IEC TR 61131-8:2017

Industrial-process measurement and control - Programmable controllers - Part 8: Guidelines for the application and implementation of programming languages

IEC TR 61131-8:2017(E) applies to the programming of program­mable controller systems using the programming languages defined in IEC 61131-3. The scope of IEC 61131-3 is applicable to this part.
This document provides
a) guidelines for the application of IEC 61131-3,
b) guidelines for the implementation of IEC 61131-3 languages for programmable controller systems,
c) programming and debugging tool (PADT) recommendations.
For further information see IEC 61131-4 which describes other aspects of the application of programmable controller systems, e.g. electromagnetic compatibility or functional safety. This third edition cancels and replaces the second edition published in 2003. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: This third edition is a compatible extension of the second edition. The main extensions are new data types and conversion functions, references, name spaces and the object oriented features of classes and function blocks (see listing in Annex B of IEC 61131-3:2013).

General Information

• Status: Published
• Publication Date: 21-Nov-2017
• ICS: 25.040.40 - Industrial process measurement and control
• ICS: 35.240.50 - IT applications in industry
• Technical Committee: SC 65B - Measurement and control devices
• Drafting Committee: WG 7 - TC 65/SC 65B/WG 7
• Current Stage: PPUB - Publication issued
• Start Date: 22-Nov-2017
• Completion Date: 03-Nov-2017

Relations

• Revises: IEC TR 61131-8:2003 - Programmable controllers - Part 8: Guidelines for the application and implementation of programming languages
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC TR 61131-8 ®
Edition 3.0 2017-11
TECHNICAL
REPORT
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Industrial-process measurement and control – Programmable controllers –
Part 8: Guidelines for the application and implementation of programming
languages
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IEC TR 61131-8 ®
Edition 3.0 2017-11
TECHNICAL
REPORT
colour
inside
Industrial-process measurement and control – Programmable controllers –

Part 8: Guidelines for the application and implementation of programming

languages
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 25.240.50 ISBN 978-2-8322-4898-0

– 2 – IEC TR 61131-8:2017  IEC 2017
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 9
4 Abbreviated terms . 9
5 Overview . 10
6 Introduction to IEC 61131-3 . 10
6.1 General considerations . 10
6.2 Overcoming historical limitations . 13
6.3 Basic features in IEC 61131-3 . 13
6.4 Language items overview . 14
6.5 Changes from IEC 61131-3:2003 (edition 2) to IEC 61131-3:2013 (edition 3) . 16
6.6 Software engineering considerations . 17
6.6.1 Application of software engineering principles . 17
6.6.2 Portability . 19
7 Application guidelines . 20
7.1 Use of data types . 20
7.1.1 Type selection . 20
7.1.2 Type versus variable initialization . 20
7.1.3 Use of enumerated and subrange types . 21
7.1.4 Use of BCD data . 22
7.1.5 Use of REAL data types . 22
7.1.6 Use of character string data types . 23
7.1.7 Use of character data types . 24
7.1.8 Use of time data types . 24
7.1.9 Declaration and use of multi-element variables . 26
7.1.10 Use of bit-string variables . 26
7.1.11 Use of partial accessing of bitstring variables . 27
7.1.12 Type assignment . 27
7.2 Data passing over POU . 27
7.2.1 General . 27
7.2.2 External variables . 29
7.2.3 In-out (VAR_IN_OUT) variables . 29
7.2.4 Formal and non-formal invocations and argument lists . 32
7.2.5 Assignment of input, output, and in-out variables of programs . 34
7.3 Use of function blocks . 35
7.3.1 Function block types and instances . 35
7.3.2 Scope of data within function blocks . 35
7.3.3 Function block access and invocation . 36
7.4 Differences between function block instances and functions . 37
7.5 Use of indirectly referenced function block instances . 37
7.5.1 General . 37
7.5.2 Establishing an indirect function block instance reference . 38
7.5.3 Access to indirectly referenced function block instances . 40

