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PROFIBUS

PROFIBUS

PROFIBUS is unique in offering a fully integrated solution for fieldbus in both discrete and process applications.

PROFIBUS DP is the profile for discrete (or factory) automation. An identical protocol, profiled as PROFIBUS PA, is used for process automation.

From the diagram you’ll see the single cable (the blue line) carrying communications between a controlling device (indicated as ‘DCS’, but it can also be a PLC or a PC) and individual IO devices. Above the blue line are typical factory devices controlled using PROFIBUS DP. Typical process layouts using PROFIBUS PA are indicated below the blue line as ‘segments’ of control.

Protocol

PROFIBUS devices communicate using the standardized PROFIBUS DP (Decentralized Periphery) communication profile which defines the rules governing communication. At the heart of the communication profile is what is known as the master/slave concept, whereby a master (active communication peer) polls the associated slaves (passive communication peers) cyclically.

When polled, a slave will react by sending a response frame to the polling master. A request frame contains the output data, e.g., set point speed of a drive and the associated response frame contains the input data, e.g., the latest measured value from a sensor. In one bus cycle, the master polls, e.g., exchanges I/O data with, all associated slaves.

This polling cycle is repeated as fast as possible. In parallel with this type of communication, which is described as cyclic and supports the regular exchange of input and output data between a master and its slaves, parameter data, e.g., device settings, can also be transmitted via PROFIBUS. This action is initiated by the master (typically under user program control) between I/O cycles to read and/or write slave parameter data. This type of communication is referred to as acyclic communication.

There can be more than one master on a PROFIBUS system. In such systems, access rights are passed from one master to the next (token passing).

In order to meet the specific requirements of the various fields of application in the best ways possible, the PROFIBUS communication system has been expanded beyond its basic functionality to include a number of additional levels supporting special functions. There are currently three such protocol levels: DP-V0, DPV1and DP-V2.

The major features of the three are as follows:

• DP-V0 supports the basic functionality of the PROFIBUS protocol. In particular, this includes cyclic I/O    communication and diagnostic reporting.

• DP-V1 adds optional functions for acyclic communication and alarm handling (enhancements to diagnostic reporting) to the PROFIBUS protocol.

• DP-V2 adds optional functions to the PROFIBUS protocol which are needed particularly in the field of drive control. These include functions for producer-consumer communication between slave devices, time synchronization and time stamping.

 

Field devices for process automation are typically slave devices which support the basic function of the PROFIBUS communication protocol (DP-V0) and are also capable of acyclic communication for the reading/ writing of device parameters (DPV1).

 

 

PURE PROCESS, AND HYBRID PLANTS TOO

The process industry’s view of fieldbus is similar to that of the discrete world, but with some physical differences because the process environment often involves explosive or incendiary processes. Power also must be delivered over the network cable - often over distances of 100s of meters!

For these reasons a PI 'Profile' known as PROFIBUS PA is used in process automation. This meets the ‘Intrinsically Safe’ (IS) and bus-powered requirements defined by IEC 61158-2, the fieldbus standard used throughout process automation. A PROFIBUS PA fieldbus network can connect up to 32 devices per 'segment' depending on type and application. 

PROFIBUS PA and PROFIBUS DP use an identical communications protocol and connecting them together is achieved easily using a simple coupler device. 

This gives PROFIBUS a big advantage in process applications because the same fieldbus can cover discrete (factory) and process applications. Most process plants have discrete functions (e.g. pumps, conveyors, actuators and robotic systems) working alongside plant instruments such as pressure and temperature transmitters, so having a single fieldbus for both means big cost savings.

We refer to this as the ‘hybrid’ capability of PROFIBUS, which means it has become the clear fieldbus leader in process automation with approximately 4.8 connected devices by the end of 2008. Of these, over 800,000 met the ROFIBUS PA Profile.

OPTIMIZED FOR DISTRIBUTED IO

PROFIBUS is the most popular fieldbus in the world for discrete automation. It’s now 20 years since it was developed by a consortium of automation companies and academic institutions and it was one of the first to achieve international standardization in 1996.

