IEC 61131-5:2000

INTERNATIONAL IEC
STANDARD
61131-5
First edition
2000-11
Programmable controllers –
Part 5:
Communications
Automates programmables –
Partie 5:
Communications
Reference number
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INTERNATIONAL IEC
STANDARD
61131-5
First edition
2000-11
Programmable controllers –
Part 5:
Communications
Automates programmables –
Partie 5:
Communications
 IEC 2000  Copyright - all rights reserved
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– 2 – 61131-5  IEC:2000(E)
CONTENTS
Page
FOREWORD . 6
Clause
1 Scope . 8
2 Normative references . 8
3 Definitions. 9
4 Symbols and abbreviations. 11
5 Models . 11
5.1 PC network communication model. 11
5.2 PC functional model . 12
5.3 PC hardware model. 14
5.4 Software model . 14
6 PC communication services. 15
6.1 PC subsystems and their status . 15
6.2 Application specific functions. 22
7 PC communication function blocks . 28
7.1 Overview of the communication function blocks . 28
7.2 Semantic of communication FB parameters. 29
7.3 Device verification. 34
7.4 Polled data acquisition . 38
7.5 Programmed data acquisition . 41
7.6 Parametric control. 51
7.7 Interlocked control . 54
7.8 Programmed alarm report . 61
7.9 Connection management. 69
7.10 Example for the use of communication function blocks . 73
8 Compliance and implementer specific features and parameters . 76
8.1 Compliance. 76
8.2 Implementation specific features and parameters . 77
Annex A (normative) Mapping to ISO/IEC 9506-5. 78
Annex B (normative) PC behavior using ISO/IEC 9506-2. 98
Figure 1 – Scope of this part of IEC 61131. 8
Figure 2 – PC communication model. 12
Figure 3 – Programmable controller functional model . 13
Figure 4 – Programmable controller hardware model. 14
Figure 5 – PC software model. 15
Figure 6 – Programmable controller power supply . 19
Figure 7 – Type description of status information . 21
Figure 8 – Interlocked control timeline. 24
Figure 9 – Function REMOTE_VAR . 31

61131-5  IEC:2000(E) – 3 –
Figure 10 – Principle of status signalling . 32
Figure 11 – Timing diagram of the ERROR and STATUS outputs . 32
Figure 12 – STATUS function block . 34
Figure 13 – USTATUS function block . 35
Figure 14 – Timing diagram of the STATUS function block . 35
Figure 15 – State diagram of STATUS function block . 36
Figure 16 – State diagram of USTATUS function block. 37
Figure 17 – READ function block . 39
Figure 18 – Timing diagram of READ function block. 39
Figure 19 – State diagram of READ function block . 40
Figure 20 – Programmed data acquisition data flow . 41
Figure 21 – USEND function block . 42
Figure 22 – URCV function block . 42
Figure 23 – Timing diagram of USEND and URCV function blocks . 43
Figure 24 – State diagram of USEND function block. 43
Figure 25 – State diagram of URCV function block . 45
Figure 26 – BSEND function block . 47
Figure 27 – BRCV function block . 48
Figure 28 – Timing diagram of BSEND and BRCV function blocks. 48
Figure 29 – State diagram of BSEND function block. 49
Figure 30 – State diagram of BRCV function block . 50
Figure 31 – WRITE function block. 52
Figure 32 – Timing diagram of WRITE function block . 53
Figure 33 – State diagram of WRITE function block . 53
Figure 34 – SEND function block . 55
Figure 35 – RCV function block. 56
Figure 36 – Timing diagram of SEND and RCV function blocks . 57
Figure 37 – State diagram of SEND function block . 58
Figure 38 – State diagram of RCV function block . 60
Figure 39 – NOTIFY function block . 62
Figure 40 – ALARM function block . 63
Figure 41 – Timing diagram of ALARM function block . 64
Figure 42 – State diagram of NOTIFY function block . 65
Figure 43 – State diagram of ALARM function block. 67
Figure 44 – CONNECT function block . 69
Figure 45 – Timing diagram of CONNECT function block . 70
Figure 46 – State diagram of CONNECT function block. 71
Figure 47 – Example in function block diagram language . 76
Table 1 – Status presenting entities . 16
Table 2 – PC summary status . 17
Table 3 – Status of I/O subsystem . 18
Table 4 – Status of processing unit . 18

