EN 60870-5-104:2006

SLOVENSKI STANDARD
01-september-2007
1DGRPHãþD
SIST EN 60870-5-104:2002
Oprema in sistemi za daljinsko vodenje – 5-104. del: Protokoli prenosa – Omrežni
dostop za transportne profile po standardu IEC 60870-5-101 (IEC 60870-5-
104:2006)
Telecontrol equipment and systems -- Part 5-104: Transmission protocols - Network
access for IEC 60870-5-101 using standard transport profiles (IEC 60870-5-104:2006)
Fernwirkeinrichtungen und -systeme -- Teil 5-104: Übertragungsprotokolle - Zugriff für
IEC 60870-5-101 auf Netze mit genormten Transportprofilen (IEC 60870-5-104:2006)
Matériels et systemes de téléconduite -- Partie 5-104: Protocoles de transmission -
Acces aux réseaux utilisant des profils de transport normalisés pour la CEI 60870-5-101
(IEC 60870-5-104:2006)
Ta slovenski standard je istoveten z: EN 60870-5-104:2006
ICS:
33.200 Daljinsko krmiljenje, daljinske Telecontrol. Telemetering
meritve (telemetrija)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60870-5-104
NORME EUROPÉENNE
November 2006
EUROPÄISCHE NORM
ICS 33.200 Supersedes EN 60870-5-104:2001

English version
Telecontrol equipment and systems
Part 5-104: Transmission protocols -
Network access for IEC 60870-5-101
using standard transport profiles
(IEC 60870-5-104:2006)
Matériels et systèmes de téléconduite Fernwirkeinrichtungen und -systeme
Partie 5-104: Protocoles de transmission - Teil 5-104: Übertragungsprotokolle -
Accès aux réseaux utilisant Zugriff für IEC 60870-5-101 auf Netze
des profils de transport normalisés mit genormten Transportprofilen
pour la CEI 60870-5-101 (IEC 60870-5-104:2006)
(CEI 60870-5-104:2006)
This European Standard was approved by CENELEC on 2006-09-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2006 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60870-5-104:2006 E

Foreword
The text of document 57/812/FDIS, future edition 2 of IEC 60870-5-104, prepared by IEC TC 57, Power
systems management and associated information exchange, was submitted to the IEC-CENELEC
parallel vote and was approved by CENELEC as EN 60870-5-104 on 2006-09-01.
This European Standard supersedes EN 60870-5-104:2001.
The main changes with respect to EN 60870-5-104:2001 are as follows: improvement of the sequences
and interoperability of the protocol and addition of new functions for the handling of redundant
connections.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2007-06-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2009-09-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60870-5-104:2006 was approved by CENELEC as a European
Standard without any modification.
__________
- 3 - EN 60870-5-104:2006
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application 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.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 60870-5-3 1992 Telecontrol equipment and systems EN 60870-5-3 1992
Part 5: Transmission protocols - Section 3:
General structure of application data

IEC 60870-5-4 1993 Telecontrol equipment and systems EN 60870-5-4 1993
Part 5: Transmission protocols - Section 4:
Definition and coding of application
information elements
IEC 60870-5-5 1995 Telecontrol equipment and systems EN 60870-5-5 1995
Part 5: Transmission protocols - Section 5:
Basic application functions
IEC 60870-5-101 2003 Telecontrol equipment and systems EN 60870-5-101 2003
Part 5-101: Transmission protocols -
Companion standard for basic telecontrol
tasks
IEC 60870-5-102 1996 Telecontrol equipment and systems EN 60870-5-102 1996
Part 5: Transmission protocols - Section 102:
Companion standard for the transmission of
integrated totals in electric power systems

ITU-T 1996 Interface between Data Terminal Equipment - -
Recommendation (DTE) and Data Circuit-terminating
X.25 Equipment (DCE) for terminals operating in
the packet mode and connected to public
data networks by dedicated circuit

IEEE 802.3 1998 Information technology - Telecommunications - -
and information exchange between systems -
Local and metropolitan area networks -
Specific requirements
Part 3: Carrier sense multiple access with
collision detection (CSMA/CD) access
method and physical layer specifications

RFC 791 1981 Internet Protocol - DARPA Internet Program - -
Protocol Specification
RFC 793 1981 Transmission Control Protocol - DARPA - -
Internet Program Protocol Specification

