EN 50170:1996/A1:2002

SLOVENSKI STANDARD
01-december-2003
General purpose field communication system - Amendment A1/Note: Includes
Corrigendum of August 2002
General purpose field communication system
Ta slovenski standard je istoveten z: EN 50170:1996/A1:2002
ICS:
35.100.01 Medsebojno povezovanje Open systems
odprtih sistemov na splošno interconnection in general
35.200 Vmesniška in povezovalna Interface and interconnection
oprema equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN 50170/A1
NORME EUROPÉENNE Volume 4
EUROPÄISCHE NORM May 2002
ICS 35.100.00; 35.200
English version
General purpose field communication system
This amendment A1 modifies the European Standard EN 50170:1996; it was approved by CENELEC on
2000-04-01. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this amendment 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 amendment exists in one official version (English). 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, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands,
Norway, Portugal, Spain, Sweden, Switzerland and 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
© 2002 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 50170:1996/A1:2002 E

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EN 50170-0-1:1996/A1:2002
Foreword
This amendment has been prepared by the British National Committee following
acceptance by technical committee CLC/TC 65CX Fieldbus at its meeting in London
on 1996-09-27 that the British Pre-standard DD 238:1996 Fieldbus met the
candidate criteria for inclusion within EN 50170 (see the paragraph headed
‘Solutions’ in clause 2.1 of EN 50170-0-1:1996).
The amendment is limited to:
� necessary editorial alterations to the published text
� new text relating to the inclusion (as A1 Volume 4) of BSI DD 238.
The following changes were made as a result of comments submitted during the
first (1997) UAP ballot:
� References to unpublished documents (for example to IEC 61158-2/Amendment 3)
have been replaced by the appropriate texts.
� References to IEC 61158 Application Layer CDV drafts have been replaced by
references to the resulting IEC Technical Specifications. For completeness,
the TSs have been included as Parts 8 and 9.
� Where appropriate cross-references to IEC 61158 have been made more precise.
The text of the draft was submitted to the formal vote and was approved by
CENELEC as amendment A1 to EN 50170:1996 on 2000-04-01.
The following dates were fixed:
(dop) 2002-11-01
� latest date by which the amendment has to be implemented
at a national level by the publication of an identical
national standard or by endorsement
(dow) 2003-04-01
� latest date by which the national standards conflicting
with the amendment have to be withdrawn

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EN 50170-0-1:1996/A1-0-1:2002
Structure of the document and overall table of contents
In paragraph 1, lines 1 and 3, delete ‘three’ and substitute ‘four’.
In paragraph 1, lines 3 and 4, delete ‘P-NET, PROFIBUS and WorldFIP’ and
substitute ‘P-NET, PROFIBUS, WorldFIP and DD 238 Foundation Fieldbus’.
In paragraph 2, line 2, delete ‘three’ and substitute ‘four’.
Add, after the list of the contents of volume 3, the following text.
‘A1 Volume 4 contains:
Part 1-4 (Informative) General description of EN 50170/A1 Volume 4
(DD 238 Foundation Fieldbus)
Part 2-4 Physical layer specification and service definitions
Part 3-4 Data link layer service definitions
Part 4-4 Data link layer protocol specifications
Part 5-4 Application layer service definitions
Part 6-4 Application layer protocol specifications
Part 7-4 Network management
Sub-part 7.1-4 System Management
Sub-part 7.2-4 Network Management’
Part 8-4 IEC/TS 61158-3:1999 Data Link layer service definition
Part 9-4 IEC/TS 61158-4:1999 Data Link layer protocol specification
Foreword
Current situation in the European market
In paragraph 4, line 1, delete ‘three’ and substitute ‘four’.
EN 50170 objective
In paragraph 1, indent 2, line 1, delete ‘three’ and substitute ‘four’.
Usage of EN 50170
Delete paragraph 1 entirely and substitute the following new paragraph.
