EN 60870-5-5:1995

SLOVENSKI STANDARD
01-avgust-1997
Telecontrol equipment and systems - Part 5: Transmission protocols - Section 5:
Basic application functions (IEC 870-5-5:1995)
Telecontrol equipment and systems -- Part 5: Transmission protocols - Section 5: Basic
application functions
Fernwirkeinrichtungen und -systeme -- Teil 5: Übertragungsprotokolle - Hauptabschnitt 5:
Grundlegende Anwendungsfunktionen
Matériels et systèmes de téléconduite -- Partie 5: Protocoles de transmission - Section 5:
Fonctions d'application de base
Ta slovenski standard je istoveten z: EN 60870-5-5:1995
ICS:
33.200 Daljinsko krmiljenje, daljinske Telecontrol. Telemetering
meritve (telemetrija)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

NORME CEI
INTERNATIONALE
I EC
870-5-5
INTERNATIONAL
Première édition
STANDARD
First edition
1995-06
Matériels et systèmes de téléconduite —
Partie 5:
Protocoles de transmission —
Section 5: Fonctions d'application de base
Telecontrol equipment and systems —
Part 5:
Transmission protocols
—
Section 5: Basic application functions
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870-5-5 © IEC:1995 – 3 –
CONTENTS
Page
FOREWORD 5
INTRODUCTION 7
Clause
1 Scope and object
2 Normative references 9
3 Definitions 11
4 Application services 13
4.1 Application service primitives 13
5 15
General concept of application functions
6 Basic application functions 17
6.1 Station initialization 21
6.2 Data acquisition by polling 49
6.3 Cyclic data transmission 53
6.4 Acquisition of events 55
6.5 Acquisition of events by quick-check procedures 57
6.6 General interrogation – Outstation interrogation 61
6.7 Clock synchronization 65
6.8 Command transmission 69
6.9 Transmission of integrated totals (telecounting) 75
6.10 Parameter loading 79
6.11 Test procedure 83
6.12 File transfer 85
6.13 Acquisition of transmission delay 99

870-5-5 © IEC:1995 - 5 -
INTERNATIONAL ELECTROTECHNICAL COMMISSION
TELECONTROL EQUIPMENT AND SYSTEMS -
Part 5: Transmission protocols -
Section 5: Basic application functions
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 cooperation 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, prepared by technical committees on
which all the National Committees having a special interest therein are represented, express, as nearly as
possible, an international consensus of opinion on the subjects dealt with.
3) They have the form of recommendations for international use published in the form of standards, 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.
International Standard IEC 870-5-5 has been prepared by IEC technical committee 57:
Power system control and associated communications.
The text of this standard is based on the following documents:
DIS Report on voting
57/227/RVD
57/200/DIS
Full information on the voting for the approval of this standard can be found in the report
on voting indicated in the above table.

870-5-5 © IEC:1995 – 7 –
INTRODUCTION
This section of IEC 870-5 specifies an assortment of basic application functions for use in
telecontrol systems.
- 9 -
870-5-5 © IEC:1995
TELECONTROL EQUIPMENT AND SYSTEMS -
Part 5: Transmission protocols -
Section 5: Basic application functions
1 Scope and object
This section of IEC 870-5 applies to telecontrol equipment and systems with coded bit
serial data transmission for monitoring and controlling geographically widespread
processes. It defines basic application functions that perform standard procedures for tele-
control systems. Basic application functions are application procedures that reside beyond
layer 7 (application layer) of the ISO reference model for open communication systems.
The defined application procedures utilize standard services of the application layer. The
e as basic standards for different companion standards
specifications of this section se rv
that will be elaborated in detail for specific telecontrol tasks. Each companion standard
may use a specific selection of the defined functions. Basic application functions, which
are not in this section but are found necessary for defining telecontrol companion
standards, should be specified in these companion standards. Only the definition of
companion standards will enable interoperability among compatible telecontrol equipment.
rvice data units (ASDUs) used by procedures
The general structure of application se
specified in this section are defined in IEC 870-5-3.
Standards specified in this section are compatible with standards defined in sections 1 to 4
of IEC 870-5 (see clause 2).
2 Normative references
The following normative documents contain provisions which, through reference in this
text, constitute provisions of this section of IEC 870-5. At the time of publication, the
editions indicated were valid. All normative documents are subject to revision, and pa rties
to agreements based on this section of IEC 870-5 are encouraged to investigate the
possibility of applying the most recent editions of the normative documents indicated
below. Members of IEC and ISO maintain registers of currently valid International
Standards.
IEC 50 (371): 1984, International Electrotechnical Vocabulary (IEV) - Chapter 371: Tele-
control
IEC 870-1-1: 1988, Telecontrol equipment and systems - Part 1: General considerations -
Section One: General principles
IEC 870-5-1: 1990, Telecontrol equipment and systems - Part 5: Transmission protocols -
Section One: Transmission frame formats

