Actuators with fieldbus interface - AUMA - #9

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Fieldbus cables/installation Cable types Data transmission via fieldbus cannot be implemented on any cable. Cable types are specified for each field­bus system. Due to the low data transmission rate, the requirements for Foundation Fieldbus are comparatively low; DeviceNet requires more complex cables as the bus specification requires a separate supply voltage within the same cable. Installation Data transmission on fieldbus systems are performed with low signal levels, e.g. +/- 5 V. The installation has to be performed thoroughly to ensure fault-free data transmission. This applies to screening, equipotential earth bonding, observance of the max. permissible cable lengths, correct setting of the bus terminations and addresses of the field devices. Observing the installation guidelines ensures that all field devices on the bus can be smoothly commissioned. Fibre optics As an alternative many fieldbus systems provide data transmission via fibre optic cables (FO). Longer distances can be covered with the cable types described in the fieldbus specifications. An electric opto-coupler converts the fieldbus signal into an optical signal and vice versa. Repeater/Extender Generally speaking, repeaters or extenders are ampli-fiers which extend the cable length of a fieldbus segment. The section between the control system and the first re-peater or between two repeaters is called a segment. The number of repeaters or segments is limited. Furthermore repeaters are also used: ■ to implement drop lines ■ to create another bus segment if the maximum number of connectable devices per segment has been reached ■ to provide galvanic separation between the segments ■ to refresh the timing of the data signals I 9