– 4 – 61131-5  IEC:2000(E)
Table 5 – Status of power supply . 19
Table 6 – Status of memory. 19
Table 7 – Status of communication subsystem . 20
Table 8 – Status of implementer specific subsystem . 20
Table 9 – Presentation of status information . 21
Table 10 – Device verification features . 23
Table 11 – Data acquisition features . 23
Table 12 – Control features . 24
Table 13 – Alarm reporting features . 25
Table 14 – Startable and stoppable units . 25
Table 15 – Meaning of I/O State . 26
Table 16 – I/O state. 26
Table 17 – Execution and I/O control features. 26
Table 18 – Loadable units. 27
Table 19 – Application program transfer features . 27
Table 20 – Connection management features . 28
Table 21 – Overview of the communication function blocks . 28
Table 22 – Semantic of communication FB parameters . 30
Table 23 – Values of the SCOPE parameter. 31
Table 24 – Value and interpretation of the STATUS output . 33
Table 25 – Transitions of the STATUS state diagram . 36
Table 26 – Action table for STATUS state diagram. 36
Table 27 – Transitions of USTATUS state diagrams. 37
Table 28 – Action table of USTATUS state diagram . 37
Table 29 – Transitions of the READ state diagram . 40
Table 30 – Action table for READ state diagram. 41
Table 31 – Transitions of the USEND state diagram. 44
Table 32 – Action table for USEND state diagram . 44
Table 33 – Transitions of URCV state diagrams . 45
Table 34 – Action table of URCV state diagram. 46
Table 35 – Transitions of the BSEND state diagram . 49
Table 36 – Action table for BSEND state diagram . 50
Table 37 – Transitions of BRCV state diagrams . 51
Table 38 – Action table of BRCV state diagram . 51
Table 39 – Transitions of the WRITE state diagram . 54
Table 40 – Action table for WRITE state diagram . 54
Table 41 – Transitions of the SEND state diagram . 58
Table 42 – Action table for SEND state diagram. 59
Table 43 – Transitions of RCV state diagrams . 60
Table 44 – Action table of RCV state diagram . 61
Table 45 – Transitions of the NOTIFY state diagram . 65
Table 46 – Action table for NOTIFY state diagram. 66
Table 47 – Transitions of the ALARM state diagram . 68

61131-5  IEC:2000(E) – 5 –
Table 48 – Action table for ALARM state diagram . 68
Table 49 – Transitions of the CONNECT state diagram . 72
Table 50 – Action table for CONNECT state diagram . 73
Table 51 – Table titles and relevant tables for compliance. 76
Table 52 – Implementation specific features and parameters . 77
Table A.1 – Type description mapping . 81
Table A.2 – Mapping of the SCOPE and SC_ID parameter . 81
Table A.3 – Size prefix of direct representation . 82
Table A.4 – Transition mapping of the STATUS state diagram . 84
Table A.5 – Action mapping for STATUS state diagram. 84
Table A.6 – Transition mapping of USTATUS state diagram . 84
Table A.7 – Action mapping of USTATUS state diagram . 84
Table A.8 – Transition mapping of the READ state diagram. 85
Table A.9 – Action mapping for READ state diagram. 85
Table A.10 – Transition mapping of the USEND state diagram . 86
Table A.11 – Action mapping for USEND state diagram . 86
Table A.12 – Transition mapping of URCV state diagram . 86
Table A.13 – Action mapping for URCV state diagram. 87
Table A.14 – Transition mapping of the BSEND state diagram . 87
Table A.15 – Action mapping for BSEND state diagram. 88
Table A.16 – Transition mapping of BRCV state diagram . 88
Table A.17 – Action mapping for BRCV state diagram . 89
Table A.18 – Transition mapping of the WRITE state diagram . 90
Table A.19 – Action mapping for WRITE state diagram . 90
Table A.20 – Transition mapping of the SEND state diagram. 90
Table A.21 – Action mapping for SEND state diagram . 91
Table A.22 – Transition mapping of RCV state diagram. 91
Table A.23 – Action mapping of RCV state diagram . 92
Table A.24 – Transition mapping of the NOTIFY state diagram. 94
Table A.25 – Action mapping for NOTIFY state diagram. 94
Table A.26 – Transition mapping of the ALARM state diagram . 95
Table A.27 – Action mapping for ALARM state diagram. 95
Table A.28 – Transitions of the CONNECT state diagram. 96
Table A.29 – Action mapping for CONNECT state diagram. 96
Table A.30 – Implementation specific features and parameters . 97
Table B.1 – CreateProgramInvocation service defaults. 98
Table B.2 – Program Invocation service defaults for I/O State parameter . 98
Table B.3 – Implementation specific features and parameters . 99