870-5-5 © IEC:1995 - 11 -
rt 5: Transmission protocols -
IEC 870-5-2: 1992, Telecontrol equipment and systems - Pa
Section 2: Link transmission procedures
Telecontrol equipment and systems - Part 5: Transmission protocols -
IEC 870-5-3: 1992,
Section 3: General structure of application data
Telecontrol equipment and systems - Part 5: Transmission protocols -
IEC 870-5-4: 1993,
Section 4: Definition and coding of application information elements
Information processing systems - Open Systems Interconnection - Basic
ISO 7498: 1984,
Reference Model
3 Definitions
For the purpose of this section of IEC 870-5, the following definitions apply:
3.1 basic application function (in telecontrol): Transmission procedure that performs
a supervisory or control function that is generally used in telecontrol systems.
Examples: command transmission, event transmission, cyclic transmission, etc.
3.2 companion standard: A companion standard adds semantics to the definitions of
the basic standard or a functional profile. This may be expressed by defining particular
uses for information objects or by defining additional information objects, service
procedures and parameters of the basic standard.
NOTE – Companion standards do not alter the standards to which they refer, but make explicit the relation-
ship between those used together for a specific domain of activity.
3.3 enhanced performance architecture (EPA): A protocol reference model that provides,
compared with the full seven-layer architecture according to the OSI basic reference
model (ISO 7498), a three-layer architecture for obtaining faster response times for the
critical information, but with service limitations.
A sequence of data fields with successive bit allocations
3.4 compound data field (CP):
that forms an information element.
The direction of transmission from the controlling station to a
3.5 control direction:
controlled station.
3.6 monitor direction: The direction of transmission from a controlled station to the
controlling station.
– 13 –
870-5-5 © IEC:1995
4 Application se rvices
Each application process can have a "primary application function" and a "secondary
application function". A "primary application function" is part of an application process that
initiates application requests to a remote application process by means of a "secondary
application function" belonging to the latter. Requested tasks are executed via communi-
cation services that involve the transmission of protocol data units (PDUs). Sequential
procedures of communication services are described by means of sequences of service
primitives.
4.1 Application service primitives
A primary application initiates a function by a service request primitive "request".
Confirmed application services require responses from the secondary application. The
secondary application returns associated responses by service response primitives that
are delivered to the primary application by service confirm primitives (see figure 1).
Service.request ( .req) The primary application initiates a request by this service
primitive to a remote secondary application function via the
communication services.
ices use this service primitive to
Service.indication ( .ind) The communication se rv
deliver the service indication request to the secondary appli-
cation function.
Service.response ( .res) The secondary application function uses this service primitive
to respond to a request from the communication se rvices.
Service.confirm ( .con) The communication se rvices use this service primitive to
deliver the response of the secondary to the primary applica-
tion function.
Unconfirmed application service:
COMMUNICATION
SERVICES
PRIMARY SECONDARY
SERVICE.req
REQUEST PDU
APPLICATION APPLICATION
SERVICE.ind
FUNCTION
FUNCTION
Confirmed application service:
REQUEST_PDU
SERVICE.req
SECONDARY
PRIMARY
SERVICE.ind
r
APPLICATION APPLICATION
SERVICE.res
SERVICE.con
FUNCTION
FUNCTION
RESPOND_PDU
Figure 1 — Basic application services