‘According to the history and the decision of TC 65CX neither to develop any new
compromise nor to re-write the actual concepts on the market, the reader should
pay his dedicated
ATTENTION
to the following notes:
-”EN 50170 contains the four following different specifications, without
attempting to develop any compromise or to mix them, as per the PAS
principles:
EN 50170 volume 1 (based on and technically identical to DK 502058
and DK 502066)
EN 50170 volume 2 (based on and technically identical to DIN 19245
series
EN 50170 volume 3 (based on and technically identical to NF C 46602,
NF C 46603, NF C 46605, NF C 46606, NF C 46606/A1, NF C 46607,
NF C 46607/A1, EN 61158-2)
EN 50170/A1 volume 4 (based on and technically identical to BSI
DD 238 series, EN 61158-2, IEC/FDIS 61158-3, IEC/FDIS 61158-4)
The concepts of these four specifications are different. For each
implementation, the user has to select the volume of EN 50170 which suits
the user’s application needs.’

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EN 50170-0-1:1996/A1:2002
Part 0-1 Introduction
Contents
Add the title of clause 6 as follows:
‘6 MAIN FEATURES OF EN 50170/A1 VOLUME 4 (DD 238 FOUNDATION FIELDBUS)’
5 Main features of EN 50170, Volume 3 (WorldFIP)
Add, at the end of clause 5, the following new clause 6.
‘6 Main features of EN 50170/A1 Volume 4 (DD 238 Foundation Fieldbus)
EN 50170/A1 Volume 4 (DD 238) has been originally developed by the Fieldbus
Foundation, an open organisation supported by a world-wide consensus of industry
suppliers for process control equipment and factory automation equipment.
Responding to strong international user demand, the Foundation has based its
specifications on the work of the IEC, and Foundation members also contribute to
the IEC committee work.
Following strong requests from user companies in UK, BSI has adopted the
Fieldbus Foundation specifications as DD 238 and made it available to CENELEC.
Technically the DD 238 Foundation Fieldbus specification is based on published
and draft international standards prepared by the IEC Fieldbus committee
IEC/SC 65C.
EN 50170/A1 Volume 4 has been published as a British pre-standard (known as
DD 238 Fieldbus) in 8 parts comprising the following main features:
DD 238: Part 1:1996 is an informative Introductory Guide to System Architecture.
It describes how the components specified in other parts of DD 238 may be
related and configured to construct and support DD 238 Fieldbus devices,
Networks and Systems. It is technically equivalent to Fieldbus Foundation
specification FF-800 revision 1.3
DD 238: Part 2:1996 is a Physical Layer profile using the EN 61158-2
specification for 31,25 kbit/s operation. Features of particular importance
are:
� Power can be supplied directly to field devices through the bus signal
conductors.
� Intrinsically safe (IS) fieldbus applications can be supported using bus
powered devices and IS barriers.
� Wiring installed for 4-20 mA devices may also be used for the DD 238
Foundation Fieldbus.
Part 2 is technically equivalent to Fieldbus Foundation specification
FF 816 revision 1.3
DD 238: Part 3:1996 is a Data Link Services Specification using the
IEC/FDIS 61158-3. It is technically equivalent to Fieldbus Foundation
specification FF-821 revision 1.3
DD 238: Part 4:1996 is a Data Link Protocol Specification using the
IEC/FDIS 61158-4. It is technically equivalent to Fieldbus Foundation
specification FF-822 revision 1.3
The IEC data link specification has been chosen because it combines the best
features of other existing protocols and adds a number of important new service
features to support modern application requirements. These include:
� An integrated access control method combining the token passing approach of
PROFIBUS (volume 2), and the time-critical approach of WorldFIP (volume 3)
with migration paths from existing applications using these protocols.

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EN 50170-0-1:1996/A1-0-1:2002
� Services needed to support multiple data models with particular features
for Client/Server, Publisher/Subscriber and Report Distributor as
implemented in DD 238: Part 5.
� Multi-segment addressing to support small and large networks assembled from
multiple DD 238 fieldbus segments.
� Tools for accurate time distribution and synchronisation across
multi-segment systems.
� Automatic backup capability to continue schedule operation if the current
bus master fails.
� On line support for flexible and time critical scheduling including
construction and revision.
� On line support for device detection, recognition and configuration.
DD 238: Part 5:1996 is a Fieldbus Access Sub-layer building on IEC data link
services to provide the needed data models for modern automation systems.