1)
RFC 894 - Standard for the Transmission of IP - -
datagrams over Ethernet Networks

1)
Undated reference.
Publication Year Title EN/HD Year
1)
RFC 1661 - Point-to-Point Protocol (PPP) - -

1)
RFC 1662 - PPP in HDLC Framing - -

RFC 1700 1994 Assigned Numbers - -

RFC 2200 1997 Internet Official Protocol Standards - -

INTERNATIONAL IEC
STANDARD 60870-5-104
Second edition
2006-06
Telecontrol equipment and systems –
Part 5-104:
Transmission protocols –
Network access for IEC 60870-5-101
using standard transport profiles

 IEC 2006 Copyright - all rights reserved
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including photocopying and microfilm, without permission in writing from the publisher.
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60870-5-104  IEC:2006 – 3 –
CONTENTS
FOREWORD.9
INTRODUCTION.13

1 Scope and object.15
2 Normative references .15
3 General architecture.17
4 Protocol structure .21
5 Definition of Application Protocol Control Information (APCI).23
5.1 Protection against loss and duplication of messages .29
5.2 Test procedures .33
5.3 Transmission control using Start/Stop .37
5.4 Portnumber .45
5.5 Maximum number of outstanding I format APDUs (k).45
6 Selection of ASDUs defined in IEC 60870-5-101 and additional ASDUs .45
7 Mapping of selected application data units and functions to the TCP services .53
7.1 Station initialization (6.1.5 to 6.1.7 of IEC 60870-5-5).53
7.2 Data acquisition by polling (6.2 of IEC 60870-5-5) .63
7.3 Cyclic data transmission (6.3 of IEC 60870-5-5) .63
7.4 Acquisition of events (6.4 of IEC 60870-5-5) .63
7.5 General interrogation (6.6 of IEC 60870-5-5).63
7.6 Clock synchronization (6.7 of IEC 60870-5-5).65
7.7 Command transmission (6.8 of IEC 60870-5-5) .67
7.8 Transmission of integrated totals (6.9 of IEC 60870-5-5) .69
7.9 Parameter loading (6.10 of IEC 60870-5-5) .69
7.10 Test procedure (6.11 of IEC 60870-5-5) .71
7.11 File transfer (6.12 of IEC 60870-5-5) Control and monitor direction .71
8 ASDUs for process information in control direction with time tag .73
8.1 TYPE IDENT 58: C_SC_TA_1 Single command with time tag CP56Time2a.75
8.2 TYPE IDENT 59: C_DC_TA_1 Double command with time tag CP56Time2a .77
8.3 TYPE IDENT 60: C_RC_TA_1 Regulating step command with time tag
CP56Time2a .79
8.4 TYPE IDENT 61: C_SE_TA_1 Set-point command with time tag CP56Time2a,
normalized value .81
8.5 TYPE IDENT 62: C_SE_TB_1 Set-point command with time tag CP56Time2a,
scaled value .83
8.6 TYPE IDENT 63: C_SE_TC_1 Set-point command with time tag CP56Time2a,
short floating point number .85
8.7 TYPE IDENT 64: C_BO_TA_1 Bitstring of 32 bit with time tag CP56Time2a .87
8.8 TYPE IDENT 107: C_TS_TA_1 Test command with time tag CP56Time2a .89
8.9 TYPE IDENT 127: F_SC_NB_1 QueryLog – Request archive file .91

60870-5-104  IEC:2006 – 5 –
9 Interoperability .93
9.1 System or device.93
9.2 Network configuration.93
9.3 Physical layer.95
9.4 Link layer .95
9.5 Application layer.97
9.6 Basic application functions .107
10 Redundant connections .115
10.1 General .115
10.2 General requirements.115
10.3 Initialisation of controlling station .119
10.4 Initialisation of controlled station .123
10.5 User data from controlling station .127
10.6 User data from controlled station.131
10.7 State transition diagrams.135