870-5-5 © IEC:1995 -15 -
5 General concept of application functions
Application processes that involve peer-to-peer communication to perform coordinated
procedures between remote locations use means that are offered in the layers 7, 2 and 1
of the EPA model (see figure 2).
More than one procedure (in different stations) may be in progress at the same time.
However the following application function procedures are described individually. The
procedures are defined in single hierarchical presentation. Additional definitions for the
use of multi-hierarchical telecontrol networks (e.g. network with concentrator station)
should be specified in the companion standards.
The individual application functions use service primitives and elements of transmission
procedures of the layers 7, 2 and 1 that are specified in the sections 1 to 5 of IEC 870-5.
APPLICATION (USER) PROCESSES
Function 1 Function 2 Function 3
Function 4 Function n
APPLICATION (LAYER 7)
EPA model
(communication
LINK (LAYER 2)
services)
PHYSICAL (LAYER 1)
Figure 2 - Location of communication services and application functions in
the EPA (enhanced performance architecture) model
Application functions are the part of application processes which perform the remote
communication procedures between application processes.
The following clauses of this section of IEC 870-5 define an assortment of basic appli-
cation functions. Each function is composed of transfer procedures of specific ASDUs
between remotely communicating application processes. Information contents, frame
formats of the various PDUs and parameter lists of service primitives are specified by the
chosen companion standards.
- 17 -
870-5-5 © IEC:1995
6 Basic application functions
This clause defines an assortment of basic application functions that utilize standard
communication services. The functions are described by presenting diagrams that indicate
the sequence of data units that are exchanged between controlling and controlled stations
and by describing the tasks of these data units to accomplish the functions. The first two
described basic application functions, namely station initialization and data acquisition by
polling represent a base for the execution of further basic application functions. These two
functions are performed by coordination of particular application and link services that are
described in detail. Other basic application functions that may involve the utilization of
polling procedures are then described without repeating details about this procedure.
Sequential transmission procedures are described by arrows. Each arrow represents a
protocol data unit PDU. A hierarchic structure of letters will be used for naming APDUs
or ASDUs which may be completed by the different companion standards. In IEC 870-5
protocol definitions ASDUs are the same as APDUs because there is no explicit APCI.
The ASDU-labels specified in the following table follow a hierarchical order, which offers
the possibility of using global labels in this standard and specific ones in the different
companion standards. The highest level distinguishes between:
Label
Kind of information Level 1
Monitored information M
Control information C
Parameter P
File transfer F
The second level defines:
Kind of information Level 2 Label
Monitored information M
Single-point information M_SP
Double-point information M_DP
Measurements M_ME
Events of protection M_EP
Integrated totals M_IT
M_ST
Step position information
Bit and octet strings M_BO
End of initialization M_El
Application layer available M_AA
Control information C
Single command C_SC
Double command C_DC
Set-point command C_SE
Regulating step command C_RC
Interrogation command C_IC
Clock synchronization command C_CS
C_CD
Delay acquisition
Counter interrogation command C_CI
C_TS
Test command
C_RP
Reset process command
C_RD
Read command
C_El
End of initialization
– 19 –
870-5-5 © IEC:1995
P
Parameter
P_ME
Parameter for measurements
Parameter activation PAC
F
File transfer
F_DR
Directory
F_SC
Select or call file, section or directory
F_LS
Last section or segment
F_AF
ACK file or section
File ready F_FR
F_SR
Section ready
F_SG
Segment
The third level is used by the different companion standards and defines the specific
type of the ASDU, the use of a time tag, etc. The first letter of the third level specifies the
availability of a time tag (N = no time tag, T = time tag), the second letter specifies
the type. Each companion standard may define its own types in alphabetical order
beginning with "A". For example:
Measurement, normalized value without time tag (type A) M_ME_NA
or
Measurement, scaled value with time tag (type B) M_ME_TB
or
Single command, type A without time tag C_SC_NA
In addition, a final number is appended to indicate which companion standard defines the
ASDU label. For example:
Companion standard 101 M_ME_NA_1 or C_SC_NA_1
Companion standard 102 M_ME_NA_2 or C_SC_NA_2
This label system is open and may be completed, if necessary, in all hierarchical levels by
the different companion standards.
ASDUs which are used in control direction may be mirrored in monitor direction. These
mirrored ASDUs are used for positive/negative acknowledgments and need to be
distinguished unambiguously in both directions. Therefore, in addition to the labels, these
ASDUs are marked with the following abbreviations in control and monitor direction.
ACT
Control direction: Activation
Monitor direction: Activation confirmation ACTCON
DEACT
Control direction: Deactivation
Monitor direction: Deactivation confirmation DEACTCON
ACTTERM
Monitor direction: Activation termination
In addition, the following abbreviations are used:

Monitor direction: Cyclic transmission CYCLIC

SPONT
Monitor direction: Spontaneous transmission
When unbalanced transmission procedures are used, the ACT may be transmitted by a link
service SEND/NO REPLY as a broadcast message (e.g. for station interrogation or clock
synchronization). Then the ACTCON has to be transmitted back individually by each
controlled station that received the ACT.