– 6 – 61131-5  IEC:2000(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PROGRAMMABLE CONTROLLERS –
Part 5: Communications
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61131-5 has been prepared by subcommittee 65B: Devices, of IEC
technical committee 65: Industrial-process measurement and control.
The text of this standard is based on the following documents:
FDIS Report on voting
65B/411/FDIS 65B/420/RVD
Full information on the voting for the approval of this standard 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 3.
This part should be read in conjunction with the other parts of IEC 61131. IEC 61131 consists
of the following parts under the general title: Programmable controllers.
Part 1:1992, General information.
Part 2:1992, Equipment requirements and tests.
Part 3:1993, Programming languages.
Part 4:1994, User guidelines (published as technical report IEC TR 61131-4)
Part 5:2000, Communications
Part 8:2000, Guidelines for the application and implementation of programming languages
(published as technical report IEC TR 61131-8)

61131-5  IEC:2000(E) – 7 –
Annexes A and B form an integral part of this standard.
Annex C is for information only.
Where a conflict exists between this and other IEC standards (except basic safety standards),
the provisions of this standard should be considered to govern in the area of programmable
controllers and their associated peripherals.
The committee has decided that the contents of this publication will remain unchanged until
2006. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual version of this standard may be issued at a later date.

– 8 – 61131-5  IEC:2000(E)
PROGRAMMABLE CONTROLLERS –
Part 5: Communications
1 Scope
This part of IEC 61131 specifies communication aspects of a programmable controller. It
specifies from the viewpoint of a PC how any device can communicate with a PC as a server
and how a PC can communicate with any device. In particular, it specifies the behavior of the
PC as it provides services on behalf of other devices and the services the PC application
program can request from other devices. It is not intended to specify how any device can
communicate with any device using a PC as a router or gateway. The behavior of the PC as a
communication client and server is specified independent of the particular communication
subsystem, but the communication functionality may be dependent on the capabilities of the
communication subsystem used.
Scope of IEC 61131-5
PC
Any device Any device
IEC  2247/2000
Figure 1 – Scope of this part of IEC 61131
The scope of this part is a subset of the "communication model" shown in figure 2 of
IEC 61131-3; namely figures 2c and 2d are included in the scope of this part. Additionally, the
means defined in this part of IEC 61131 may be used for communications within a program or
between programs.
The mapping of the PC behavior to some particular communications subsystems is provided in
the annexes.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 61131. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 61131 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of ISO and IEC maintain
registers of currently valid International Standards.
IEC 60050-351:1998,
International Electrotechnical Vocabulary – Part 351: Automatic control
IEC 61131-1:1992, Programmable controllers – Part 1: General Information
IEC 61131-2:1992, Programmable controllers – Part 2: Equipment requirements and tests
IEC 61131-3:1993, Programmable controllers – Part 3: Programming languages
ISO/IEC 2382-1:1993, Information technology – Vocabulary – Part 1: Fundamental terms
ISO/IEC 9506-1:1990, –
Industrial automation systems Manufacturing Message Specification –
Part 1: Service definition
61131-5  IEC:2000(E) – 9 –
ISO/IEC 9506-2:1990, Industrial automation systems – Manufacturing Message Specification –
Part 2: Protocol specification
3 Definitions
For the purpose of this part of IEC 61131, the following definitions apply.
This part of IEC 61131 is based on the concepts of parts 1 to 3 of IEC 61131 and makes use of
the following terms defined in other international standards.
Definitions from other publications
IEC 60050-351
control
monitoring
IEC 61131-1
application program (2.1)
application program archiving (4.6.4)
cold restart (2.56)
input (2.25)
main processing unit (2.32)
modifying the application program (4.6.2.6)
output (2.40)
programmable controller (2.50)