Figure 1 – General architecture (example) .19
Figure 2 – Selected standard provisions of the defined telecontrol companion standard.21
Figure 3 – Selected standard provisions of the TCP/IP protocol suite RFC 2200
(example) .23
Figure 4 – APDU of the defined telecontrol companion standard.25
Figure 5 – APCI of the defined telecontrol companion standard .25
Figure 6 – Control field of type Information transfer format (I format) .27
Figure 7 – Control field of type numbered supervisory functions (S format) .27
Figure 8 – Control field of type unnumbered control functions (U format) .27
Figure 9 – Undisturbed sequences of numbered I format APDUs .29
Figure 10 – Undisturbed sequences of numbered I format APDUs acknowledged by an
S format APDU .31
Figure 11 – Disturbed sequence of numbered I format APDUs.31
Figure 12 – Time-out in case of a not acknowledged last I format APDU.33
Figure 13 – Undisturbed test procedure .35
Figure 14 – Unconfirmed test procedure .35
Figure 15 – Start data transfer procedure .37
Figure 16 – Stop data transfer procedure.39
Figure 17 – State transition diagram for Start/Stop procedure (controlled station) .41
Figure 18 – State transition diagram for Start/Stop procedure (controlling station) .43
Figure 19 – TCP connection establishment and close .55
Figure 20 – Initialization of the controlling station .57
Figure 21 – Local initialization of the controlled station .59
Figure 22 – Remote initialization of the controlled station .61
Figure 23 – ASDU: C_SC_TA_1 Single command with time tag CP56Time2a .75
Figure 24 – ASDU: C_DC_TA_1 Double command with time tag CP56Time2a.77
Figure 25 – ASDU: C_RC_TA_1 Regulating step command with time tag CP56Time2a .79

60870-5-104  IEC:2006 – 7 –
Figure 26 – ASDU: C_SE_TA_1 Set-point command with time tag CP56Time2a,
normalized value.81
Figure 27 – ASDU: C_SE_TB_1 Set-point command with time tag CP56Time2a, scaled
value .83
Figure 28 – ASDU: C_SE_TC_1 Set-point command with time tag CP56Time2a, short
floating point number .85
Figure 29 – ASDU: C_BO_TA_1 Bitstring of 32 bit with time tag CP56Time2a .87
Figure 30 – ASDU: C_TS_TA_1 Test command with time tag CP56Time2a .89
Figure 31 – ASDU: F_SC_NB_1 QueryLog – Request archive file.91
Figure 32 – Initialisation of controlling station with redundant connections .121
Figure 33 – Initialisation of controlled station with redundant connections.125
Figure 34 – Redundant connections – User data from controlling station .129
Figure 35 – Redundant connections – User data from controlled station .133
Figure 36 – State transition diagram for redundant connections (controlled station) .137
Figure 37 – State transition diagram for redundant connections (controlling station) .139

Table 1 – Process information in monitor direction.47
Table 2 – Process information in control direction .49
Table 3 – System information in monitor direction.51
Table 4 – System information in control direction .51
Table 5 – Parameter in control direction.51
Table 6 – File transfer.51

60870-5-104  IEC:2006 – 9 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
TELECONTROL EQUIPMENT AND SYSTEMS –

Part 5-104: Transmission protocols –
Network access for IEC 60870-5-101 using
standard transport profiles
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of 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, IEC publishes International Standards, Technical Specifications,
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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 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 IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
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.
International Standard IEC 60870-5-104 Ed.2 has been prepared by IEC technical
committee 57: Power systems management and associated information exchange.
This second edition cancels and replaces the first edition published in 2000 and constitutes a
technical revision. The main changes of this second edition with respect to the previous
edition are as follows: improvement of the sequences and interoperability of the protocol and
addition of new functions for the handling of redundant connections.