- 21 -
870-5-5 ©IEC:1995
6.1 Station initialization
Station initialization procedures are required to set stations into correct operating states
before the beginning of application-dependent telecontrol operations. It is necessary
to distinguish between cold-start and warm-start procedures. A cold start is a primary
bootstrap procedure of a station which means that information on process variables is
cleared before starting to update the data base to the actual states. A warm start is a
rebootstrapping procedure of a station that is reset or reactivated, which means that the
information on process variables that was acquired before the reactivation is not cleared.
A further distinction is the initialization of controlling and controlled stations. The following
specifications consider mainly initialization procedures that involve data transmission
between stations.
Controlling stations are frequently equipped with redundant control and data-base
equipment that guarantees a switch-over without loss of information in case of failures
of the active control equipment. In this case, there is no need to initialize a general
interrogation to update the data base of the controlling station. However, after a power-up
or a general reset of the whole controlling station, the general interrogation (see 6.6
and IEV 371-04-05) and, in some systems, clock synchronization (see 6.7) procedures are
indispensable.
The controlled station may be reset by a local command or via a request from the
controlling station.
6.1.1 Description of general initialization procedure (see figure 3)
Figure 3 describes the initialization of the controlling and controlled station in general.
Detailed definitions including the communication services used are given in subsequent
figures.
Initialization of controlling station
After the internal initialization of the controlling station the link layer establishes connec-
tions to the controlled stations (see 6.1.2, 6.1.5 and IEC 870-5-2). When the controlling
station is ready to process information of the controlled stations, it may send C_El (End
of Initialization) to the connected controlled stations (optional). After receiving this C_El
PDU the controlled stations may send process information to the controlling station. The
controlling station then proceeds with the function general interrogation (see 6.6) and
optionally clock synchronization (see 6.7).
Initialization of controlled station
If necessary, after the internal initialization of the controlled station, the link layer
establishes the connection to the controlling station (see 6.1.3, 6.1.6 and IEC 870-5-2).
If the controlled station is ready to process information of the controlling station, it may
send a M_EI PDU to the controlling station (optional). After receiving this PDU the
controlling station proceeds with general interrogation (see 6.6) and - in some systems -
with clock synchronization (see 6.7).

870-5-5 © IEC:1995 - 23 -
Communication Application function of
Application function of
services CONTROLLED STATION
CONTROLLING STATION
INITIALIZATION OF
CONTROLLING STATION
Start of initialization of
controlling
station
Initialization of
communication
services
Initialization
time
Communication
initialized
Controlling
A_ENDINIT.req
station
C_El
initialized
A_ENDINIT.ind
Controlling
If necessary station
initialized
Following functions:
general interrogation
dock
synchronization (opt)
INITIALIZATION OF
CONTROLLED STATION
Reset may be
local
A_RESET_PROCESS.req
Reset of
C RP
controlled
station
A RESET PROCESS.ind
Reset of
controlled station
If necessary
Start of
Initialization of
controlled
Initialization of
station
communication
services
Communication
Initialization
initialized
time
Controlled
M
EI
station
A_ENDINIT.req
initialized
A_ENDINIT.ind
Controlled
If necessary
station
initialized
Following functions:
general interrogation
dock
synchronization (opt)
Figure 3 - Sequential procedure, general Initialization procedure