programmable controller system (2.51)
testing the application program (4.6.2.5)
warm restart (2.56)
IEC 61131-3
access path (1.3.2)
direct representation (1.3.23)
invocation (1.3.43)
program (verb, 1.3.60)
sub-element (2.3.3.1)
ISO/IEC 2382-1
data
ISO/IEC 9506-1
client
download
event (clause 15)
server
uninterruptible variable access (12.1.1.1)
upload
variable
– 10 – 61131-5  IEC:2000(E)
Definitions of this part
3.1
alarm
event which signals a specific condition
3.2
data acquisition
collection of data for the purpose of process monitoring and report generation
3.3
direct operator interface
when the client can communicate to the operator interface via the communication system with
no application program interaction
3.4
device verification
allows other devices to determine if the PC is able to perform its intended function in the
control system
3.5
health
the health of a PC or its subsystems is specified by returning one, and only one, of the three
possible values. They are, in order of decreasing health: GOOD, WARNING and BAD
3.6
interlocked control
control through the synchronization of data exchanges between two parties. At various points in
time, one party is waiting for the other party to deliver some expected data
3.7
local
internal to the PC; opposite of remote
3.8
parametric control
control by the client writing to control variables residing in the PC
3.9
processing unit
part of the main processing unit. It is the portion of a PC system which is responsible for the
storage of the application program and data and the execution of the application program.
A PC system has one or more processing units
3.10
program verification
testing of a PC application program to verify that it performs the function(s) it was designed to
do in the process environment
3.11
recipe
description of procedures, or data for those procedures, or both, for making a product which
uses the process or machinery that the controller is attached to, which is different from a
previous product
61131-5  IEC:2000(E) – 11 –
3.12
remote
external to the PC; opposite of local
3.13
state
the state of the PC system is indicated by a list of attributes, each of which may be TRUE or
FALSE. Zero, one, or more of these attributes may be TRUE at the same time
3.14
unsolicited
performed without an explicit request
4 Symbols and abbreviations
These are some abbreviations frequently used in this part of IEC 61131. These terms are
defined or referenced in clause 3 of this part of IEC 61131.
CFB Communication function block
FB Function block
I/O Input and output
IEC International Electrotechnical Commission
ISO International Organization for Standardization
MMS Manufacturing Message Specification, ISO/IEC 9506-1 and ISO/IEC 9506-2
OSI Open Systems Interconnection
PADT Programming and debugging tool
PC Programmable controller
PU Processing unit
5 Models
This clause specifies the models which are used in the remainder of this part of IEC 61131.
5.1 PC network communication model
A programmable controller supplies some specific application functions to the rest of the
control system. It may also request functions from other programmable controllers. The
communication functions defined in this part of IEC 61131 are based on a communication
subsystem that can report communication errors to the signal processing function of the PC
(see 5.2).
The following diagram illustrates the devices in a communication network, showing three
possible devices that request PC functions (clients) from PC 2. The two highlighted PCs are in
the scope of this part of IEC 61131.

– 12 – 61131-5  IEC:2000(E)
Supervisory
controller
Client
Communication system
Client Client Server
Other-end system Programmable Programmable
which talks to PC controller 1 controller 2
Machinery or
process
IEC  2248/2000
NOTE From the communication viewpoint the 'supervisory controller' and the 'other-end system which talks to PC'
mentioned in this figure exhibit the same behavior to a PC communication server, i.e., they submit requests to the
PC2.
Figure 2 – PC communication model
A PC may use its client function to communicate with any device if it behaves like a PC.
5.2 PC functional model
A PC consists of several functions (see figure 3). For a PC within the scope of this part of
IEC 61131, at least one communication function is present.
The following diagram is taken from IEC 61131-1, figure 1. It is designed to illustrate some of
the subsystems of a typical PC.

