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Communication in Automation

The communication capability of devices and subsystems and consistent information methodology are indispensible components of future-oriented automation concepts. Communications are increasingly occuring horizontally at the field level as well as vertically through several hierarchical levels simultaneously. Layered and coordinated industrial communication systems, such as PROFIBUS with lower-level interfacing to AS-Interface and upper level interfacing to Ethernet (over PROFInet) (see Figure 1), offer ideal preconditions for transparent networking in all areas of the production process.

1.1 Industrial Communication

At the sensor-actuator level signals of binary sensors and actuators are transmitted over a sensor actuator bus. This provides a simple and cost-effective technology where data and power are transmitted over a shared medium. AS-Interface offers a suitable bus system for this field of application.

At field level distributed devices such as I/O modules, transducers, drive units, analysis devices, valves or operator terminals, communicate with automation systems over a powerful, real-time communication system. Transmission of the process data is cyclic, while additional interrupts, configuration data and diagnosis data are transmitted acyclically as required. PROFIBUS fulfills these criteria and offers an universal solution for both factory and process automation.

At the cell level programmable controllers such as PLCs and IPCs, communicate with each other and with IT systems of the office world using standards such as Ethernet, TCP/IP, Intranet and Internet. This information flow requires large data packets and a range of powerful communication functions.

As well as PROFIBUS, Ethernet-based PROFInet offers a trendsetting solution for this purpose.

In the following PROFIBUS will be described in detail as the central connecting link for the information flow in the automation industry. For information on AS-Interface please refer to the relevant literature. Furthermore PROFInet will be briefly presented in chapter 10.

Fieldbuses are industrial communication systems that use a range of media such as copper cable, fiber optics or wireless, with bit-serial transmission for coupling distributed field devices (sensors, actuators, drives, transducers, etc.) to a central control or management system. Fieldbus technology was developed in the 80s with the aim of replacing the commonly used central parallel wiring and prevailing analog signal transmission (4-20 mA- or +/- 10V interface) withdigital technology. Due, in parts, to the different industry specific demands and preferred proprietary solutions of large manufacturers, several bus systems with varying properties were established in the market. The key technologies are now included in recently adopted standards IEC 61158 and IEC 61784. PROFIBUS is an integral part of these standards.

Communication in automation technology

Figure 1: Communication in automation technology


Recently, Ethernet-based communication systems have emerged in industrial automation. They offer wide-ranging options for communications between the different levels of industrial automation and the office world. PROFInet is an example of one such Ethernet-based communication system.

A need for the coordinated development and distribution of these fieldbus systems in the market has seen the emergence of a number of User Organizations comprising manufacturers, users and institutes, such as the PROFIBUS User Organization (PNO) and its parent organization PROFIBUS International (PI) for PROFIBUS and PROFInet technologies.

User benefits are the motivation for the emergence and continual development of fieldbus technology. This ultimately manifests itself as a reduction of the total cost of ownership, as well as an increase in performance and quality improvement during the setup and operation of automation plants. The benefits are achieved during configuration, cabling, engineering, documentation, assembly and commissioning, as well as during production. An additional benefit is achieved by the reduction of the total life-cycle costs in the form of easy modification and continuous availability due to regular diagnosis information, preventive maintenance, simple parameter assignment, consistent data flow and asset management.

The OSI reference model

Figure 2: The OSI reference model

Fieldbuses increase the productivity and flexibility of automated processes compared to conventional technology and they create the basic prerequisite for the configuration of distributed automation systems.

Today PROFIBUS is used in virtually all areas of automation, in factory automation and process automation,

but also in traffic engineering, power generation and power distribution.


1.2 Fieldbus Technology Terms

The ISO/OSI reference model describes communications between the stations of a communication system. In order for it to run effectively, defined rules and transfer interfaces need to be used for the communications protocol. In 1983, the International Organization for Standardization (ISO) developed the OSI reference model ("Open Systems Interconnection Reference Model") for just this purpose. This protocol defines the elements, structures and tasks required for communication and arranges them into seven layers with each layer building upon the layer beneath (Fig. 2). Each layer has to fulfill specified functions within the communication process. If a communication system does not require some of those specific functions, the corresponding layers have no purpose and are bypassed. PROFIBUS uses layers 1, 2 and 7.

Communications Protocols define how two or more stations exchange data using message frames. A data frame contains different fields for messages and control information. The actual data field is preceded by the header information (source and destination address and details of the subsequent message) and followed by the data security part containing check information with regard to the correctness of the transmission (fault recognition).

A feature of fieldbuses is that they enable optimum transmission of small, time-critical data volumes and simplify the transmission process.