870-5-5 © IEC:1995 - 25 -
6.1.2 Initialization of the controlling station in unbalanced transmission systems
(description of sequential procedure, see figure 4)
If a "Start local initialization" occurs just after data have been requested from the
controlled station (for example as shown dotted in figure 4), then the link of the controlling
station cannot receive the requested data because it is no longer available. After the start
of initialization of the controlling station, the link layer is usually reset and made available
earlier than the other internal functions of the controlling station during its initialization.
The link of the controlling station then establishes connection to the link of the controlled
station by transmitting a "Request status of link" that is answered by "Status of link". To
accomplish link synchronization, the controlling station transmits a "Reset of remote link"
that is answered by an "ACK". This "ACK" confirms the start condition of the link layer of
the controlled station (next frame count bit FCB = 1 expected, see 5.1.2 of IEC 870-5-2).
The state of the remote link layer may optionally be interrogated by a "Request status of
link". After complete initialization of the application functions in the controlling station, the
connection between the application functions may be established by transmitting a C_El
PDU to the controlled station. The transmission of the C_El PDU is not required in
systems that establish the link connection after the complete initialization of the appli-
cation functions in the controlling station. After the initialization the controlling station
is updated by performing a general interrogation (see 6.6) and proceeds - in some
systems - with clock synchronization (see 6.7). After this, the ordinary telecontrol
operations may begin.
870-5-5 © IEC:1995 - 27 -
Application function of Communication Application function of
CONTROLLING STATION services CONTROLLED STATION
Request data
A_DATA.req
^.—
START of local initialization
e.g. Power on
r--^
r Data
No response,
link is not available,
reset or switched off
Link is
available
Request status
of link
Status of
link
Initialization of
the controlling station
Reset of
remote link
Reset of
the link
ACK
Next FCB = t
expected
Request
status of link
OPTIONAL
Status of
link
A_ENDINIT.req s r data:
C
Controlling station
is initialized
A_ENDINIT.ind
OPTIONAL
Controlling station
is initialized
Following functions:
- - - - represents an example
general interrogation and
clods synchronization (opt)
Figure 4 - Sequential procedure, initialization of the controlling station in
unbalanced transmission systems

870-5-5 © IEC:1995 - 29 -
6.1.3 Local initialization of the controlled station in unbalanced transmission systems
(description of sequential procedure, see figure 5)
After the start of a local initialization of a controlled station during operations with the
controlling station, the controlling station determines that its link is disconnected from
the controlled station due to unconfirmed services. Upon a specified number of unsuccess-
ful repetitions (see IEC 870-5-2, annex A) the controlling station tries to establish the link
connection by transmitting repeated "Request status of link" at specified time-out intervals.
When the link of the controlled station is available, it answers with the "Status of link".
Then the controlling station transmits a "Reset of remote link". The controlled station
confirms the reset condition by an "ACK" to the controlling station (expected frame count
bit FCB = 1, see 5.1.2 of IEC 870-5-2). Then the controlling station may interrogate the
controlled station by repeated "Request status of link". When this is answered by a "Status
of link" which indicates that data class 1 are available, data are requested by a "Request
user data class 1" and may be confirmed either by M_AA (application layer available) or a
M_El (End of initialization). The complete initialization of the application functions in
the controlled station may be indicated to the controlling station by a M_El PDU. Then the
controlling station is updated by performing a general interrogation (see 6.6) and proceeds
- in some systems - with clock synchronization (see 6.7). Then the ordinary telecontrol
operations may begin.
NOTE — M_AA is used when the controlling station has to be informed about the availability of the complete
communication system in addition to the availability of the link layer (which is indicated by the link service
"Status of the link").
870-5-5 ©IEC:1995 - 31 -
Communication Application function of
Application function of
CONTROLLED STATION
se rvices
CONTROLLING STATION
START of local initialization
A_DATA.req
e.g. Power on
User data
Time out and
unsuccessful
retries
No response,
link is not available,
Request
reset or switched off
status of link
Time out
Request
status of link
Time out
Request
status of link
Link
available
Initialization
Status of link
of the
controlled station
Reset of
remote link
Reset of
the link
ACK
Next FCB = 1
expected
Request
status of link
OPTIONAL
Status of link
•
Application
Request user
•
layer
data Bass 1
•
is available
OPTIONAL
User data:
M AA
Request user
data class 1
A_ENDINIT.req
OPTIONAL
Controlled station is
available after local
User data:
initialization
M El
A ENDINIT.ind
Controlled station is
available after local
initialization
Following functions:
general interrogation and
dock synchronization (opt) - - - - represents an example
Figure 5 - Sequential procedure, local initialization of the controlled station in
unbalanced transmission systems