� Client/Server. This is the model used by pairs of application entities
using one-to-one communications, for example, a traditional master station
issuing confirmed requests and commands to a slave.
� Publisher/Subscriber. This is the model to support efficient simultaneous
transfer of data from one data-source (publisher) to multiple data-sinks
(subscribers) using retentive buffers.
� Report Distributor. This model supports multi-cast (one source to many
sinks) queued and prioritised communication for efficient transfer of
messages and data without immediate confirmation.
Part 5 is technically equivalent to Fieldbus Foundation specification
FF-875 revision 1.3
DD 238: Part 6:1996 is a Foundation Fieldbus Message Specification (FFMS)
defining communication objects and standard services to support user
applications such as function blocks. The main service categories include
Context Management, Object Dictionary, Variable Access, Event Services,
Upload/Download and Program Invocation. Messages are formatted using the
Abstract Syntax Notation ASN1.
Part 6 is technically equivalent to Fieldbus Foundation specification
FF-870 revision 1.3.
DD 238: Part 7:1996 Fieldbus Management is specified in two parts, sub-part 7.1
System Management and sub-part 7.2 Network Management, which are technically
equivalent to Fieldbus Foundation specifications FF-880 revision 1.0 and
FF-801 revision 1.3.
The System Management Kernel in each device maintains the information base for
distributed inter-operation of device applications. This includes a secure
configuration sequence at start-up with assignment of data link address based on
unique manufacturer assigned codes and user assigned tag names. Procedures are
included for recognising and supporting temporary devices such as Hand Held
Terminals and Workstations. System management also provides scheduling services
for user applications and a common application time reference among all devices
to ensure that user tasks and function blocks can execute in the proper time
relationship across the network.
The Network Management agent supports vertical integration of Fieldbus layers by
providing capabilities for configuration management, performance management and
fault management. These capabilities are accessed by the same services as other
device applications, they do not use a specialised management protocol.’
Data Link layer
For completeness, the following documents are included as Parts 8 and 9
respectively:
IEC/TS 61158-3:1999 Digital data communications for measurement and control -
Fieldbus for use in industrial control systems - Part 3: Data link service
definition.
Page 6
EN 50170-0-1:1996/A1:2002
IEC/TS 61158-4:1999 Digital data communications for measurement and control -
Fieldbus for use in industrial control systems - Part 4: Data link protocol
specification’
Volume 3
Add, at the end of volume 3, the following new A1 volume 4.

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EN 50170-0-1:1996/A1-0-1:2002
General Purpose Field Communication System
Volume 4
Page 8
EN 50170-0-1:1996/A1:2002
Table of Contents of Volume 4 .Page
Part 1-4 General description of EN 50170/A1 Volume 4
(DD 238/Foundation Fieldbus).9
Part 2-4 Physical layer specification and service definitions.65
Part 3-4 Data link layer service definitions.111
Part 4-4 Data link layer protocol specifications.133
Part 5-4 Application layer service definitions.201
Part 6-4 Application layer protocol specifications.275
Part 7-4 Network management
Sub-Part 7.1-4 System Management.401
Sub-Part 7.2-4 Network Management.459
Part 8-4 IEC/TS 61158-3:1999 Digital data communications for measurement and
control - Fieldbus for use in industrial control systems – Part 3:
Data link service definition.509
Part 9-4 IEC/TS 61158-4:1999 Digital data communications for measurement and
control - Fieldbus for use in industrial control systems – Part 4:
Data link protocol specification.653

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EN 50170:1996/A1-1-4:2002
General Purpose Field Communication System
Part 1-4
General Description of EN 50170/A1 Volume 4

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EN 50170:1996/A1-1-4:2002
Contents
Page
