Profibus - Is PROFIBUS A Field Bus Communication Standard
PROFIBUS is the abbreviation for Process Field Bus and is the standard for field bus communication in automation technology. The PROFIBUS communication protocol was created in 1989 by a consortium of companies and institutions and promoted by the BMBF (german department of education and research). PROFIBUS has since become the most popular fieldbus in discreet manufacturing and process control, with over 20 million installed devices world wide.
PROFIBUS is divided into two variations, the more commonly used DP (Decentralized Peripheral) version that replaced the first complex communication protocol version FMS (Fieldbus Message Specification) in 1993 and the less commonly used PA (Process Automation) protocol version.
PROFIBUS DP is the high speed solution of PROFIBUS. It has been designed and optimized especially for communication between automation systems and decentralized devices. It can operate at data rates of up to 12Mbit/s over twisted pair cables or fiber optic links. The communication is via cyclic data traffic exclusively, and each field device needs to exchange its data with the control device (Master) within a given cycle time. Operation and monitoring tasks for a visualization device separate from the control device was not supported by this specification and appropriate function extensions were defined in 1997. The new extended version is referred to as PROVIBUS-DPV1 and is compatible with all earlier versions of the existing DP protocol. PROVIBUS-DPV1 also offers, in addition to the cyclic DP communication services, an acyclic service for alarm messages, diagnostics and control of field devices.
PROFIBUS AP is used to monitor measuring equipment via a process control system. It also meets the two special requirements of process automation, an intrinsically safe transmission technique and field device power over bus cable. The disadvantage of this protocol is its slow data rate of only 31.25kbit/s. Weak current flow through the bus lines makes it intrinsically safe and ideal for use in explosion-hazardous areas.
The FROFIBUS protocol (OSI reference model)
For the User Interface of PROFIBUS various service levels were defined for the DP protocol:
·DP-V0 provides the service for cyclic exchange of data and diagnosis.
·DP-V1 provides the service for acyclic and cyclic exchange and alarm handling.
·DP-V2 provides the service for isochronous mode data exchange broadcast (slave-to-slave communication).
Data Link layer:
The Field Bus Data Link Layer (FDL) work with a hybrid bus access protocol. It combines token passing with a master slave method. The token passing procedure guarantees that the bus access gets assigned to a specific master node for a defined time frame. In this time frame the master node can access the bus and request data from any master or slave device. After the time frame expires the current master node will pass the token to the next master node on the bus. This layer also includes the handling of data security and the transmission protocol of telegrams.
Physical layer or bit-transmission layer:
Three different methods are specified for the physical layer of FROFIBUS
·Electrical wire transmission EIA-485, twisted wire pair with impedance of 150 ohms. Specified for Bit rates 9.6 kbit/s to 12Mbit/s. This transmission method is usually used with PROFIBUS DP. The cable length between repeaters is limited from 100 to 1200 m, depending on the bit rate.
·Fiber optical transmission uses, star-, bus- and ring topologies and the distance between repeaters can be up to 15 km.
·With Manchester Bus Powered transmission technology the data and device power are fed through the same cable. The bit rate is a fixed 31.25 kbit/s. This technology was specifically developed for PROFIBUS PA.
Other transmission mediums products, like radio waves, sliding contacts etc. can be obtained from various manufacturers, but this does not mean they conform to any standards.