870-5-5 © IEC:1995 - 33 -
6.1.4
Remote initialization of the controlled station in unbalanced transmission
systems (description of sequential procedure, see figure 6)
After receiving a remote RESET_PROCESS command C_RP ACT the controlled station
responds with a RESET_PROCESS confirmation C_RP ACTCON. Upon recognition, or
optional confirmation of the RESET_PROCESS command, all application processes above
layer 7, as defined in figure 2, are reset and initialized. Any messages pending trans-
mission are discarded. The controlling station interrogates the link by transmitting a
"Request status of link". When the link of the controlled station is available, it answers with
"Status of link". The controlling station may optionally transmit a "Reset of remote link" in
addition to the RESET_PROCESS command C_RP ACT. The controlled station confirms
the start condition by an "ACK" to the controlling station (expected frame count bit FCB = 1,
see 5.1.2 of IEC 870-5-2). Then the controlling station may interrogate the controlled
station by repeated "Request status of link".
NOTE — With use of the optional "Reset of remote link", a remote initialization of the complete controlled
station has been performed.
When the "Request status of link" is answered by a "Status of link" which indicates that
data class 1 are available, data are requested by a "Request user data class 1" and may
be confirmed either by M_AA (Application layer available) or a M_EI (End of initialization).
Both of these services are optional in controlled stations where the link is only available
after the end of the complete initialization.
NOTE — The described remote initialization procedure restarts a process function when the application
function of the controlled station is available. In case of unavailability, the complete application user process
(application layer, application functions and application processes) may be restarted via the link service
function "Reset of user process".

$70-5-5 © IEC:1995 -
35 -
Communication
Application
Application function of function of
services
CONTROLLING STATION CONTROLLED STATION
A_RESET_PROCESS.req
C_RP
ACT
A RESET PROCESS.ind
A_RESET_PROCESS.res
C_RP
ACTCON
A_RESET_PROCESS.con OPTIONAL
Request
status of link
Link is available
Status of link
Reset of
remote link
OPTIONAL
Reset of
the link
ACK Initialization of
Next FCB = 1 controlled station
expected
Request
status of link
OPTIONAL
Status of link
•
•
•
Request user
data Gass 1
OPTIONAL
User data:
M AA
Request user
data class 1
A_ENDINIT.req
OPTIONAL
Controlled station is
available after
User data:
remote initialization
M El
A_ENDINIT.ind
Controlled station is
available after
remote initialization
Following functions:
general interrogation and
clock synchronization (opt)
Figure 6 - Sequential procedure, remote initialization of the controlled station in
unbalanced transmission systems

870-5-5 © IEC:1995 - 37 -
6.1.5 Initialization of the controlling station in balanced transmission systems
(description of sequential procedure, see figure 7)
After the start of the initialization of the controlling station, the controlled station deter-
mines that its link is disconnected from the controlling station due to unconfirmed se
rvices.
The controlled station then tries to establish the link connection by transmitting "Request
status of link" at specified time-out intervals. When the link layer of the controlling station
is available, it confirms this condition by a "Status of link" to the controlled station. The
controlled station then transmits a "Reset of remote link" that is answered by an "ACK"
which confirms the reset condition of the link layer of the controlling station (next frame
count bit FCB = 1 expected, see 5.1.2 of IEC 870-5-2). Then the controlling station
synchronizes its link connection with the controlled station by transmitting "Request status
of link" and "Reset of remote link" to it. Upon receipt of the "ACK", the link connection is
established in both directions. The state of the link may be interrogated by "Request
status of link" by both stations (figure 7 shows only interrogation by the controlling
station). After complete initialization of the controlling station, it may transmit C_El (End of
initialization) to the controlled station. The transmission of the C_El PDU is optional in
systems that establish the link connection after the complete initialization of the appli-
cation functions in the controlling station. After the initialization, the controlling station is
updated by performing a general interrogation (see 6.6) and proceeds - in some systems -
with clock synchronization (see 6.7). Then the normal telecontrol operations may begin.