7.5.4 Invocation of indirectly referenced function block instances . 41
7.5.5 Recursion of indirectly referenced function block instances . 43
7.5.6 Execution control of indirectly referenced function block instances . 44
7.5.7 Use of indirectly referenced function block instances in functions . 44
7.6 Use of programs . 44
7.6.1 Difference to function block . 44
7.6.2 Communication with other programs . 44
7.7 Object orientation . 45
7.7.1 General introduction . 45
7.7.2 Usage of methods . 45
7.7.3 Usage of class variable . 49
7.7.4 Usage of inheritance . 50
7.7.5 Usage of override . 53
7.7.6 Usage of interfaces . 53
7.8 Recursion within programmable controller programming languages . 54
7.9 Multiple invocations of a function block instance . 54
7.10 Language specific features . 55
7.10.1 Edge-triggered functionality . 55
7.10.2 Edge-triggering in LD language . 55
7.10.3 Use of edge-triggered function blocks . 56
7.10.4 Use of EN/ENO in functions and function blocks . 56
7.10.5 Language selection. 57
7.11 Namespaces . 58
7.11.1 General . 58
7.11.2 Usage of global namespace . 58
7.11.3 Usage of INTERNAL . 58
7.12 Use of SFC elements . 59
7.12.1 General . 59
7.12.2 Action control . 59
7.12.3 Boolean actions . 60
7.12.4 Non-SFC actions . 64
7.12.5 SFC actions . 65
7.12.6 SFC function blocks . 66
7.13 Scheduling, concurrency and synchronization mechanisms . 67
7.13.1 Operating system issues . 67
7.13.2 Task scheduling . 68
7.13.3 Semaphores . 69
7.13.4 Messaging . 70
7.13.5 Time stamping . 70
7.14 Communication facilities in ISO/IEC 9506-5 and IEC 61131-5 . 71
7.14.1 Overview . 71
7.14.2 Data representation . 71
7.14.3 Communication channels . 73
7.14.4 Reading and writing variables . 73
7.14.5 Communication function blocks . 74
7.15 Deprecated programming practices . 75
7.15.1 General . 75
7.15.2 Global variables . 75
7.15.3 Jumps in FBD/LD language . 75

– 4 – IEC TR 61131-8:2017  IEC 2017
7.15.4 Dynamic modification of task properties . 75
7.15.5 Execution control of function block instances by tasks . 76
7.15.6 WHILE and REPEAT constructs for interprocess synchronisation . 76
7.15.7 Expecting programs associated with a task to be executed sequentially . 77
7.16 REAL_TO_INT conversion functions . 78
7.17 Implementation dependant parameters . 78
8 Implementation guidelines . 80
8.1 General . 80
8.2 Resource allocation . 80
8.3 Implementation of data types . 80
8.3.1 REAL and LREAL data types . 80
8.3.2 Bit strings . 81
8.3.3 Character strings . 81
8.3.4 Time data types . 81
8.3.5 Multi-element variables . 82
8.4 Execution of functions and function blocks . 83
8.4.1 General . 83
8.4.2 Functions . 83
8.4.3 Function blocks . 83
8.5 Object oriented features . 84
8.5.1 Classes . 84
8.5.2 Methods . 84
8.5.3 Dynamic binding and virtual method table . 85
8.5.4 Interfaces . 85
8.6 Implementation of SFCs . 85
8.6.1 General considerations . 85
8.6.2 SFC evolution . 85
8.6.3 SFC analysis . 86
8.7 Task scheduling . 87
8.7.1 General . 87
8.7.2 Classification of tasks . 88
8.7.3 Task priorities . 88
8.8 Error handling . 88
8.8.1 Error-handling mechanisms . 88
8.8.2 Run-time error-handling procedures. 90
8.9 System interface . 92
8.10 Compliance . 92
8.10.1 General . 92
8.10.2 Compliance statement . 92
8.10.3 Compliance testing . 92
Annex A (informative) Relationships to other standards . 93
INDEX . 94
Bibliography . 101

Figure 1 – A distributed application . 11
Figure 2 – Stand-alone applications . 12
Figure 3 – Cyclic or periodic scanning of a program . 13
Figure 4 – Programming Modelm . 15