1 Overview.12
1.1 Key Components of Fieldbus Systems.13
1.1.1 Function Block Application Processes .13
1.1.2 Object Dictionary (OD) and Device Descriptions (DD) .14
1.1.3 Network Communications .14
1.1.4 Network Management .15
1.1.5 System Management .16
1.2 Relationship to the ISO OSI Reference Model.17
1.3 Relationship to the International Electro-technical Commission (IEC)
Fieldbus Standards.17
1.4 EN 50170/A1 Volume 4 Documents.18
2 Network Topology.18
2.1 Single Link Topology.19
2.2 Bridged Networks.19
2.3 Device Architecture.21
3 Application Process (AP).22
3.1 Overview.22
3.2 Network Visible Objects.22
3.3 Application Layer Interface.23
3.4 Object Dictionary.23
3.5 AP Directory.24
3.5.1 AP Directory Header .24
3.5.2 Object References .25
3.5.3 AP Architecture Summary .26
4 Function Block APs.27
4.1 Overview.27
4.2 Block Types.27
4.2.1 Resource Blocks .29
4.2.2 Transducer Blocks .29
4.2.3 Function Blocks .29
4.2.4 View and Trend Objects .29
4.2.5 Alert Objects .29
4.2.6 Link Objects .29
4.3 Types of Function Blocks.30
5 System Management Kernel (SMK).30
5.1 Overview.30
5.2 SMK Communications.31
5.3 SMK Services.31
5.3.1 SMIB Access .31
5.3.2 Tag and Address Assignment .31
5.3.3 Device Identification .33
5.3.4 Locating Remote Devices and Objects .33
5.3.5 Clock Synchronization .33
5.3.6 Function Block Scheduling .34
6 Communications Entity.34
6.1 Overview.34
6.2 Network Management Agent.35
6.3 Foundation Fieldbus Messaging Service (FFMS).36
6.4 Fieldbus Access Sublayer (FAS).38
6.5 Data Link Layer (DLL).39
6.5.1 DLL Lower Level (Media Access) Functions .40
6.5.2 DLL Upper Level (Data Transfer) Functions .41
6.6 Physical Layer.43
7 VCR and SMKP Operation.43
7.1.1 Report Distribution VCRs .44
7.1.2 Publisher/Subscriber VCRs .46
7.1.3 Client/Server VCRs .48
7.1.4 System Management Kernel Protocol (SMKP) Communications .51

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EN 50170:1996/A1-1-4:2002
8 Specification of Fieldbus Systems.52
8.1 Common Model.52
8.2 Device Descriptions.53
8.2.1 Device Description Language (DDL).53
8.2.2 Device Description Services.54
8.3 Communications Profiles.55
8.3.1 Passive Devices.55
8.3.2 Remote Input/Output (I/O) Devices.55
8.3.3 Temporary Devices.56
8.3.4 Field Devices.56
8.3.5 Display and Control Devices.56
9 Configuration Information.56
9.1 Level 1 Configuration: Manufacturer Device Definition.56
9.2 Level 2 Configuration: Network Definition.57
9.3 Level 3 Configuration: Distributed Application Definition.58
9.4 Level 4 Configuration: Device Configuration.59
10 System Configuration and Startup.60
10.1 Offline Configuration.60
10.2 Link and System Startup.60
10.3 LAS Schedule Loading.61
10.4 Configuration Updates.61
Annex A (informative) Standard Data Types and Data Structures.62

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EN 50170:1996/A1-1-4:2002
General information on licensing
CENELEC calls attention to the fact that patent rights are linked to EN 50170
Amendment 1 (Foundation Fieldbus). CENELEC takes no position concerning the
evidence, validity and scope of this patent right. The following release of
rights was made available to CENELEC. This release of rights for EN 50170/A1
contains the following statement (extract):
“… Rosemount Inc. is willing to grant licenses under fair, reasonable and non-
discriminatory terms to the invention claimed und Unites States patent number
5,76489 and corresponding foreign patents for purposes of implementing products
that are compliant with EN 50170/A1. …”
Rosemount’s letter and Fieldbus Foundation’s undertakings (policy letter on
licensing) in this respect are on file with CENELEC and available for inspection
by all interested parties at the CENELEC Central Secretariat.
The license details may be obtained from:
Fisher-Rosemount European Headquarters AG
Blegistrasse 21
CH-6341 Baar
Switzerland
If during the application of this Standard Intellectual Property Rights may appear
and will not be made available on reasonable and non discriminatory terms and
conditions to anyone wishing to obtain such a license, applying the rules of
CEN/CENELEC Memorandum 8, this fact shall be brought to the attention of CENELEC
Central Secretariat for further action.