60870-5-104  IEC:2006 – 11 –
The text of this standard is based on the following documents:
FDIS Report on voting
57/812/FDIS 57/819/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 2.
IEC 60870-5 consists of the following parts, under the general title Telecontrol equipment and
systems – Part 5: Transmission protocols
Part 5: Transmission protocols – Section One: Transmission frame formats
Part 5: Transmission protocols – Section 2: Link transmission procedures
Part 5: Transmission protocols – Section 3: General structure of application data
Part 5: Transmission protocols – Section 4: Definition and coding of application
information elements
Part 5: Transmission protocols – Section 5: Basic application functions
Part 5-6: Guidelines for conformance testing for the IEC 60870-5 companion standards
Part 5-101: Transmission protocols – Companion standard for basic telecontrol tasks
Part 5: Transmission protocols – Section 102: Companion standard for the
transmission of integrated totals in electric power systems
Part 5-103: Transmission protocols – Companion standard for the informative interface of
protection equipment
Part 5-104: Transmission protocols – Network access for IEC 60870-5-101 using standard
transport profiles
Part 5-601: Conformance test cases for the IEC 60870-5-101 companion standard
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site 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.
60870-5-104  IEC:2006 – 13 –
INTRODUCTION
IEC 60870-5-101 provides a communication profile for sending basic telecontrol messages
between a central telecontrol station and telecontrol outstations, which uses permanent
directly connected data circuits between the central station and individual outstations.
In some applications, it may be required to send the same types of application messages
between telecontrol stations using a data network containing relay stations which store and
forward the messages and provide only a virtual circuit between the telecontrol stations. This
type of network delays messages by varying amounts of time depending on the network traffic
load.
In general, the variable message delay times mean that it is not possible to use the link layer
as defined in IEC 60870-5-101 between telecontrol stations. However, in some cases it is
possible to connect telecontrol stations having all three layers of the companion standard
IEC 60870-5-101 to suitable data networks using Packet Assembler Disassembler (PAD) type
stations to provide access for balanced communication.
In all other cases this companion standard, which does not use the link functions of
IEC 60870-5-101, may be used to provide balanced access via a suitable transport profile.

60870-5-104  IEC:2006 – 15 –
TELECONTROL EQUIPMENT AND SYSTEMS –

Part 5-104: Transmission protocols –
Network access for IEC 60870-5-101 using
standard transport profiles
1 Scope and object
This part of IEC 60870 applies to telecontrol equipment and systems with coded bit serial data
transmission for monitoring and controlling geographically widespread processes. It defines a
telecontrol companion standard that enables interoperability among compatible telecontrol
equipment. The defined telecontrol companion standard utilizes standards of the IEC 60870-5
series. The specifications of this part present a combination of the application layer of
IEC 60870-5-101 and the transport functions provided by a TCP/IP (Transmission Control
Protocol/Internet Protocol). Within TCP/IP, various network types can be utilized, including
X.25, FR (Frame Relay), ATM (Asynchronous Transfer Mode) and ISDN (Integrated Service
Data Network). Using the same definitions, alternative ASDUs (Application Service Data Unit)
as specified in other IEC 60870-5 companion standards (for example, IEC 60870-5-102) may
be combined with TCP/IP, but this is not described further in this part.
NOTE Security mechanisms are outside the scope of this standard.
2 Normative references
The following referenced documents are indispensable for the application 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 60870-5-3:1992, Telecontrol equipment and systems – Part 5: Transmission protocols –
Section 3: General structure of application data
IEC 60870-5-4:1993, Telecontrol equipment and systems – Part 5: Transmission protocols –
Section 4: Definition and coding of application information elements
IEC 60870-5-5:1995, Telecontrol equipment and systems – Part 5: Transmission protocols –
Section 5: Basic application functions
IEC 60870-5-101:2003, Telecontrol equipment and systems – Part 5-101: Transmission
protocols – Companion standard for basic telecontrol tasks
IEC 60870-5-102:1996, Telecontrol equipment and systems – Part 5: Transmission protocols
– Section 102: Companion standard for the transmission of integrated totals in electric power
systems
ITU-T Recommendation X.25:1996, Interface between Data Terminal Equipment (DTE) and
Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and
connected to public data networks by dedicated circuit
IEEE 802.3:1998, Information technology – Telecommunications and information exchange
between systems – Local and metropolitan area networks – Specific requirements – Part 3:
Carrier sense multiple access with collision detection (CSMA/CD) access method and
physical layer specifications
60870-5-104  IEC:2006 – 17 –
RFC 791, Internet Protocol, Request for Comments 791 (MILSTD 1777) (September, 1981)
RFC 793, Transmission Control Protocol, Request for Comments 793 (MILSTD 1778)
(September, 1981)
RFC 894, Internet Protocol on Ethernet Networks
RFC 1661, Point-to-Point Protocol (PPP)
RFC 1662, PPP in HDLC Framing
RFC 1700, Assigned Numbers, Request for Comments 1700 (STD 2) (October, 1994)
RFC 2200, Internet Official Protocol Standards, Request for Comments 2200 (June, 1997)
3 General architecture
This standard defines the use of an open TCP/IP-interface to a network, containing for
example a LAN for telecontrol equipment, which transports IEC 60870-5-101 ASDUs. Routers
which include the different WAN-types (for example, X.25, Frame Relay, ISDN, etc.) may be
connected via a common TCP/IP-LAN-interface (see figure 1). Figure 1 shows a redundant
configuration in the central station in addition to a non-redundant system.
Motivations:
The use of separate routers offers the following advantages.
– There is no need for network-specific software in end systems.
– There is no need for routing functionality in end systems.
– There is no need for network management in end systems.
– It facilitates obtaining end systems from manufacturers that specialize in telecontrol
equipment.
– It facilitates obtaining individual separate routers, to suit a variety of networks from
manufacturers specializing in this non-telecontrol specific field.
– It is possible to change the network type by replacing only the router type, without
affecting the end systems.
– It is particularly suitable for converting existing end systems that conform to
IEC 60870-5-101.
– It is suitable for present and future implementations.