Figure 5 – Software Modelm . 16
Figure 6 – ST example of time data type usage . 26
Figure 7 – Example of declaration and use of array types . 26
Figure 8 – Examples of VAR_IN_OUT usage . 32
Figure 9 – Hiding of function block instances . 36
Figure 10 – Graphical use of a function block name . 40
Figure 11 – Access to an indirectly referenced function block instance . 40
Figure 12 – Invocation of an indirectly referenced function block instance. 43
Figure 13 – Standard FB CTUD according to IEC 61131-3 . 46
Figure 14 – Functionblock CTUD object oriented version . 47
Figure 15 – Call of standard and OO-Functionblock in ST . 48
Figure 16 – Call of standard function block in FBD . 48
Figure 17 – All of a method in FBD . 48
Figure 18 – Synthesis: a traditional function block derived from an OO-function block . 49
Figure 19 – Use of function blocks derived from ETrig-Base, the base function block
interface is highlighted . 53
Figure 20 – Timing of edge triggered functionality . 56
Figure 21 – Execution control example . 57
Figure 22 – Timing of Boolean actions . 64
Figure 23 – Example of a programmed non-Boolean action. 64
Figure 24 – Use of the pulse (P) qualifier . 65
Figure 25 – An SFC function block . 66
Figure 26 – Example of incorrect and allowed programming constructs . 77
Figure 27 – Reduction steps . 86
Figure 28 – Reduction of SFCs . 87

Table 1 – Available data passing of variables to POU . 28
Table 2 – Examples of textual invocations of functions and function blocks . 33
Table 3 – Characteristics of the languages . 58
Table 4 – Differences between multi-user and real-time systems . 68
Table 5 – Implementation-dependent parameters . 78
Table 6 – Recommended run-time error-handling mechanisms . 89

– 6 – IEC TR 61131-8:2017  IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL-PROCESS MEASUREMENT AND CONTROL –
PROGRAMMABLE CONTROLLERS –
Part 8: Guidelines for the application
and implementation of programming languages

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example “state of the art”.
IEC 61131-8, which is a technical report, has been prepared by subcommittee
65B: Measurement and control devices, of IEC technical committee 65: Industrial-process
measurement, control and automation.
This third edition cancels and replaces the second edition published in 2003. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
This third edition is a compatible extension of the second edition. The main extensions are
new data types and conversion functions, references, name spaces and the object oriented
features of classes and function blocks (see listing in Annex B of IEC 61131-3:2013).
The text of this technical report is based on the following documents:
DTR Report on voting
65B/1058/DTR 65B/1073/RVDTR
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61131 series, published under the general title Industrial-process
measurement and control – Programmable controllers, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 8 – IEC TR 61131-8:2017  IEC 2017
INTRODUCTION
This part of IEC 61131 is being issued as a technical report in order to provide guidelines for
the implementation and application of the programming languages defined in
IEC 61131-3:2013.
The content of this document answers a number of frequently asked questions about the
intended application and implementation of the normative provisions of IEC 61131-3.

INDUSTRIAL-PROCESS MEASUREMENT AND CONTROL –
PROGRAMMABLE CONTROLLERS –
Part 8: Guidelines for the application
and implementation of programming languages