0 Scope
This document is an informative description of the architecture of the
EN 50170/A1 Volume 4 Fieldbus. It describes how the components specified in the
other Parts of EN 50170/A1 Volume 4 may be related and configured to construct
and support EN 50170/A1 Volume 4 Fieldbus devices, networks, and systems.
1 Overview
A fieldbus system is a distributed system composed of field devices and
control/monitoring equipment integrated into the physical environment of a plant
or factory. Fieldbus devices work together to provide I/O and control for
automated processes and operations. Fieldbus systems may operate in
manufacturing and process control environments that include intrinsic safety
requirements. In these environments, devices operate with limited memory and
processing power and with networks that have low bandwidth. The
EN 50170/A1 Volume 4 System Architecture provides a framework for describing
these systems as a collection of physical devices interconnected by a fieldbus
network. Fieldbus networks may be composed of one or more links interconnected
by bridges. Bridges transfer messages transparently between links. Communication
between physical devices and across bridges is provided by physical layer and
data link layer protocols.
Each physical device performs a portion of the total system operation by
implementing one or more application processes. Application processes perform
one or more time-critical functions, such as providing sensor readings or
implementing control algorithms. These and other elementary field device
functions are modelled as function blocks. Their activities are coordinated
through configuration of their operating parameters, execution schedules, and
communications.
NOTE  Application Process (AP) is a term defined by the International Standard
“Information technology - Open systems interconnection - Basic reference model
(ISO/IEC 7498) to describe the portion of a distributed application that is
resident in a single device. The term has been adapted to the fieldbus environment
to describe entities within devices that perform a related set of functions, such
as function block processing, network management, and system management.

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EN 50170:1996/A1-1-4:2002
Communication between application processes occurs through application layer
protocols. The layers between the data link layer and the application layer, as
defined by ISO/IEC 7498, are not present in the EN 50170/A1 Volume 4 System
Architecture.
1.1 Key Components of Fieldbus Systems
The EN 50170/A1 Volume 4 System Architecture is simple and open. It has been
designed to be simple and to meet the functional, environmental, and technology
needs of distributed manufacturing and process control systems. It is open so
that distributed automation systems can be constructed from control and
measurement devices manufactured by different vendors.
This architecture represents a departure from vendor specific control systems
architectures that have dominated the marketplace in the past. In these
architectures, proprietary interfaces define how devices communicate with each
other. Devices that work in one vendor’s control system, therefore, do not work
in that of another. Furthermore, because of these proprietary interfaces, the
upgrade and evolution of a system is controlled more by its vendor than by what
the industry as a whole has to offer.
The EN 50170/A1 Volume 4 System Architecture has been designed to change this.
Its purpose is to open the interface definitions to promote interoperability
between and among devices, regardless of vendor. It addresses application and
communications interfaces, and provides for network and system management. The
basic structure of this architecture is illustrated in Figure 1, followed by an
overview of its key components.
NOTE  EN 50170/A1 Volume 4 specifies the communication layers, shown respectively
as PhY (EN 50170/A1-2-4), DLL (EN 50170/A1-3-4 and EN 50170/A1-4-4), FAS
(EN 50170/A1-5-4), FFMS (EN 50170/A1-6-4), and their Management components for
System Management (EN 50170/A1-7.1-4) and Network Management (EN 50170/A1-7.2-4).
The other ‘User Layer’ components are included and described as informative
material to illustrate a full integrated system based on the EN 50170/A1 Volume 4
communication specification.
DD
System Function
Function Network Mgmt.
Mgmt. Block
OD
OD SMIB
Block APs Agent
Kernel
Objects
OD
FFMS FFMS FFMS LME
SMKP
FAS FAS LME
NMIB
DLL DL LME
PHY PHY LME
Figure 1 - Key Components of EN 50170/A1 Volume 4 System Architecture
1.1.1 Function Block Application Processes
Automated manufacturing and process control systems perform a variety of
functions. Because each system is different, the mix and configuration of
functions are different. Therefore, the EN 50170/A1 Volume 4 System Architecture
has been designed to support a range of functional models, each addressing a
different need.
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EN 50170:1996/A1-1-4:2002
One of these models, the function block model, has been specified within the
architecture to support low level functions found in manufacturing and process
control. Function blocks model elementary field device functions, such as
analogue input (AI) functions and proportional integral derivative (PID)
functions. The function block model has been supplemented by the transducer
block model to decouple function blocks from sensor and actuator specifics.