60870-5-104  IEC:2006 – 19 –
Application 101
Application 101
CENTRAL
STATION
Transport interface
Transport interface
End system
TCP/IP TCP/IP
LAN interface*
LAN interface*
Router (X.25, FR, ISDN.)
Router
Router
Network
Network
X.25, FR, ISDN.
X.25, FR, ISDN.
Router (X.25, FR, ISDN.)
Router (X.25, FR, ISDN.)
LAN interface* LAN interface*
OUTSTATION
TCP/IP TCP/IP
End system
Transport interface
Transport interface
Application 101 Application 101

Without redundancy  With redundancy

IEC  2785/2000
* The LAN interface may be redundant.

Figure 1 – General architecture (example)

60870-5-104  IEC:2006 – 21 –
4 Protocol structure
Figure 2 shows the protocol structure of the end system.
Initialization
Selection of application functions of
User process
IEC 60870-5-5 according to IEC 60870-5-101
Selection of ASDUs from IEC 60870-5-101 and IEC 60870-5-104
Application (layer 7)
APCI (Application Protocol Control Information)
Transport Interface (user to TCP interface)
Transport (layer 4)
Selection of
Network (layer 3)
TCP/IP protocol suite (RFC 2200)
Link (layer 2)
Physical (layer 1)
NOTE Layers 5 and 6 are not used.

IEC  2786/2000
Figure 2 – Selected standard provisions of the defined telecontrol companion standard
Figure 3 shows the recommended selection of the TCP/IP Protocol suite (RFC 2200) used in
this standard. At the time of publication, the RFCs indicated were valid, but may have been
replaced in the meantime by equivalent, relevant RFCs. The relevant RFCs are available at
the Internet address http://www.ietf.org.
The Ethernet 802.3 stack shown may be used by a telecontrol station end system or DTE
(Data Terminal Equipment) to drive a separate router as shown in the example in figure 1. If a
redundant configuration is not required, a point-to-point interface (for example, X.21) to the
separate router may be used instead of a LAN interface, thus retaining more of the original
hardware when converting end systems originally conforming to IEC 60870-5-101.
Other compatible selections from RFC 2200 are also permitted.
This standard uses the TCP/IP transport profile as defined in other referenced standards,
without alteration.
60870-5-104  IEC:2006 – 23 –
Transport Interface (user to TCP interface)
Transport
RFC 793 (Transmission control protocol)
(layer 4)
Network
RFC 791 (Internet protocol)
(layer 3)
RFC 1661
RFC 894
(PPP)
(Transmission of
IP datagrams
Data link
over ethernet
RFC 1662 (layer 2)
networks)
(PPP in HDLC-like
framing)
Physical
X.21 IEEE 802.3
(layer 1)
Serial line Ethernet
IEC  2787/2000
Figure 3 – Selected standard provisions of the TCP/IP
protocol suite RFC 2200 (example)
5 Definition of Application Protocol Control Information (APCI)
The transport interface (User to TCP interface) is a stream-oriented interface which does not
define any start or stop mechanism for the ASDUs of IEC 60870-5-101. In order to detect the
start and the end of the ASDUs, each APCI includes the following delimiting elements: a start
character, the specification of the length of the ASDU, plus the control field (see figure 4).
Either a complete APDU (or, for control purposes, only the APCI fields) may be transferred
(see figure 5).
NOTE The abbreviations used above are taken from clause 5 of IEC 60870-5-3 as follows.
APCI Application Protocol Control Information
ASDU Application Service Data Unit
APDU Application Protocol Data Unit