870-5-5 © IEC:1995 - 39 -
Communication Application function of
Application function of
services CONTROLLED STATION
CONTROLLING STATION
Start of local initialization A_DATA.req
^ r
User data
e.g. Power on
Time out
•
Request status
of link
Time out
•
Request status
of link
Time out
No response,
link is not
•
Request status
available, reset
of link
or switched off
Link is
available
Status of link
Reset of
remote link
Reset
of the link
ACK
Next FCB =1
expected
Request status
of link
Initialization of the
controlling station
Status of link
Reset of
remote link
Reset
of the link
ACK
Next FCB = 1
expected
Request status
of link
OPTIONAL
Status of link
A_ENDINIT.req
User data:
C El
Controlling station is
initialized A_ENDINIT.ind
OPTIONAL
Controlling station
Following functions:
is initialized
general interrogation and
clock synchronization
- - - - represents an example
(opt)
Figure 7 - Sequential procedure, initialization of the controlling station
in balanced transmission systems

870-5-5 © IEC:1995 - 41 -
6.1.6 Local initialization of the controlled station in balanced transmission systems
(description of sequential procedure, see figure 8)
After the start of a local initialization of a controlled station during operations with the
controlling station, the controlling station determines that its link is disconnected from
the controlled station due to unconfirmed services. Upon a specified number of unsuccess-
annex A), the controlling station tries to establish the link
ful repetitions (see IEC 870-5-2,
connection by transmitting "Request status of link" at specified time-out intervals. When
the link of the controlled station is available, it answers with the "Status of link". Then the
controlling station transmits "Reset of remote link". The controlled station confirms the
reset condition by an "ACK" to the controlling station (expected frame count bit FCB = 1,
see 5.1.2 of IEC 870-5-2). Then the controlled station also synchronizes its link connection
to the controlling station by transmitting "Request status of link" and "Reset of remote link"
to it. Upon receipt of the "ACK", the link connection is established in both directions.
Optionally the controlled station may then indicate the availability of the application layer
and/or the end of the initialization by transmitting M_AA (Application layer available) and
M_EI (End of initialization) to the controlling station. Both these services are optional in
controlled stations that release the availability of the link after the end of the complete
initialization.
870-5-5 © IEC:1995 - 43 -
Communication
Application function of Application function of
services
CONTROLLING STATION
CONTROLLED STATION
START of local initialization
A_DATA.req
e.g. Power on
DATA PDU
Time out and
unsuccessful
retries No response,
link is not
Request
available, reset
status of link
or switched off
Time out
Request
status of link
Time out
Request
status of link
Link is available
Status of link
Reset of
remote link
Reset
of the link
ACK
Next FCB =1
Initialization of the
expected
controlled station
Request
status of link
Link is available
Status of link
Reset of
remote link
Reset
of the link
ACK
Next FCB = 1
expected
Application
layer is
available
User data:
M_AA
OPTIONAL
A_ENDINIT.req
User data:
M_El
A_ENDINIT.ind OPTIONAL Controlled station is
available after local,
initialization
Controlled station is
available atter local
initialization
Following functions:
general interrogation and
- - - - represents an example
synchronization (opt)
Figure 8 - Sequential procedure, local initialization of the controlled station in
balanced transmission systems
870-5-5 © IEC:1995 - 45 -
6.1.7 Remote initialization of the controlled station in balanced transmission systems
(description of sequential procedure, see figure 9)
After receiving a remote RESET_PROCESS command C_RP ACT, the controlled station
responds with a RESET_PROCESS confirmation C_RP ACTCON and starts initialization
of the processes. The controlling station interrogates the link by transmitting a "Request
status of link". When the link of the controlled station is available, it answers with the
"Status of link". Then the controlling station may transmit a "Reset of remote link" in
addition to the RESET_PROCESS command. The controlled station confirms the start
condition by an "ACK" to the controlling station (expected frame count bit FCB = 1,
see 5.1.2 of IEC 870-5-2). The controlled station synchronizes its link connection to the
controlling station by transmitting "Request status of link" and "Reset of remote link". Then
the controlled station may optionally transmit M_AA (Application layer available) and M_EI
(End of initialization) to the controlling station. Both these services are optional in
controlled stations that release the availability of the link after the end of the complete
initialization.
NOTE — The described remote initialization procedure restarts a process function when the application
function of the controlled station is available. In case of unavailability the complete application user process
may be restarted via the link service function "Reset of user process".