Additional models, such as the “exchange block” model, are defined for remote
input/output and programmable devices.
The function block model provides a common structure for defining function block
inputs, outputs, algorithms and control parameters and combining them into an
Application Process that can be implemented within a single device. This
structure simplifies the identification and standardization of characteristics
that are common to function blocks.
The Fieldbus Foundation has taken a first step in the standardization of
function blocks by defining a small set of parameters used in all function
blocks called universal parameters. A second level of standardization has
occurred through the definition of a standard set of function block classes,
such as input, output, control, and calculation blocks. Each of these classes
also has a small set of parameters standardized for it.
A third level of standardization provides definitions for transducer blocks
commonly used with standard function blocks. Examples include temperature,
pressure, level, and flow transducer blocks.
This classification scheme provides for a fourth level of standardization by
allowing vendors to add their own parameters by importing and subclassing
standard classes. This approach provides for the extensibility of function block
definitions as new requirements are discovered and as technology advances.
1.1.2 Object Dictionary (OD) and Device Descriptions (DD)
Associated with the function block model are standardized tool definitions, that
are used to support function blocks. Two of these tools are the Object
Dictionary (OD) and the Device Description (DD). They provide for the definition
and description of network visible objects of a device, such as function blocks
and their parameters.
To promote consistency of definition and understanding of these objects,
descriptive information, such as data type and length, is maintained in an
Object Dictionary (OD) by device applications. The OD contains a description for
each network visible object of an application, and makes this information
available over the network.
OD descriptions of device applications can be supplemented by machine readable
Device Descriptions (DD). DDs are written using the Device Description Language
(DDL). The DDL is a programming language used to extend OD descriptions by
providing type information about objects. The same DD type description may be
used to describe a number of objects, located in either a single device or in
multiple devices.
Once written, the DDL source is translated into a machine readable form, so that
it may be loaded into the device it describes, or stored on an external medium,
such as a floppy disk. Users of a device may then read its DD directly from the
device, or obtain it from the external medium. The function block model is used
in conjunction with the OD and DD to simplify the process of achieving device
interoperability. Parameter transfers between function blocks can be easily
verified, because all parameters are described using the OD and DD.
Additionally, human interface devices do not have to be programmed specifically
for each type of device on the network. Instead, their displays and their
interactions with devices can be driven from OD and DD descriptions.
1.1.3 Network Communications
The EN 50170/A1 Volume 4 network transfers information between devices using
preconfigured communications channels called Virtual Communications
Relationships (VCR). Three types of VCRs are defined. Their characteristics are
defined below, and their operation is described in Section 7 below.

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EN 50170:1996/A1-1-4:2002
Publisher/Subscriber VCRs: These VCRs are used by data producers (publishers)
to post data into a network buffer that may be
read, on demand, by users (subscribers) of the
data. The network buffer is distributed among the
publisher and its subscribers, such that a single
copy of the data is maintained in all buffers.
Updates to publisher buffers are transferred
simultaneously to all subscriber buffers in a
single broadcast. Transfers of this type can be
scheduled on a precisely periodic basis. Using
system management services, updates to the
publisher buffer can be synchronized with the
buffer transfers to reduce the delays between data
production and data transfer.
Report Distribution VCRs: These VCRs are used to broadcast and multicast
event and trend reports. The destination address of
a VCR may be predefined so that all reports are
sent to same address, or it may be provided
separately with each report. Transfers of this type
are queued. They are delivered to the receivers in
the order transmitted, although there may be gaps
due to corrupted transfers. These transfers are
unscheduled and occur in between scheduled
transfers at a given priority. A token passing
mechanism is used to share the unscheduled time
between devices.
Client/Server VCRs: These VCRs are used for request/response exchanges
between pairs of applications. Like Report
Distribution VCRs, their transfers are queued,
unscheduled, and prioritized. However, unlike
Report Distribution VCRs, they are flow controlled
and employ a retransmission procedure to recover
from corrupted transfers.
