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GEC ALSTHOM T&D PROTECTION & CONTROL LIMITED, St Leonards Works, Stafford ST17 4LX, EnglandTel: 01785 223251 Telex: 36240 Fax: 01785 212232

IEC870 Interface Guide

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2 Preface

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Preface 3

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Preface

This document details the IEC870 communication standard used on GEC ALSTHOM T&DProtection & Control relays in sufficient detail to enable third parties to interface these relays toother systems and to develop other slave devices (relays) which utilise this communicationsystem. It describes how messages are formed and transmitted from one device to another. Itdoes not describe what the messages mean as this is dependent on the application to whichIEC870 is put and the particular format of the data that is transmitted.

This guide should be used with the appropriate user guide for the particular application thatthe IEC870 system will be used with.

The information and illustrations found in this book are not binding. GEC ALSTHOM T&DProtection & Control Ltd. reserve the right to modify products in line with a policy ofcontinuous product improvement. Information in this document is subject to change withoutnotice.

4 Table of Contents

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Table of Contents 5

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Table of Contents Page

Preface ..................................................................................3

Chapter 1: IEC870 Communication Standard..........................71.1. PHYSICAL LAYER..................................................................................................................7

1.1.1. TRANSMISSION MEDIUM....................................................................................................71.1.2. IEC870 ELECTRICAL STANDARD: ........................................................................................81.1.3. TRANSMISSION METHOD...................................................................................................8

1.2. DATA LINK LAYER ................................................................................................................91.2.1. MEDIA ACCESS ...............................................................................................................91.2.2. LOGICAL LINK ................................................................................................................9

List of Figures Page

Figure 1. IEC870 message frame............................................9

Figure 2. Information field format. ..........................................9

6 Chapter 1: IEC870 Communication Standard

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Chapter 1: IEC870 Communication Standard 7

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Chapter 1: IEC870 Communication Standard

IEC870 is an international standard communication system which was developed fortelecontrol applications. It defines two areas of application, unbalanced operation (master toslave) and balanced operation (peer to peer). GEC ALSTHOM T&D P & C (Stafford) havedecided to implement frame format FT1.2 with variable length in the unbalanced mode.

The IEC870 standard (References 1 & 2) defines layers 1 & 2 in the Open System Interface(OSI) communication 7 layer model. The GEC ALSTHOM T&D P & C (Stafford)implementation uses the "enhanced performance architecture" which combines the functionsof layers 3 to 7 in a single application layer. This application layer is referred to as the Couriercommunication protocol.

This document describes the frame structure of the IEC870 FT1.2 messages as implementedby GEC ALSTHOM T&D P & C (Stafford) together with an overview of the communicationprocedures. Full details of the transmission and reception procedures to be followed by anoutstation may be found in the document "Standard Coding Model for an IEC870 FT1.2Outstation" (Reference 3).

1.1. PHYSICAL LAYER

This part of the model defines the physical medium of the communication standard and how itis connected.

1.1.1. TRANSMISSION MEDIUM

Recommended cable:Multi-core with outer screen, minimum conductor size 7/0.2mm, generally to DEF STANDARD16-12 Part 4e.g. Belden 9933

UL 2919

Connection MethodIEC870 is normally connected in a point to point manner, although multi-drop systems maybe achieved by utilising the splitter units (Model GT0023) supplied by GEC ALSTHOM T&D P& C (Stafford). The IEC870 network thus fashioned must contain one Master Control Unitlocated at the top of the tree. The total IEC870 network from the Master Control Unit to thefarthest slave device is known as a spur.


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Cable Termination:IEC870 connections will typically be by means of 25pin D-type connectors.

Typical Connector Pinout:

PinNumber

Signal Name Signal Function SignalDirection

Essential Signals1 PG - Protective Ground Screen connection N/A2 TXD - Transmitted Data Serial data output Output3 RXD - Received Data Serial data input Input7 SG - Signal Ground Signal reference level N/AControl Signals4 RTS - Ready to Send Unit has data to send Output5 CTS - Clear to Send Intermediate device can accept

data to sendInput

6 DSR - Data Set Ready Intermediate device cancommunicate

Input

8 DCD - Data Carrier Detected Intermediate device has receivedvalid carrier signal

Input

20 DTR - Data Terminal Ready Unit is ready to communicate OutputPower Signals9 +12V Power for intermediate units Input/Output10 - 12V Power for intermediate units Input/Output

The two data lines and the two ground connections are essential for the operation of thecommunication link. The other signals provide control or power for intermediate devices suchas modems or splitter units and may not be required when these devices are not used. Itshould be noted that not all intermediate devices will supply all the control signals and acertain amount of flexibility in connection of these signals will be required.

