5.1.5.1)	93
A.3.10	Functions of time data types (2.5.1.5.6)	93
A.3.11	Function blocks (2.5.2) - Extended initialisation facilities	93
A.3.12	Pulse action qualifiers (2.6.4.4)	94
A.3.13	Action control (2.6.4.5)	94
A.3.14	Configuration initialisation (2.7.1)	94
A.3.15	Instruction List (3.2)	95
A.3.16	Formal specification of language elements (Annex B)	97
A.3.17	Further amendments	97
ANNEX B	SOFTWARE QUALITY MEASURES	99
ANNEX C	INDEX	101