60870-5-104  IEC:2006 – 25 –
START 68H
Length of the APDU (max. 253)
Control field octet 1
APCI
Control field octet 2
Control field octet 3
Control field octet 4
APDU
Length
ASDU defined
in IEC 60870-5-101
ASDU
and IEC 60870-5-104
IEC  2788/2000
Figure 4 – APDU of the defined telecontrol companion standard

START 68H
Length of APDU
Control field octet 1
APCI
Control field octet 2
Length = 4
Control field octet 3
Control field octet 4
IEC  2789/2000
Figure 5 – APCI of the defined telecontrol companion standard
START 68H defines the point of start within the data stream.
The length of the APDU defines the length of the body of the APDU, which consists of the four
control field octets of the APCI plus the ASDU. The first counted octet is the first octet of the
control field, the last counted octet is the last octet of the ASDU. The maximum length of
the ASDU is limited to 249 because the maximum value of the field length of APDU is
253 (APDU = 255 minus start and length octet) and the length of the control field is

max
4 octets.
The control field defines control information for the protection against loss and duplication of
messages, start and stop of message transfers and the supervision of transport connections.
The counter mechanism of the control field is defined according to 2.3.2.2.1 to 2.3.2.2.5 of the
ITU-T X.25 recommendation.
60870-5-104  IEC:2006 – 27 –
Figures 6, 7 and 8 show the definition of the control field.
Three types of control field formats are used to perform numbered information transfer (I format),
numbered supervisory functions (S format) and unnumbered control functions (U format).
Control field octet 1 bit 1 = 0 defines the I format. I format APDUs always contain an ASDU.
The control information of an I format is shown in figure 6.

Bit 18 7 6 5 4 3 2
Send Sequence Number N(S) LSB
octet 1
Send Sequence Number N(S)
MSB
octet 2
Receive Sequence Number N(R) LSB 0
octet 3
MSB Receive Sequence Number N(R)
octet 4
IEC  2790/2000
Figure 6 – Control field of type Information transfer format (I format)
Control field octet 1 bit 1 = 1 and bit 2 = 0 defines the S format. S format APDUs consist of
the APCI only. The control information of an S format is shown in figure 7.

Bit 18 7 6 5 4 3 2
0 1
0 octet 1
0 octet 2
LSB
Receive Sequence Number N(R) 0
octet 3
MSB
Receive Sequence Number N(R)
octet 4
IEC  2791/2000
Figure 7 – Control field of type numbered supervisory functions (S format)
Control field octet 1 bit 1 = 1 and bit 2 = 1 defines the U format. U format APDUs consist of
the APCI only. The control information of a U format is shown in figure 8. Only one function –
TESTFR, STOPDT or STARTDT – may be active at the same time.

Bit 18 7 6 5 4 3 2
STOPDT STARTDT
TESTFR
octet 1
1 1
con act con act con act
octet 2
0 0 octet 3
octet 4
IEC  2792/2000
Figure 8 – Control field of type unnumbered control functions (U format)

60870-5-104  IEC:2006 – 29 –
5.1 Protection against loss and duplication of messages
The use of the Send Sequence Number N(S) and the Receive Sequence Number N(R) is
identical to the method defined in ITU-T X.25. For simplification purposes, the additional
sequences are defined in figures 9 to 12.
Both sequence numbers are sequentially increased by one for each APDU and each direction.
The transmitter increases the Send Sequence Number N(S) and the receiver increases the
Receive Sequence Number N(R). The receiving station acknowledges each APDU or a
number of APDUs when it returns the Receive Sequence Number up to the number whose
APDUs are properly received. The sending station holds the APDU or APDUs in a buffer until
it receives back its own Send Sequence Number as a Receive Sequence Number which is a
valid acknowledge for all numbers <= the received number. Then it may delete the correctly
transmitted APDUs from the buffer. In case of longer data transmission in one direction only,
an S format has to be sent in the other direction to acknowledge the APDUs before buffer
overflow or time out. This method should be used in both directions. After the establishment
of a TCP connection, the send and receive sequence numbers are set to zero.
The following definitions are valid for figures 9 to 16:
V(S) = Send state variable (see ITU-T X.25);
V(R) = Receive state variable (see ITU-T X.25);
Ack = Indicates that the DTE has received correctly all I format APDUs numbered up to and including this number;
I(a,b) = Information format APDU with a = send sequence number and b = receive sequence number;
S(b) = Supervisory format APDU with b = receive sequence number;
U  = Unnumbered control function APDU.