61131-5  IEC:2000(E) – 13 –
Other systems
MAN-MACHINE
INTERFACE
Operator
Communication functions
functions
Mains Power
Programming,
supply supply APPLICATION
debugging, and
function programmer
testing functions
Signal
OPERATING
processing
SYSTEM
function
functions
APPLICATION APPLICATION
PROGRAM PROGRAM
execution storage functions
DATA
storage
functions
INTERFACE functions to
sensors and actuators
Machine  /  Process
IEC  2249/2000
Figure 3 – Programmable controller functional model
There is a function that is part of the PC system, but usually external to the PC itself, known as
the programming and debugging tool (PADT). The PADT is modelled as interacting with the PC
via the communications function.
The Interface Function to Sensors and Actuators can have I/O which are local or remote to the
Main Processing Unit (see 5.3 for the hardware model). The Interface Function to Sensors and
Actuators has two attributes for each Application Program which defines how the PC is
monitoring and controlling the machine/process. The input attribute has the following states:
• inputs provided to the Application Program are being supplied by the sensors,
• inputs provided to the Application Program are being held in the current state.
The output attribute has the following states:
• the actuators are being controlled by the Application Program,
• the actuators are being held in the current state.

– 14 – 61131-5  IEC:2000(E)
5.3 PC hardware model
The following figure shows the PC hardware model. It shows the modules that make up a PC.
A PC subsystem consists of one or more modules. The following figure corresponds to figure
B.1 of IEC 61131-1 and figure 1 of IEC 61131-2.
Peripherals
Remote I/O station(s)
Main processing unit
Input module(s)
Memory(ies)
and Output module(s)
processing unit(s)
Communication module(s)
Power supply unit(s)
Implementer-specific subsystem(s)
IEC  2250/2000
Figure 4 – Programmable controller hardware model
5.4 Software model
Figure 5 shows the PC software model defined in IEC 61131-3, figure 1. It illustrates the basic
high-level language elements of the PC programming languages and their interrelationships.
These consist of elements which are programmed using the languages defined in IEC 61131-3,
i.e. programs and function blocks; and configuration elements, namely, configurations,
resources, tasks, global variables, and access paths, which support the installation of
programmable controller programs into programmable controller systems.
A configuration is the language element which corresponds to a programmable controller
system as defined in IEC 61131-1. A resource corresponds to a "signal processing function"
and its "man-machine interface" and "sensor and actuator interface" functions (if any) as
defined in IEC 61131-1. A configuration contains one or more resources, each of which
contains one or more programs executed under the control of zero or more tasks. A program
may contain zero or more function blocks or other language elements as defined in
IEC 61131-3.
Configurations and resources can be started and stopped via the "operator interface",
"programming, testing, and monitoring", or "operating system" functions defined in
IEC 61131-1. The mechanisms for the starting and stopping of configurations and resources
via communication services are defined in this part of IEC 61131.
Programs, resources, global variables, access paths (and their corresponding access
privileges), and configurations can be loaded or deleted by the "communication function"
defined in IEC 61131-1. The loading or deletion of a configuration or resource shall be
equivalent to the loading or deletion of all the elements it contains.
Access paths and their corresponding access privileges allow to access variables of a PC via

communication services.
61131-5  IEC:2000(E) – 15 –
Configuration
Resource Resource
Task Task Task Task
Program Program Program Program
FB FB FB FB
Global and directly represented variables
Access paths
Communication function
Execution control path
or Variable access paths
FB
Function block
Variable
IEC  2251/2000
NOTE 1 This figure is illustrative only. The graphical representation is not normative.
NOTE 2 In a configuration with a single resource, the resource need not be explicitly represented.
Figure 5 – PC software model
6 PC communication services
This clause describes the concept of status information of a PC and provides a specification of
the services the PC provides to the control system via the communication subsystem. (The
next clause specifies how the PC application program can use the communication subsystem
to interact with other devices.)
6.1 PC subsystems and their status
A PC can provide status, which includes state information and fault indications.
Status can be reported on some of the subsystems identified in the following figure. In addition,
there is a summary status that provides general information about the PC.