870-5-5 © IEC:1995 - 47 -
Application function of Communication Application function of
CONTROLLED STATION
CONTROLLING STATION services
A_RESET_PROCESS.req
C_RP
ACT
A RESET PROCESS.ind
A_RESET_PROCESS.res
C_RP
ACTCON
OPTIONAL
A_RESET_PROCESS.con
Request
status of link
Link is
available
Status of link
Reset of
remote link
OPTIONAL
Reset
of the link
ACK
Next FCB = 1
expected
Request Initialization of the
status of link controlled station
Link is
available
Status of link
Reset of
remote link
Reset
of the link
ACK
Next FCB = 1
expected
Application
layer is
User data:
available
M AA
OPTIONAL
A_ENDINIT.req
User data:
Controlled station is
M El
A_ENDINIT.ind available after remote
OPTIONAL
initialization
Controlled station is
available after remote
initialization
Following functions:
general interrogation and
clock synchronization (opt)
Figure 9 - Sequential procedure, remote initialization of the controlled station
in balanced transmission systems

870-5-5 © IEC:1995 – 49 –
6.2 Data acquisition by polling
Data acquisition by polling is used in SCADA systems operating with unbalanced data
transmission procedures to update the controlling station with actual states of process
variables in controlled stations. The controlling station performs polling by interrogating
controlled stations sequentially. Controlled stations may transmit only when they are
polled.
Poll sequences are system-dependent. Quiescent telecontrol systems (IEV 371-07-08) use
exclusively sequential polling for events, while pure cyclic data transmission systems use
exclusively sequential polling for cyclic transmission of data. In general, systems admit
both types of polling. Defined poll sequences have to admit dynamic changes caused by
the application process. A usual method is to run a sequential poll procedure for cyclic
data with low priority in the controlling station, which means that it can be interrupted with
event-triggered communication requests such as command transmissions, application-
dependent data requests, etc. A variety of methods may be used to acquire events that
occur in controlled stations. Some systems use alternating or interleaved poll sequences
for events and for cyclic data. Other systems use cyclic poll sequences with the facility of
announcing the presence of events in the returned cyclic data responses.
The selected poll procedure must be transparent to the application process. Therefore
the polling function is performed by the communication services. In the case of balanced
communication systems the polling function is omitted.
6.2.1 Description of sequential procedure (see figure 10)
Figure 10 shows different POLL procedures that may occur in cyclic or non-cyclic POLL
sequences.
The first procedure shows a "Request user data class 1" that is generated by the communi-
cation services of the controlling station which is answered by a NACK. This procedure
occurs in case of event acquisition if no events wait for transmission.
The next procedure shows a "Request user data class 2", however directed to a destina-
tion that returns data. This answer is delivered to the application function in the controlling
station as an A_USER_DATA_CLASS2.ind. The ACD-bit = 1 (see 5.1.2 of IEC 870-5-2)
indicates to the controlling station the availability of class 1 data in the controlled station
which are requested by a "Request user data class 1".
In the third procedure, the application function of the controlling station produces
an A_RD_DATA request which is transmitted by a C_RD PDU (send/confirm link service)
to the controlled station. Then the requested data are polled by a "Request user data
class 1", transmitted as an M_ PDU and delivered to the controlling station by an
A M DATA.ind.
870-5-5 © IEC:1995 - 51 -
Application function of
Application function of
Communication
CONTROLLED STATION
CONTROLLING STATION services
Request user
data class 1
NACK
A_USER_DATA_CLASS_2.req
Request user
data class 2
A_USER_DATA_CLASS_1.req
User data
class 2
A USER DATA CLASS 2.ind
Request user
data class 1
Class 1 contains spontaneous data
Class 2 contains cyclic data
User data
class 1
A USER DATA CLASS 1.ind
A_RD_DATA.req
Send (C_RD)
A RD DATA.ind
ACK
Request user
data class 1 or 2
NACK
•
•
•
Request user
data class 1 A_M_DATA.req
RESP_Data (M)
class 1
A M DATA.ind
Figure 10 - Sequential procedure, polling procedure

870-5-5 © IEC:1995 - 53 -
6.3 Cyclic data transmission
Cyclic data transmission is used to provide a continuous updating function of current values of
process variables in telecontrol systems operating with balanced and unbalanced transmission
procedures. This procedure is usually executed with low priority which means that it can be
interrupted by event-triggered communication requests.
6.3.1 Description of sequential procedure (see figure 11)
The application process in the controlled station overwrites cyclically the actual values of
process variables into a buffer memory. The actual value of this buffer memory is trans-
mitted to the controlling station at cyclic intervals, see figure 11. The arrival of the data is
indicated to the process in the controlling station by an A_CYCLIC_DATA.ind.
Communication Application function of
Application function of
services CONTROLLED STATION
CO
...