To support these VCRs, the EN 50170/A1 Volume 4 System Architecture defines a
three layer communications architecture. First, the physical layer specifies how
signals are sent. Second, the data link layer specifies how the network is
shared and scheduled among devices. Third, the application layer defines the
message formats available to applications for the exchange of commands and
responses, data, and event messages.
The Fieldbus physical layer is composed of the physical medium and signalling
protocol used to transmit data. Two power options are defined for twisted wire
pair. The bus powered option allows devices to draw their power directly from
the bus. This option provides the ability to implement low power devices for use
in intrinsically safe environments. The second power option is referred to as
self-powered, and requires devices to draw their power from an external source.
It does not provide power on the bus.
Three transmission speeds may be used in fieldbus networks, 31.25 kbit/s, 1
Mbit/s, and 2.5 Mbit/s. The 31.25 kbit/s and 1 Mbit/s speeds have been specified
to support intrinsically safe environments.
1.1.4 Network Management
To provide a means for integrating the layer 2 and 7 protocols (the
communication stack protocols) within a device, and for controlling and
monitoring their operation, the EN 50170/A1 Volume 4 System Architecture
includes a network management agent for each device. The network management
agent provides capabilities that support configuration management, performance
management, and fault management. The capabilities are accessed through the same
communication protocols used to access other device applications, instead of
requiring the use of a specialized network management protocol.

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EN 50170:1996/A1-1-4:2002
Using configuration management capabilities of the network management agent,
parameters are set within the communication stack to support data exchanges with
other devices in the system. This process normally involves defining the
transfers between devices and then selecting the desired communications
characteristics to support the transfers. These characteristics are loaded into
the device using the configuration management capabilities of the network
management agent.
As part of this configuration, the network management agent can be configured to
collect performance and fault related information for selected transfers. This
information is accessible during run-time, making it possible to view and
analyze the behaviour of device communications. If problems are detected,
performance is to be optimized, or device communications are to be changed, then
reconfiguration can be performed while the device is still operating. The nature
of the reconfiguration depends upon whether or not communication with the other
devices has been interrupted.
This configuration, performance, and fault information is contained in the
Network Management Information Base (NMIB), although much of it actually resides
within the communication stack itself. Like function block application
information, the NMIB is represented by a Management VFD and described by an OD.
The System Management Information Base also is represented by this VFD to
provide a central point of access to management information within the device.
1.1.5 System Management
The EN 50170/A1 Volume 4 System Architecture includes a System Management Kernel
(SMK) for each device. SMKs in all devices maintain information and a level of
coordination that provides a distributed platform for the execution and
interoperation of device applications. The information maintained by the SMK is
defined as the System Management Information Base (SMIB) and is described by its
OD.
One responsibility of the SMK is to provide for the configuration of basic
system information into its SMIB, prior to device operation. A SMK startup
procedure takes a device through a set of predefined phases as it prepares the
device for operation.
During this procedure, a special system configuration device, such as a
handheld, may pass system information to the SMIB through its standard fieldbus
port. This permits the device to be configured in an off-line workbench network
or on the operational network, depending on the particular needs of the system.
Once the device is configured, system management defines procedures for
assigning it a permanent data link address, based on its configured tag (system
specific name), and for bringing it to an operational state without affecting
the operation of other devices on the network. A similar procedure permits
temporary devices, such as hand-held terminals or portable workstations, to join
and leave the network for short periods of time as needed.
After a device has been added to the network, its applications may find it
necessary to locate remote devices and function blocks. To meet this need, the
SMK provides a directory service to its applications. This capability may be
used by an application to obtain network access information about an object by
broadcasting its name to all devices on the network and waiting for the device
containing the object to respond .
To become fully operational, the activities of devices and their function blocks
may have to be synchronized with those of the other devices on the network. Two
mechanisms are defined by system management to support this. First, system
management provides for the synchronization of application clocks across devices
to a common application time reference so that each device shares a common sense
of time. Second, and related to the first, system management uses scheduling
objects that control when function blocks execute. This ensures that each
function block executes at the appropriate time in relationship to other
function blocks in the system and in relationship to their scheduled data
transfers.
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SMKs communicate using two separate application layer protocols. FFMS is used
for access to the SMIB using a standardized Management VCR. This VCR is used
also to access Network Management Information contained in the joint SM/NM VFD.
