Profibus

PROFIBUS (Process Field Bus) is a standard for field bus communication in automation technology and was initially (1989 ) funded by the BMBF. It is not to be confused with the PROFINET standard for Industrial Ethernet.

  • 3.1 Application layer
  • 3.2 Link Layer
  • 3.3 Physical Layer
  • 4.1 Process Equipment
  • 4.2 drives
  • 4.3 Functional Safety

Formation

The history of PROFIBUS goes back to a 1987 launched in Germany publicly funded joint projects, for which 21 companies and institutions had drawn up a project framework " Fieldbus". The goal was the realization and distribution of a bit-serial field bus, for which the standardization of field device interface should create the basic requirement. To this end, relevant member companies agreed to support a common technical concept for the manufacturing and process automation. In a first step, the complex communication protocol PROFIBUS FMS (Fieldbus Message Specification) is specified, which was tailored for demanding communication tasks. In further steps, the specification of the simple structure making it much faster protocol PROFIBUS DP ( Decentralized Peripherals ) was carried out from 1993.

Use

Variants

PROFIBUS exists in three variants, where DP is the most widely used:

  • PROFIBUS DP (Decentralized Periphery ) for control of sensors and actuators through a central controller in production technology. Here in particular are also many standard diagnostic possibilities in the foreground. Other applications include the connection of "distributed intelligence", ie the networking multiple controllers to each other (similar to PROFIBUS FMS). Data rates up to 12 Mbit / s on twisted pair cables and / or fiber optic cable. The use of drive units is specified in the profile Profidrive.
  • PROFIBUS PA (Process Automation) is or PLC / DCS used for communication between measurement and process equipment, actuators and process control in the process and process engineering. PROFIBUS PA is a PROFIBUS variant with suitable for process automation Physical Layer ( transfer layer ) at which to PROFIBUS DP via so-called coupler multiple segments ( PA segments ) can be connected to field devices. The two-wire bus line these segments does not only communication, but also the energy supply of the participants ( MBP transmission technology ). PROFIBUS PA represents a digital alternative to traditional analog connection of devices ( 4 to 20mA ) technique and provides additional functionality through the bi-directional communication. PROFIBUS PA is suitable by restricting output also for hazardous areas. Depending on the application, different types of explosion protection are used. Particularly established is the degree of protection intrinsic safety. It allows access to field device during operation without a hot work permit ( FISCO specification). The data transfer rate of PROFIBUS PA is located at 31.25 kbit / s, which ensures high immunity to electromagnetic interference and allows long cable runs. This speed is adequate for process engineering applications. Another special feature of PROFIBUS PA is the widely used device profile " PA Devices " ( PA profile ) in which the main features of the field devices are standardized across manufacturers, which is for the user of significant benefit.
  • PROFIBUS FMS (Fieldbus Message Specification) was primarily intended for use in complex machines and systems. This protocol variant was replaced by DP and today is no longer part of the international fieldbus standard.

Scenarios

In the manufacturing industry PROFIBUS DP is used for rapid communication between controllers and sensors / actuators of all types in machinery and equipment.

The development comes in the manufacturing industry significantly towards higher speeds by use of Industrial Ethernet ( PROFINET) as a communication technology. Result, investments are largely protected by eg Figure of existing PROFIBUS devices profiles ( as Profidrive ) on PROFINET.

In the process industry, a duality exists between

  • Fieldbus (PROFIBUS PA and FOUNDATION fieldbus H1 ) for newly constructed facilities.

The benefits of fieldbus technology with respect to, for example, easier and more cost -efficient topology, bi-directional communication, extensive sources of information and standardization of the device functions ( Device Profile ) is fully located on the user side. Further developments are anticipated in the direction of easier handling of the field devices in operation and maintenance.

PROFIBUS is the only fieldbus that can be used in the manufacturing industry and in the process industry alike and has since become the world market leader. Worldwide, 43.8 million devices with PROFIBUS in use.

Technology

PROFIBUS protocols ( OSI model )

Application layer

DP application layer is defined in three steps. The original 1993 uncommitted DP protocol is now colloquially referred to as "DP -V0 ," the two extensions corresponding to " DP-V1" and " DP-V2 ". In each stage the following functions are defined:

  • In DP -V0, the cyclic exchange of data and diagnoses. Devices that support this functionality, mainly found in the general automation and machine control applications.
  • In DP -V1 acyclic data exchange and alarm handling. Devices supporting these extensions can be found mainly in process engineering.
  • In DP -V2, the isochronous data exchange, the slave - to-slave communication and time synchronization. With this expansion, especially requirements from production technology and robot control was taken into account.

The PA protocol has been defined during the development stage DP -V1.

Link layer

The data link layer FDL (Fieldbus Data Link) uses a hybrid access method that combines token passing with a master - slave method. In a PROFIBUS network, the control or process control systems are the master and the sensors and actuators the slaves.

There are different message types used, the (SD) can be distinguished by the start delimiter (PROFIBUS DP):

Abbreviations:

  • SD - Start Delimiter, to distinguish the type of message
  • LE - Length of the net data (including DA, SA, FC, DSAP, SSAP )
  • LEr - Repeat the net data length ( Hamming distance = 4! )
  • FC - Function Code
  • DA - Destination Address
  • SA - Source Address
  • DSAP - Destination Service Access Point
  • SSAP - Source Service Access Point
  • PDU - Protocol Data Unit ( net data )
  • FCS - Frame Check Sequence
  • ED - End Delimiter ( = 0x16 )

The FCS is calculated by simply adding up the bytes within the specified length. An overflow is ignored. Each byte is secured with an even parity and transmitted asynchronously with start and stop bit.

