Difference between profibus and foundation field bus


ProfiBus (PROcess FIeld BUS) is a widely accepted international networking standard, commonly found in process control and in large assembly and material handling machines. It supports single-cable wiring of multi-input sensor blocks, pneumatic valves, complex intelligent devices, smaller sub-networks (such as ASi), and operator interfaces. It is an open, vendor independent standard. It adheres to the OSI model, ensuring that devices from a variety of different vendors can communicate easily and effectively. It has been standardized under the German National standard as DIN 19 245 Parts 1 and 2 and, in addition, has also been ratified under the European national standard EN 50170 Volume 2.

The bus interfacing hardware is implemented on ASIC (Application Specific Integrated Circuit) chips produced by multiple vendors, and are based on RS-485 as well as the European EN50170 Electrical specification. ProfiBus uses 9-Pin D-type connectors (impedance terminated) or 12mm round (M12-style) quick-disconnect connectors. The number of nodes is limited to 127.

The distance supported is up to 24km (with repeaters and fiber optic transmission), with speeds varying from 9600bps to 12Mbps. The message size can be up to 244 bytes of data per node per message (12 bytes of overhead for a maximum message length of 256 bytes), while the medium access control mechanisms are polling and token passing. ProfiBus supports two main types of devices, namely, masters and slaves.

  • Master devices control the bus and when they have the right to access the bus, they may transfer messages without any remote request. These are referred to as active stations
  • Slave devices are typically peripheral devices i.e. transmitters/sensors and actuators. They may only acknowledge received messages or, at the request of a master, transmit messages to that master. These are also referred to as passive stations

Foundation Fieldbus

Foundation Fieldbus allows end-user benefits such as:

  • Reduced wiring
  • Communications of multiple process variables from a single instrument
  • Advanced diagnostics
  • Interoperability between devices of different manufacturers • Enhanced field level control
  • Reduced start-up time
  • Simpler integration.

The concept behind Foundation Fieldbus is to preserve the desirable features of the present 4-20mA standard while taking advantage of the new digital technologies. This provides the features noted above because of:

  • Reduced wiring due to the multi-drop capability
  • Flexibility of supplier choices due to interoperability
  • Reduced control room equipment due to distribution of control functions to the device level
  • Increased data integrity and reliability due to the application of digital communications.

Foundation Fieldbus implements four OSI layers. Three of them correspond to OSI layers 1, 2 and 7. The fourth is the so-called ‘user layer’ that sits on top of layer 7 and is often said to represent OSI ‘layer 8’.The user layer provides a standardized interface between the application software and the actual field devices.

Also read

Smart Fieldbus approach to instrumentation system

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