Bus access control (MAC, Medium Access Control) is a specific procedure that determines at which point in time a station can send data. While active stations can start the exchange of information, passive stations may only begin communication when prompted by an active station.

A distinction is made between a controlled, deterministic access procedure with real-time capability (master-slave with PROFIBUS) and a random, non-deterministic access procedure (CSMA/CD with Ethernet).

Addressing is necessary to selectively identify a station. For this purpose, station addresses are assigned

either by an address switch (hard addresses) or during parameter assignment during commissioning (soft addresses).

Communication Services fulfill communication tasks of the station either cyclic or acyclic user data communication. The number and type of these services are criteria for the application area of a communications protocol. A distinction is made between connection-oriented services (that means with handshake procedure and monitoring) and connectionless services. The second group includes multicast and broadcast messages that are sent either to a specific group or to all stations.

Profiles are used in automation technology to define specific properties and behavior for devices, device families or entire systems. Only devices and systems using a vendor-independent profile provide interoperability on a fieldbus, thereby fully exploiting the advantages of a fieldbus.

Application profiles refer primarily to devices (field devices, controls and integration tools) and comprise both an agreed selection of bus communications and the specific device application. This type of profile serves manufacturers as a specification for the development of profile-conforming and interoperable devices. System profiles describe classes of systems that include functionality, program interfaces and integration tools.

 

 

IEC 61158 document

Contents

OSI layer

IEC 61158-1

Introduction

 

IEC 61158-2

Physical layer specification and service definition

1

IEC 61158-3

Data-link service definition

2

IEC 61158-4

Data-link protocol specification

2

IEC 61158-5

Application layer service definition

7

IEC 61158-6

Application layer protocol specification

7

 Table 1: Breakdown of IEC 61158

 

Profile set

Data link

Physical layer

Implementation

Profile 3/1

IEC 61158 subsets;

asynchronous

transmission

RS485

Plastic fiber

Glass fiber

PCF fiber

PROFIBUS

Profile 3/2

IEC 61158 subsets;

synchronous

transmission

MBP

PROFIBUS

Profile 3/3

ISO/IEC8802-3

TCP/UDP/IP/Ethernet

ISO/IEC

8802-3

PROFInet

Table 2: Properties of the Communication Profile Family CPF 3 (PROFIBUS)

 

1.3 International Standardization

International standardization of a fieldbus system is necessary for its acceptance, establishment and its benefits. PROFIBUS achieved national standardization in 1991/1993 in DIN 19245, Part 1-3 and Europe-wide standardization in 1996 in EN 50170.

Together with other fieldbus systems, PROFIBUS has been standardized in IEC 61158 since 1999. 2002 saw the completion of activities to update IEC 61158. In the course of these activities, the latest PROFIBUS and PROFInet developments were incorporated in this standard.

IEC 61158 bears the title "Digital Data Communication for Measurement and Control – Fieldbus for Use in Industrial Control Systems “and is broken down into 6 parts that are designated 61158-1, 61158-2 etc. The contents of Part 1 deal with introductory subjects, while the subsequent parts are oriented towards the OSI reference model (layers 1, 2 and 7); see Table 1.

The various parts of IEC 61158 define, among other things, the numerous services and protocols for communication between stations which are regarded as the total available set, from which a specific selection (subset) is made for specific fieldbus systems.

The fact that a wide range of different fieldbus systems is available on the market is acknowledged in IEC 61158 by the definition of 10 “fieldbus protocol types“ with the designation Type 1 to Type 10.

PROFIBUS is Type 3 and PROFInet Type 10.

IEC 61158 notes that bus communication (by definition) is only possible between devices that belong to the same protocol type.

IEC 61784 bears the title "Profile Sets for Continuous and Discrete Manufacturing Relative to Fieldbus Use in Industrial Control Systems“. Assignment to IEC 61158 is established through the following introductory comment: "This international standard (i.e. IEC 61784) specifies a set of protocol specific communication profiles based on IEC 61158, to be used in the design of devices involved in communications in factory manufacturing and process control“.

IEC 61784 depicts which subsets of the total available set of "services“ and “protocols” specified in IEC 61158 (and other standards) are used by a specific fieldbus system for communication. The fieldbus-specific "communication profiles” determined in this manner are summarized in the "Communication Profile Families (CPF)“ according to their implementation in the individual fieldbus systems.

The profile sets implemented with PROFIBUS are summarized under the designation "Family 3” with subdivisions 3/1, 3/2 und 3/3. Table 2 shows their assignment to PROFIBUS and PROFInet.

Go back to the header article PROFIBUS Technology and Application

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