The individual bytes of a telegram must slip-free, ie be no pause between the stop and the next start bit is transmitted. The master signals the beginning of a new message with a SYN - break of at least 33 bits (logic " 1" = bus quiet state ).

For PROFIBUS PA, the equal division of message types exist, they have another telegram frame (except SD ) is equal, in contrast to DP for all frame types, including with special bit symbols of the Manchester code and a preamble for synchronization. A 16 -bit long CRC replaced as the FCS checksum, but it is used for each message type. The pause time between telegrams are also specified different. Wherein the DC- free signal level of the bus quiet state is characterized by the absence of a signal is determined ( a signal level change ).

Physical layer

At the physical layer, three different methods are defined:

  • For electrical transmission, EIA - 485 twisted pair cables are used with a wave impedance of 150 ohms in a bus topology. It can be configured to s 12 Mbit / s bit rate of 9600 bit /. Depending on the bit rate, the cable length between two repeaters is limited to 100 to 1200 meters. This transmission method is mainly used for PROFIBUS DP and FMS. For electrical transmission Two types of cables are specified (type A and type B). The most popular cable is the purple of type A.
  • In the optical transmission over fiber optic star, bus, and ring topologies are used. The distances between the repeaters can be up to 15 km. The ring topology can be configured with redundant architecture.
  • When MBP ( Manchester Bus Powered) transmission technology are transmitted over the same cable data and the feeding of the field devices. The power can be limited so that a use in potentially explosive atmospheres is possible. Then we speak of a so-called intrinsically safe area. The application of the FISCO concept is recommended. For this range have the bus cable that has a different specification than type A cable (EIA -485), a blue coat. The bus topology can be up to 1900 meters in length capable branches to the field devices with a maximum of 120 meters length. The bit rate is fixed here 31.25 kbit / s This technology has been specifically defined for use in process automation for PROFIBUS PA.

For the data transfer via sliding contacts, for mobile subscribers or for optical or harmful data transmission in free space products from several manufacturers are available, but they are not fixed in any standard (Wireless Profibus).

Inner workings of a plug

Cable for PROFIBUS DP in a characteristic color

Application profiles

For a smooth interaction of the bus nodes involved in an automation solution, this must be the same in their fundamental features and services. The unification is achieved by " profiles " with binding targets for the functions and services. The possible functions of communication are restricted to the Profibus and prescribed additional requirements on the operation of the field device. This may be to device classes cross- properties as a safety-related behavior (Common Application Profiles ) or to device class-specific properties ( Specific Application Profiles ) act. These distinction is made between

  • Device profiles (Device Profiles ) for example, Robot drives ( Profidrive ), process equipment, encoders, pumps
  • Industry profiles (Industry Profiles ) for example, Laboratory systems and rail vehicles
  • Integration Profiles ( Profiles integration ) for the integration of subsystems such as HART or IO-Link Systems

Process equipment

Modern process equipment have their own intelligence and can take over a part of the information processing or the overall functionality in automation systems. The profile PA Devices defined for different classes of process equipment all functions and parameters in process equipment typical of the signal flow from the sensor signal from the process to the preprocessed process value is read to the control system together with a measured value status. The PA Device profile contains device data sheets for

  • Pressure and differential pressure
  • Level, temperature and flow
  • Analog and digital inputs and outputs
  • Valves and actuators
  • Analyzers

The current version 3.02 of the PA device profile represents a novelty in the process industry is: Classified diagnostic information, device- neutral configuration or self - adapting field devices ( when replacing an earlier unit version out ) are a few examples.

Drives

Pro Drive is the device profile for modular drive units. It was jointly developed by manufacturers and users in the 1990s and since then covers in conjunction with PROFIBUS and PROFINET later addition with the entire range from the simplest to the most demanding drive solutions from.

Functional Safety

PROFIsafe defines how safety-related devices ( emergency stop pushbuttons, light curtains, overflow protection, ...) via PROFIBUS with safety controllers communicate so sure that they can be used in safety -related automation tasks up to SIL3 ( Safety Integrity Level). It implements the secure communication over a profile, ie a specific format of the user data and a specific protocol. PROFIsafe is set for PROFIBUS and PROFINET in the standard IEC 61784-3.

Standardization

PROFIBUS was 1991/1993 defined in DIN 19245, has been convicted in 1996 in EN 50170 and established since 1999 in IEC 61158/IEC 61784.

Organization

Since 1989, the Profibus User Organization (PNO ), in which manufacturers and users have come together in Germany exists. 1992 has been founded in Switzerland in the first regional Profibus Switzerland organization. In the following years further RPAs (Regional Profibus & Profinet Association) have been added. Today Profibus is represented by 27 RPAs in the world. In 1995, all RPAs have joined the international umbrella organization Profibus & Profinet International ( PI).

RS-485 Medical diagnosis Manchester-Code Bitrate International Electrotechnical Commission
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