Profibus (Process Field Bus)
PROFIBUS (process field bus) is a standard for fieldbus communication in automation technology and was distributed for the first time in 1989 by the BMBF (Federal Ministry for Education and Research) and later used by Siemens. It should not be confused with the PROFINET standard for industrial Ethernet. It is not an openly published / free protocol like older protocols such as MODBUS.
Origin of Profibus
The history goes back to a plan for an association that was publicly marketed in Germany in 1987. For this plan, 21 companies and institutes devised a master project/plan with the name “Fieldbus”. The aim was to spread the use of a bit-serial fieldbus. This should be based on the field device interfaces. For this reason, the companies agreed to support a common technological concept for production and process automation. At first, the complex communication protocol PROFIBUS FMS (fieldbus message specification) was specified. This was intended for demanding communication tasks. Afterwards, from 1993 onwards, the specifications for the simpler and faster PROFIBUS DP protocol was completed. FMS is used for (non-targeted) data communication between several masters. DP is a protocol that is used for (targeted) communication between masters and their I/O slaves.
Nowadays, two variants are being used, the most frequent of which is DP. The application-specific PA is used less frequently.
- DP is used to operate actuators and sensors with centralized controllers in production automation applications.
- PA is used to control measuring equipment with the aid of a process control systems in production applications. This variant is used in hazardous areas. The physical level (for example the cable) corresponds to the IEC 61158-2 standard which ensures that current is directed to field devices via the bus without creating explosive situations. PA has a data transfer rate of 31.25 kbps. However, PA uses the same protocol as DP and as such, it can be connected with a DP network via a connector.
In order to be able to apply these functions, several different service levels of the DP protocol have been defined:
- DP-V0 is for cyclic data / diagnosis exchange
- DP-V1 is for anti-cyclic / cyclic data exchange and alarm processing
- DP-V2 is for isochronous data exchange and slave-to-slave communication.
The security layer FDL (fieldbus data link) works with a hybrid access procedure that connects token passing with a master-slave method. In a PROFIBUS DP network, the controllers or the process controllers are the masters and the sensors and actuators are the slaves.
Bit Transmission Layer
There are three different methods for the bit transmission layer.
- Cable pairs with resistances of 150 Ohm are used in the bus topology for electrical transmission in accordance with EIA-485. Bit rates of 9.6 kbps to 12 Mbps can be used. The cable length between two repeaters extends from 100 to 1200 m, always depending on the bit rate that is used. This transmission method is especially used with PROFIBUS DP.
- Star, bus and ring topologies are used for optical data transmission. The distance between the repeaters can be up to 15 km. The ring topology can also be used redundantly.
- With MBP (Manchester bus-powered) data transmission, the data and fieldbus are supplied via the same cable. The power supply can be influenced so that use in hazardous areas is possible. The bus topology can be up to 1900 m long and connected with other field devices. The bit rate is fixed at 31.25 kbps. This technology is especially used in the process automation of PROFIBUS PA.
Products of different manufacturers can be used for data transmission via sliding contacts in the case of mobile devices or optical / wireless data transmission in open areas.
Profiles are predefined configurations of the functions and characteristics of the PROFIBUS and are destined for use in specific devices and applications. They are set by PI working groups and are made public by PI. Profiles are important for openness, interoperability and exchangeability so that the end user can be sure that a similar configuration from a different brand or a different seller will function in the same way. The choice that the user makes promotes competition, which motivates sellers and producers to offer better performance at a lower price.
PROFIBUS was defined in 1991/1993 in accordance with DIN 19245 and in 1996, it was integrated into EN 50170. Since 1999 it has belonged to IEC 61158/IEC 61784.
The PROFIBUS Nutzerorganisation e.V. (PNO) was founded in 1989. The group is composed mainly of different manufacturers and users from Europe. The first regional organization of this type in Europe (PROFIBUS Switzerland) was founded in 1992. In the following years other PROFIBUS & PROFINET associations were established so that meanwhile, quite a few different ones have become established. In 1995, these regional associations joined together under the international parent organization PROFIBUS & PROFINET International (PI). Today, it is represented by 25 regional associations all over the world. They have 1400 members, which include most, if not all large automation sellers and suppliers, as well as end users.