Station A
Station B
Internal counters V after
Internal counters V after
APDU was sent or received
APDU was sent or received
Ack V(S) V(R) V(S) V(R) Ack
0 0 0 0 0 0
I (0,0) 1
I (1,0) 2
I (2,0) 3
I (0,3)
I (1,3)
2 1 3
I (3,2) 4
2 4
IEC  2793/2000
Figure 9 – Undisturbed sequences of numbered I format APDUs

60870-5-104  IEC:2006 – 31 –
Station B
Station A
Internal counters V after Internal counters V after

APDU was sent or received APDU was sent or received

Ack V(S) V(R) V(S) V(R) Ack
0 0 0 0 0 0
I (0,0) 1
I (1,0) 2
I (2,0) 3
Time-out t2
S(3)
IEC  929/06
Figure 10 – Undisturbed sequences of numbered I
format APDUs acknowledged by an S format APDU

Station A Station B
Internal counters V after
Internal counters V after
APDU was sent or received APDU was sent or received

Ack V(S) V(R) V(S) V(R) Ack
0 0 0 0 0 0
I (0,0)
2 1
I (2,0)
Sequence error
S(1) (optional)
Active close
Active open follows
(see Figures 19
to 22)
IEC  930/06
NOTE To avoid retransmission of APDU’s that have already been accepted, an S-frame should, if possible, be
sent prior to the active close.
Figure 11 – Disturbed sequence of numbered I format APDUs

60870-5-104  IEC:2006 – 33 –
Station B
Station A
Internal counters V after
Internal counters V after
APDU was sent or received
APDU was sent or received
Ack V(S) V(R) V(S) V(R) Ack
0 0 0 0 0 0
I (0,0)
Time-out
cancelled
S (1)
X
Time-out
t1
Active close
Active open follows
(see Figures 19
to 22)
IEC  931/06
Figure 12 – Time-out in case of a not acknowledged last I format APDU
5.2 Test procedures
Unused, but open, connections may be periodically tested in both directions by sending test APDUs
(TESTFR = act) which are confirmed by the receiving station sending TESTFR = con. Both stations
may initiate the test procedure after a specified period of time in which no data transfers occur
(time out). The reception of every frame – I frame, S frame or U frame – retriggers timer t3.
Station B has to supervise the connection independently. However, as long as it receives test
frames from station A, it will not send test frames.
The test procedure can also be initiated on "active” connections where absence of activity is
possible for extended times and connectivity needs to be assured.

60870-5-104  IEC:2006 – 35 –
Station B
Station A
Internal counters V after Internal counters V after

APDU was sent or received APDU was sent or received

Ack V(S) V(R) V(S) V(R) Ack
0 0 0 0 0 0
I (0,0) 1
I (1,0) 2
S (2)
Time-out t3
U (TESTFR act)
U (TESTFR con)
IEC  932/06
Figure 13 – Undisturbed test procedure

Station A Station B
Internal counters V after Internal counters V after
APDU was sent or received APDU was sent or received

Ack V(S) V(R) V(S) V(R) Ack
0 0 0 0 0 0
I (0,0)
I (1,0) 2
S (2)
Time-out t3
U (TESTFR act)
Time-out
t1
Active close
Active open follows
(see Figures 19
to 22)
IEC  933/06
Figure 14 – Unconfirmed test procedure

60870-5-104  IEC:2006 – 37 –
5.3 Transmission control using Start/Stop
STARTDT (Start Data Transfer) and STOPDT (Stop Data Transfer) are used by the controlling
station (for example, Station A), to control the data transfer from a controlled station (Station
B). This is useful, for example, when more than one connection between the stations is open
and therefore available, but only one connection at a time is used for the data transfer. The
defined functionality for STARTDT and STOPDT avoids loss of data in the case of switchover
from one connection to another. STARTDT and STOPDT are also used with single
connections between the stations to control the traffic on the connection.
When the connection is established, user data transfer is not automatically enabled from the
controlled station on that connection, i.e. STOPDT is the default state when a connection is
established. In this state, the controlled station does not send any data via this connection,
except unnumbered control functions and confirmations to such functions. The controlling
station must activate the user data transfer on a connection by sending a STARTDT act via
this connecti
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