1 Scope
This part of IEC 61131, which is a technical report, applies to the programming of program-
mable controller systems using the programming languages defined in IEC 61131-3. The
scope of IEC 61131-3 is applicable to this part.
This document provides
a) guidelines for the application of IEC 61131-3,
b) guidelines for the implementation of IEC 61131-3 languages for programmable controller
systems,
c) programming and debugging tool (PADT) recommendations.
For further information IEC 61131-4 describes other aspects of the application of
programmable controller systems, e.g. electromagnetic compatibility or functional safety.
NOTE Neither IEC 61131-3 nor this document explicitly addresses safety issues of programmable controller
systems or their associated software. The various parts of IEC 61508 can be consulted for such considerations.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 61131-3:2013, Programmable controllers – Part 3: Programming languages
IEC 61131-5, Programmable controllers – Part 5: Communications
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Abbreviated terms
ED Early Detection
ETrig Edge triggered function
EW Early Warning
– 10 – IEC TR 61131-8:2017  IEC 2017
FB Function Block
FBD Function Block Diagram
FC Function
IL Instruction List
LD Ladder Diagram
MMS Manufacturing Message Specification
NaN Not a number
OO Object Orientation
OOP Object Oriented Programming
PLC Programmable Logic Controller
POU Program Organization Unit
PADT Programming And Debugging Tool
RT RunTime
SFC Sequential Function Chart
ST Structured Text
VMD Virtual Manufacturing Device
5 Overview
The intended audience for this document consists of
– users of programmable controller systems as defined in IEC 61131-3, who shall program,
configure, install and maintain programmable controllers as part of industrial-process
measurement and control systems; and
– implementers of programming and debugging tools (PADT)as defined in IEC 61131-3, for
programmable controller systems. This can 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 is mainly oriented toward the implementers 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.
Clause 6 of this document provides a general introduction to IEC 61131-3, while
Clause 7 provides complementary information about the application of some of the
programming language elements specified in IEC 61131-3. Clause 8 provides information
about the intended implementation of some of these programming language elements.
Hence, it is expected that users of programmable controllers will find Clauses 6 and 7 of this
part most useful, while programming language implementers will find Clause 8 more useful,
referring to the background material in Clauses 6 and 7 as necessary.
6 Introduction to IEC 61131-3
6.1 General considerations
In the time before the IEC 61131-3 was existing, the limited capabilities of expensive
hardware components imposed severe constraints on the design process for industrial-
process control, measurement and automation systems. Software design and implementation
were tightly tailored to the selected hardware. This required specialists who were highly
skilled, both in solving process automation problems and in dealing with complicated, often
hardware-specific computer programming constructs.

With the rapid innovation in microelectronics and related technologies, the cost/performance
ratio of system hardware has decreased dramatically. At present, a programmable controller
can cost many times less than the cost of programming it.
Driven by rapidly decreasing hardware cost, a trend has become established of replacing
large, centrally installed process computers or other comparatively large, isolated controllers
by systems with spatially and functionally distributed parts.
As illustrated in Figure 1, the essential backbone of such systems is the communication
subsystem, which provides the mechanism for information exchange between the distributed
automating devices. Connected to this backbone are the devices, such as programmable
controllers, which deliver the distributed processing power of the system. Each device, under
the control of its own software, performs a dedicated subtask to achieve the required overall
system functionality. Each device is chosen with the size and performance required to meet
the demands of its particular subtask.
In a different environment, programmable controllers are used in stand-alone applications as
illustrated in Figure 2. Users of these applications also stand to gain by the evolution outlined
above. Due to the present low cost of hardware components, many new, relatively small,
automation tasks can be solved profitably and flexibly by programmable controllers.
Operating
monitoring
Communication subsystem
I
I I O I
Loop Logic Mixed
O O O
control control control
Automated process
IEC
Figure 1 – A distributed application

– 12 – IEC TR 61131-8:2017  IEC 2017
Operating Operating
monitoring monitoring
I
I I
O
Logic Mixed
O O
control control
Press Pump
IEC
Figure 2 – Stand-alone applications
In addition to their low hardware price, the intensive use of programmable controllers in
solving automation tasks is also advanced by their straightforward operating and
programming principles, which are easily understood and applied by the shop-floor personnel
involved in programming, operation and maintenance.
Programmable controllers typically employ the principles of cyclic or periodic program
execution illustrated in Figure 3. Cyclically running programs restart execution as fast as
possible after they have terminated execution, in IEC 6113-3 such programs are not assigned
to a task and executed with the lowest priority. Periodic execution of a program is triggered by
a clock mechanism at equidistant points in time. The same controller may execute such
programs quasi-simultaneous with different periodic times. Another kind (defined by
IEC 61131-3 but not shown in Figure 3) is the non-periodic execution, which is based on
events and will be executed upon each occurrence of such events. These principles are well
known and applied in the operation of digital signal processing systems to simulate the
operation of continuously operating analogue or electromechanical systems. Process values
are read into the device and written out to the process as discrete samples at random or
equidistant points in time, depending on the control task that has to be fulfilled.
The advantage of these operating principles is that they allow the construction of programs for
programmable controllers using elements closely related to the principles of hard-wired logic
or continuous control circuits previously used for the same purpose.
The operating principles of programmable controllers thus enable the provision of application-
specific, graphical programming languages. Combined with appropriate man-machine inter-
faces, these languages enable the control engineer to concentrate on solving the problems of
the application, without extensive training in software engineering. The control engineer’s
technological specifications can be mapped directly to the corresponding language elements.
Another particular advantage of such programming languages is that the representation they
offer can be used not only for program input and documentation, but also for on-line test and
diagnosis as well. Thus, programming and debugging tools (PADT) for programmable
controllers are able to provide the graphically oriented representation and documentation that
are already familiar to the application engineer and shop-floor personnel.

