profibus-introduction
Profibus - Introduction
Profibus
Profibus (Process Field Bus) is the most popular type of fieldbus with more than 25 million nodes (2008) in use worldwide. It is widely used for its fully integrated solution in discrete and process applications. Usage ranges from automated robotic manufacturing, building automation and food production to power, water and waste treatment plants. There are approximately 3000 products from 300 different vendors and over 1100 member organizations worldwide.
Profibus was developed in 1989 as the result of a German research project involving 21 companies and institutes. The group which had taken over this task was named ZVEI (Central Association for the Electrical Industry). ZVEI completed its goal_ with the creation of Profibus FMS (Fieldbus Message Specification). In 1993 the group introduced a new standard, Profibus DP (Decentralized Periphery). This new version featured more simplicity, easier configuration, faster messaging and has replaced the FMS standard in most applications. Profibus was introduced as German standard DIN 19245, later adopted as International Standard EN 50170 and then incorporated into IEC 61158 as an international fieldbus standard.
Profibus Family
There are three variants of Profibus:
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Profibus DP - Decentralized Periphery - is used to operate sensors and actuators via a centralized controller in production technology. It is a low cost variant with a high speed up to 12 Mbit/s. For its simple field level communication and the previously mentioned reasons, about 90 percent of current Profibus applications are DP.
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Profibus FMS - Field Message Specification - is for high end, application level communication - Used at cell or controller level. It provides object oriented transmission of structured data, loading and control of programs, alarm services etc. By definition the associated hardware was complex and expensive to implement. This standard is obsolete and no longer supported by Profibus International.
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Profibus PA - Process Automation - developed specifically for the process industry. The standard uses a two-wire connection carrying both power and data. It is used in applications requiring intrinsic safety. This is the reason why it is more expensive and therefore required in only specific applications.
Profibus Architecture
The Profibus standard is based on the well known ISO/OSI layers architecture. Profibus defines only three of the seven layers of the model: physical layer, link layer and application layer. The remaining of layers are partly included in these three layers.
Physical Layer
The physical layer defines properties of the transmission system like electrical levels, wiring, network topology, data coding and communication speed. Data is transmitted asynchronously at baud rates:
- 9.6, 19.2, 45.45, 93.75, 187.5 and 500~kbit/s; 1.5, 3.0, 6.0 and 12.0~Mbit/s
Profibus provides three possible transmission systems, each with its own
area of applicability:
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RS-485 (Profibus-H2) - is an electrical specification of a two wire, half-duplex, multi-point serial communication. Since it uses a differential balanced line over a twisted pair, it can span relatively large distances (up to 1200 m) . The RS-485 standard is administered by the TIA (Telecommunications Industry Association) and is registered as standard EIA-485. The standard only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any data protocol. Four wire connection with a power supply is also possible. The maximal number of stations per segment is 32.
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Fibre optic (Profibus-FO) - plastic or glass fibre-optic cable can be used giving high speed, interference free communication with electrical isolation.
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IEC 1158-2 (Profibus-H1) - shielded twisted pair wire with up to 32 stations per segment. Power supply and data is transmitted over the same cable. This transmission system can be used in hazardous environments - used by Profibus PA.
Link Layer
The Link layer takes control over the Medium Access. Data is exchanged in all Profibus systems using messages or telegrams that pass between stations. The telegrams are encoded and decoded in this layer. The Link layer consists of the FDL (Field Data Link), a part realizing the main function, and the FMA (Fieldbus Management), a part controlling the layer.
Profibus devices can be Masters or Slaves. Master devices (also called active stations) control the bus for a limited period. During this period they send requests to other devices and receive back responses. Slave devices (also called passive stations) can only respond to master requests - they can never send a message without first receiving a request. Slave stations are often field mounted devices, e.g. I/O blocks, transmitters, sensors, actuators, valves, drives, etc. Therefore at least one master must be present in every Profibus network.
Master stations control the network communications. Slave stations respond to telegrams from their controlling master. A network can have one or more master stations. Each master can control one or more slaves. All devices on the bus have the same priority, no master or slave station is more important than any other.
Application Layer
The application layer is used by the user applications. In most cases the application layer is implemented as user libraries used to simplify communication with the link layer. It uses the FDL part for accessing the medium and the FMA part for accessing services such as setting bus parameters or activating SAP (Service Access Point) services.
References
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