– 16 – 61131-5  IEC:2000(E)
Table 1 – Status presenting entities
No. Status presenting entities
1 PC (as a whole)
2 I/O subsystem (includes Input and Output modules and other intelligent I/O devices)
3 Processing unit
4 Power supply subsystem
5 Memory subsystem
6 Communication subsystem
7 Implementer specific subsystems
NOTE The status is intended to provide information about the controller including its
hardware and firmware subsystems, not considering configuration information. It is not
intended to provide information about the controlled process nor the PC application program.
The status data contains information concerning the state and the health of the PC and its
subsystems.
There are two concepts used in this part of IEC 61131 related to status: health and state. The
"health" of a PC or its subsystems is specified by returning one and only one of the three
possible values. The semantics associated with each value is specified below. They are, in
order of decreasing health:
a) GOOD – If TRUE, the PC (or the specified subsystem) has not detected any problems
which would prohibit it from performing the intended function;
b) WARNING – If TRUE, the PC (or the specified subsystem) has not detected any problems
which would prohibit it from performing the intended function, but it has detected at least
one problem which could place some limits on its abilities. The limit may be time,
performance, etc. (see the following statements for further definition of these limits);
c) BAD – If TRUE, the PC (or the specified subsystem) has detected at least one problem
which could prohibit it from performing the intended function.
The "state" of the PC system is indicated by a list of attributes, each of which may be TRUE or
FALSE. Zero, one, or more of these attributes may be TRUE at the same time. The semantics
associated with each attribute is specified in the remainder of this clause.
Each of the status information can also have implementer specified attributes. Some examples
of implementer specified attributes are:
a) additional error diagnostics (e.g. EEPROM write cycles exceeded);
b) additional operational states (e.g. auto-calibrate enabled);
c) local key status (e.g. auto-restart required).
Implementations are not required to provide subsystem status. All instances of similar types of
subsystems present in a system are reported separately. The name of the subsystem can be
provided to allow differentiating subsystems of the same type.
6.1.1 PC summary status
The PC provides the following summary status information.

61131-5  IEC:2000(E) – 17 –
Table 2 – PC summary status
No. Item Description
1 Health GOOD All subsystems in the PC indicate a GOOD health condition
2 WARNING At least one subsystem indicates a WARNING health condition and no sub-
system indicates a BAD health condition
3 BAD At least one subsystem indicates a BAD health condition
4 Running If TRUE, this attribute indicates if at least one part of the user application has been loaded
and is under control of the PC
5 Local control If TRUE, this attribute indicates if local override control is active. If active, the ability to
control a PC and its subsystems from the network may be limited. For example, this could
be closely tied to the use of a local key switch
6 No outputs If TRUE, this attribute indicates that the PC can change the physical state of all outputs as
disabled a result of application program execution or other means. If not TRUE, the physical state of
some of the outputs are not affected (logical state may be affected). This is typically used
in the testing and modifying of application programs in the PC
7 No inputs If TRUE, this attribute indicates that the PC can access the physical state of all inputs as a
disabled result of application program execution or other means. If not TRUE, the physical state of
some inputs cannot be accessed. This is typically used in the testing and modifying of
application programs where the inputs can be simulated
8 Forced If TRUE, this attribute indicates that at least one I/O point associated with the PC has been
forced. When an Input is forced, the application program will receive the value specified by
the PADT instead of the actual value from the machine or process. When an output is
forced, the machine or process will receive the value specified by the PADT instead of the
value generated by execution of the application program. When a variable is forced, the
application program will use the value specified by the PADT instead of that generated by
the normal program execution
9 User application If TRUE, this attribute indicates that the Processing Unit has at least one user application
present present
10 I/O subsystem If TRUE, this attribute indicates "WARNING" or "BAD" which is caused by an I/O subsystem
11 Processing unit If TRUE, this attribute indicates "WARNING" or "BAD" which is caused by a processing unit
subsystem subsystem
12 Power supply If TRUE, this attribute indicates "WARNING" or "BAD" which is caused by a power supply
subsystem subsystem
13 Memory If TRUE, this attribute indicates "WARNING" or "BAD" which is caused by a memory
subsystem subsystem
14 Communication If TRUE, this attribute indicates "WARNING" or "BAD" which is caused by a communication
subsystem subsystem
15 Implementer If TRUE, this attribute indicates "WARNING" or "BAD" which is caused by an implementer
specified specified subsystem
subsystem
6.1.2 I/O subsystem
The PC provides the following status information of its I/O subsystem.

– 18 – 61131-5  IEC:2000(E)
Table 3 – Status of I/O sub
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