Clock trigger:
e.g. every 80 ms
Read Read
Cyclic Periodic
inputs inputs
execution execution
OR
Execute Execute
algorithms algorithms
Write Write
outputs outputs
IEC
Figure 3 – Cyclic or periodic scanning of a program
6.2 Overcoming historical limitations
Automation system designers are often required to use programmable controllers from various
manufacturers in different automation systems, or even in the same system. However, the
hardware of programmable controllers from different manufacturers may have very little in
common. This has resulted in significant differences in the elements and methods of
programming the software as well and has led to the development of manufacturer-specific
programming and debugging tools, which generally carried very specialized software for
programming, testing and maintaining particular controller “families”.
Changing from one controller family to another often required the designer to read large
manuals for both the hardware and software of the new family. Often, the manual had to be
reviewed several times in order to understand the exact meaning and to use the new
controller family in an appropriate way. Due to the concentrated, tedious work necessary to
read and understand the new, vendor-specific material, few people did it. For this reason,
many people regarded the design and the programming of such controllers as some black
magic to be avoided. Thus, the knowledge of how to use such systems effectively was
concentrated in one or a few specialists and could not be transferred effectively to those
responsible for system operation, maintenance, and upgrade.
A major goal of IEC 61131-3 was to remove such barriers to the understanding and
application of programmable controllers. Thus, IEC 61131-3 introduced numerous facilities to
support the advantages of programmable controllers described in 6.1, even if controllers of
different vendors are concerned. It has turned out that the resulting expansion of the
application domains of programmable controllers, and the increasing demand of customers
fed through this expansion, stimulated a lot of vendors to make their programming systems
compliant to the standard.
Vendor and user organizations like PLCopen accelerated this process by promoting the
benefits and advantages of standardizing PLC programming to a large extent.
6.3 Basic features in IEC 61131-3
From the point of view of the application engineer and the control systems configurator, the
most important features introduced by IEC 61131-3 can be summarized as follows.

– 14 – IEC TR 61131-8:2017  IEC 2017
a) Well-structured, “top-down” or “bottom-up” program development is facilitated by language
constructs for the definition of typed functional objects (program organization units) such
as functions, function blocks, classes and programs.
b) Strong data typing is not only supported but inherently required, thus eliminating a major
source of programming errors.
c) A sufficient set of features for the execution control of program organization units is
included; those features associated with steps, transitions and action blocks offer
excellent means to represent complicated sequential control solutions in a concise form.
d) The functionality for designing the communication using the VAR_ACCESS keyword
between application programs is provided. Independent of the mapping of programs onto a
single device or different devices, identical communication features can be used between
two programs. This facilitates the reuse of software in different environments.
e) Graphical or textual languages may be chosen by the designer, according to the
requirements of the application. These languages, plus a set of textual and graphical
common elements, support software design methodologies based on well-understood
models.
1) The graphical Ladder Diagram (LD) language models networks of simultaneously
functioning electromechanical elements such as relay contacts and coils, timers,
counters, etc.
2) The graphical Function Block Diagram (FBD) language models networks of
simultaneously functioning electronic elements such as adders, multipliers, shift
registers, gates, etc.
3) The Structured Text (ST) language models typical information processing tasks such
as numerical algorithms using constructs found in general-purpose high level
languages such as Pascal.
4) The Instruction List (IL) language models the low-level programming of control systems
in assembly language.
In IEC 61131-3:2013, IL is marked as deprecated for the next release, because an
assembler like language is not up-to-date in modern development environments.
5) A set of graphical and textual common elements provides rules for defining values and
variables, features for software configuration and object declaration. The common
elements include graphical and textual elements for the construction of program
organization units.
6) Sequential Function Charts (SFCs), which model time- and event-driven s
...