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Field bus systemsModern automation techniques are distinguished by decentralization and distributed data processing.
Field buses enable the specific communication of automation components - bus nodes - among themselves. Since the beginning of the 90's bus systems are increasingly used, in particular open field bus systems such as Profibus, Interbus and CAN. Unlike conventional techniques these systems are economically interesting. Field buses are not only economically favorable they also represent a new technology which opens another dimension for the planning of system concepts incorporating decentralized solutions. Data exchangeDifferent bus systems use different principles to transfer data. Shift register with sum framework protocolThe bus master transmits the output data in each cycle to all slaves and receives as response the input data of all slaves. The small data range of this procedure becomes balanced by a high log efficiency. The bus cycle time is calculable. Master Slave principle with token passingThe data communication is controlled by the master. Slaves at the bus answer only on request of the masters. Every master has a determined time to exchange data with the slaves. The bus cycle time is therefore calculable. If several masters are available, the bus access right is regulated by exchanging tokens. The master which receives the token, possesses the exclusive right to access the bus. Delegated tokenA central bus arbiter regulates the data communication. It distributes the token, according to certain algorithms, to the individual bus users. If a user is in the possession of the token, it can transmit messages. Subsequently, it returns the tokens to the arbiter. Priority-controlled data communicationWith this method each user can transmit data at each point in time. To avoid collisions, or to resolve created collisions, there must be mechanisms responsible for arbitration. For example, CSMA/CA procedure (carrier sense multiple access with collision avoidance) prevents the emergence of a collision by simultaneous transmitting, the CSMA/CD PROCEDURE (Carrier Sense Multiple Access with Collision Detection) resolves developed collisions. Selection of the bus systemThe selection of a bus system depends very strongly on the application. Individual systems are optimized for the main application. Therefore a universal bus system is not possible. The following technical criteria can assist for the selection of a bus system:
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