A special purpose management protocol, the System Management Kernel Protocol
(SMKP), is used to support all other functions of the SMK. This protocol is
integrated into the SMK, and operates directly over the data link layer (no VCR
is used).
1.2 Relationship to the ISO OSI Reference Model
The ISO OSI Reference Model defines a general seven layer communications
architecture for the interconnection of open systems. This general architecture
has been optimized for the fieldbus environment by removing the middle layers
(which are normally associated with general purpose, non-time critical
applications such as file transfer and electronic mail). This three-layer
architecture, and its relationship to the ISO OSI seven layer architecture is
shown in the following Figure 2.
As shown in Figure 2, the Application Layer for the fieldbus is composed of two
sublayers. The Message Sublayer provides communication services to fieldbus APs,
system management APs, and to network management APs.
The Fieldbus Access Sublayer (FAS) provides different types of communication
channels, called Virtual Communication Relationships (VCRs). VCRs are mapped
onto the underlying network by the FAS to insulate application processes from
changes in the network technology.
The middle layers of the OSI architecture are not present in the system
architecture, while the Fieldbus Data Link and Physical Layer definitions
conform to the OSI definitions for these layers.
Message Sublayer
Application Layer
Fieldbus Access Sublayer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
OSI Layers Fieldbus Layers
Figure 2 - Relationship of the EN 50170/A1 Volume 4 Communication Architecture
to the ISO OSI Reference Model
1.3 Relationship to the International Electro-technical Commission (IEC)
Fieldbus Standards
The EN 50170/A1 Volume 4 System Architecture is the same as that proposed in
IEC/SC 65C/WG 6. The EN 50170/A1 Volume 4 Application Layer Protocols are
similar to those being specified in the IEC/SC 65C/WG 6 Application Layer
Committee. The Data Link Layer is a subset of the IEC/TS 61158-3 Data Link Layer
specifications. The EN 50170/A1 Volume 4 Physical Layer is a subset of the
EN 61158-2 Physical Layer specification.

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1.4 EN 50170/A1 Volume 4 Documents
The other parts of the EN 50170/A1 Volume 4 define standards for user
applications, network protocols, system and network management. A summary of
each of the documents in the EN 50170/A1 Volume 4 Specifications is provided
below, with additional informative material about Fieldbus Foundation User Layer
components (available from the Fieldbus Foundation, 9390 Research Boulevard,
Suite II-250, Austin, Texas 78759, USA) suitable for making an integrated
application based on EN 50170/A1 Volume 4.
Guide to System Architecture -- provides an introductory overview of the
EN 50170/A1 Volume 4 Fieldbus. It describes the Architectural Model for Fieldbus
Systems, Fieldbus Devices, and Fieldbus Communications, including the rules for
naming and addressing.
Function Block Application Process (FBAP) -- describes function blocks and the
application processes that contain them.
Device Description Language (DDL) -- describes the language used to specify
fieldbus applications and their interactions. This is one of the tools used
within the Fieldbus Foundation to achieve interoperability.
System Management (SM) -- specifies the data, procedures, and protocols used to
manage basic fieldbus device capabilities.
Network Management (NM) -- specifies data and procedures used to manage the
communications stack of a fieldbus device.
Foundation Fieldbus Message Specification (FFMS) -- specifies the
EN 50170/A1 Volume 4 application layer services and protocols used by fieldbus
applications. This protocol supports both client-server and publisher-subscriber
(distributed data base services) models for direct control of applications and
for distribution of data and events.
Fieldbus Access Sublayer (FAS) -- specifies the services, protocols, and
application layer communication relationship types used to convey application
layer protocol data units. It also specifies the mapping of communication
relationships to the data link layer.
Fieldbus Data Link Layer (DLL) Services Subset -- specifies a subset of
IEC/TS 61158-3:1999 Digital data communications for measurement and control -
Fieldbus for use in industrial control systems – Part 3 : Data link service
definitions (see EN 50170/A1–8-4).
Fieldbus Data Link Layer Protocol Subset -- specifies a subset of
IEC/TS 61158-4:1999 Digital data communications for measurement and control -
Fieldbus for use in industrial control system
...