Maximum length of Single Connection:15m or 2500pF total capacitance (core to core and core to screen)

Maximum Slave Devices per Spur:32.

1.1.2. IEC870 ELECTRICAL STANDARD:

Voltage Signals:Based on RS232 voltage levels:

Loaded driver output = ±5V to ±15V (3kΩ load).Driver short-circuit current ≤ 500mAReceiver input range = ±25V.Receiver input sensitivity = ±3V

1.1.3. TRANSMISSION METHOD

Transmission :Half Duplex asynchronous transmission.


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Transmission Rate:Variable up to 20Kbits/Sec with the following preferred rates600 Bits/Sec1200 Bits/Sec2400 Bits/Sec4800 Bits/Sec9600 Bits/Sec19200 Bits/Sec

1.2. DATA LINK LAYER

1.2.1. MEDIA ACCESS

The media access layer defines how access is obtained on the media to transport data.

IEC870 messages are transmitted as frames as defined below. These frames consist of eightbit data values transmitted least significant bit first, with an even parity bit appended. Thecharacters are framed by one start bit and one stop bit. The characters that constitute a frameshould be transmitted without any idle time between them. There should be a minimuminterval of three character periods (33 bit periods) between frames transmitted by the masterstation.

A frame begins with a start byte of 68h and is terminated by an end byte of 16h. The startbyte is followed by a single byte indicating the number of bytes in the information field whichis repeated. The frame header is completed by a second start byte.

The byte immediately preceding the end byte is a checksum calculated over the whole of theinformation field. This checksum may be calculated by performing a modulo 256 addition ofall the bytes involved (i.e. the addition of the byte values ignoring any carries).

The information field is located between the second start byte and the checksum field.

1.2.2. LOGICAL LINK

The logical link layer defines the additions to a valid IEC870 frame for device addressingpurposes as well as the operation of the control byte. Received frames that fail any of thechecks should be discarded.

Information Field:

Control ByteThis byte contains the following fields in messages transmitted from the master device

StartByte

LengthByte

LengthByte

StartByte

Information field Checksum EndByte

68h LL Repeated 68h xx xx ...... xx xx xx 16hFigure 1. IEC870 message frame.

Control Byte Unit Address Network Address Data fieldCC UU AA..AA 00 dd..dd

Figure 2. Information field format.


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MSB Bit Number LSB7 6 5 4 3-0

Reserved Primary Frame CheckBit

Frame CheckValid

Function Code

(0) PRM (1) FCB FCV

In messages transmitted to the master device the control byte has the following formatMSB Bit Number LSB

7 6 5 4 3-0Reserved Primary Access Data Flow

ControlFunction Code

(0) PRM (0) ACD (0) DFC (0)

The ACD and DFC bits are not used by the Courier implementation, and will always be set tozero.

The Courier protocol only supports a sub-set of the functions included within the IEC870standard, in messages from the master device the supported functions are:

Function Code Value Frame Check ValidReset Remote Link 00h No*Send - No Reply 04h NoRequest Class 2 Data 0Bh Yes

The reset remote link function expects both the FCV bit and the FCB bit to be zero, the frame isrejected otherwise.

In messages to the master the functions supported by Courier are:

Function Code ValueConfirm 00hData Reply 08h

The unit address field designates the destination device in request messages from a mastercontrol unit to a slave device and the source device in response messages from a slave deviceto a master control unit. A slave device should only receive messages with a unit address fieldmatching its own device address, a programmed group address or the global address of 255.Master control units should accept all messages from slave devices regardless of the addressfield. The network address field is between 1 and 6 bytes in size and is terminated by a zerobyte. The network address is used to identify subservient units in a multi-tier hierarchicalsystem as described in the network address section of the Courier protocol. A slave deviceexpects to find a zero terminator as the only element of the network address field, and willreject the frame should any other value be found.

The Data Field of the information field is totally transparent and the information can take onany form and contain any binary bit combination.


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References

1. IEC870 standard Part 5 Section 1 "Transmission frame formats"Published by:- International Electrotechnical Committee

2. IEC870 standard Part 5 Section 2 "Link transmission procedures"Published by:- International Electrotechnical Committee

3. "Standard Coding Model for an IEC870 Outstation"Internal GEC ALSTHOM T&D P & C (Stafford) document


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AMENDMENTS

Issue Date ChangesA 06/05/93 First IssueB 20/05/93 Changed headers.C 12/04/95 Changed fonts

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