PROFIBUS DP is optimized for distributed I/O applications. Up to 126 I/O devices can be connected to a PROFIBUS DP cable. Since each I/O device can handle hundreds of connection points, this provides very large number connection possibilities for a single PLC or controller. PROFIBUS DP is suited to just about any manufacturing automation use, from simple conveyor systems and assembly lines right up to complex printing machines and robotic cells.

As well as the basic control of inputs and outputs, PROFIBUS DP supports a range of standardized automation application services, or ‘Profiles’ - Motion Control and Functional Safety are just two examples - that add significantly to the value proposition and business benefits.

In other words, PROFIBUS-enabled plants run more effectively and with lower life cycle costs, enhancing profitability and improving manufacturing across the enterprise.

REASONS FOR USING PROFIBUS

v  It's the preferred fieldbus for end users and it's used in the largest number of applications worldwide.

v  It offers huge vendor and product choice.

v  It delivers true openness and total interoperability, allowing changes/updates at low cost.

v  Its protocol is optimized for factory and process control using standardized interfaces, and therefore it's ideal for hybrid applications too.

v  Less hardware needed, which means less weight and space also lowered installation and life cycle costs.

v  There's easy and consistent integration of functional safety and motion control in factory and process automation applications.

v  There's flexible media redundancy.

v  There's tightly-managed technology development, including test & certification processes.

v  It's supported by PI (PROFIBUS International) is the world’s largest fieldbus organization.

v  There's easy migration to PROFINET, PI's Industrial Ethernet solution.

 

Power and communication via a single cable

 

Like conventional 4-20 mA or HART communication technologies, fieldbus technology supports the simultaneous transmission of power and communication data via a single cable, even in potentially explosive atmospheres. Furthermore, with wiring overheads significantly reduced, it meets the requirements for simpler and safer installation and boasts all the benefits of digital transmission.

IEC 61158-2 defines MBP (Manchester-encoded, Bus Powered), as a transmission technology satisfying all requirements of and developed specifically for the needs of process automation. This transmission technology makes it possible to supply power to the connected devices directly via the bus medium. MBP is characterized by the following features:

v  Transfer rate: 31.25 kbps

v  Transmission technology: Half duplex, synchronous, self clocking, with Manchester biphase L encoding

v  CRC (cyclic redundancy check)

v  Data security: Preamble, failsafe start-end delimiters

v  Cable: Shielded, twisted pair line (type A or type B)

v  Topology: Line and tree topology with termination; combined topology possible

v  Number of stations: Up to 32 stations per segment

v  Ignition protection: Several methodologies and technologies

Explosion protection is implemented via power limiting of the incoming bus supply as well as installation components in the field. Live maintenance on field devices during plant operation is made possible, for example, by means of intrinsically safe explosion protection. The easiest way to verify intrinsic safety of a segment is to use the FISCO model. In this case, since all components used comply with FISCO standards simple comparisons of power voltage and current eliminate further calculations.

Topology

PROFIBUS PA offers quite flexible installation concepts which, thanks to the advanced installation technologies available on the market, can lead to incredibly robust systems. In principle, all topologies are supported.

However, in practice, the trunk & spur topology has established itself as the de facto standard due to the fact that it is so clear and easy to understand and maintain. The total length of a segment in the most ideal situation must not exceed 1,900 meters.

Transmission conditions for PROFIBUS PA can be optimized by using type A cable consisting of a single shielded, twisted pair. All segments must be terminated correctly with terminating resistors (“T”). These terminations are very important for reliable operation due to their effect on signal quality.

 

 

 

 

PROFIBUS, PLANT-WIDE

PROFIBUS is used in more than 500,000 applications to solve all kinds of automation challenges in manufacturing and process control. Below are a number of application reports which show how PROFIBUS can be used to resolve an automation task. These examples in different industries clearly demonstrate the benefits of PROFIBUS in comparison with traditional technology.

v Food & Beverage.

v Water & Wastewater.

v  Oil & Gas.

v Energy Pulp & Paper.

v Paints, Chemical, Pharma.

v Metal, Mining, Glass, Cement.

v Machine.

v Traffic, Infrastructure.

v Car manufacturer.

v Identification

 

http://www.profibus.com/technology/profibus/case-studies/

 

Web link: www.profibus.com ;

PDF: PROFIBUS-PA-system-descr_e_Aug07.pdf