What is PROFIBUS ?
PROFIBUS is an international fieldbus communications standard for linking process control and plant automation modules. Instead of running individual cables from a main controller to each sensor and actuator, a single multi-drop cable (or other communications link eg fibre or wireless) is used to connect all devices, with high speed, bi-directional, serial messaging used for transfers of information.
This arrangement can offer huge savings on cabling costs, when compared with older methods, particularly for large sites. It allows simple linking of devices from many different vendors and, with care, can reduce installation, commissioning, fault-finding and associated plant downtime costs.
Due to the use of serial messaging for transfer of sensor information to a controller and return of any required actuator commands over one common bus link, special tools are necessary to view what is happening on this vital communications artery. These tools are needed to verify the bus integrity, the communication waveform shapes and content quality of the signals from each separate device for verification of optimal operation and location of weaknesses and faults. For most effective use, these tools need trained and experienced personnel able to apply the appropriate tests, at the right location and to successfully understand and interpret the results presented, so allowing rapid and accurate identification of any errors, and their correction.
There are two different versions of PROFIBUS. These are known as PROFIBUS DP and PROFIBUS PA. Simplistically, PROFIBUS DP is usually run over violet sheathed, two core screened cable, at speeds from 9.6 kbps up to 12 Mbps, using RS 485 balanced transmission. The required speed for any one network is chosen to give an appropriate cycle time for communication with all connected devices, to effectively and accurately achieve their respective operational role in a timely manner. Slower communication speed allows longer cable runs and are suited to more slowly changing operational systems faster speeds and shorter cycle times are needed for quickly changing systems. Note that the RS 485 standard allows connection of only 32 devices larger networks may be built by adding more, electrically isolated, segments using repeaters and/or hubs.
PROFIBUS PA is normally run over heavier gauge, two core screened cables, often with blue outer sheath, at fixed 31.25 kbps communications speed, with near identical message formats to those of PROFIBUS DP, but here with both slave power and communications signals often carried over the same wires. PROFIBUS PA communication may be employed for systems needing intrinsically safe equipment where risks of explosion must be minimised.
Neither PROFIBUS DP nor PROFIBUS PA should be confused with ProfiNet which is a totally different Ethernet based communications standard used for process measurement and control over Cat 5 (or similar) twisted pair cables, as most frequently seen for linking office computers.
The above notes are intended as a simple introduction to PROFIBUS and why special tool are needed for associated commissioning and maintenance work. This widely used International Standard is evolving all the time - See http://en.wikipedia.org/wiki/Profibus for a more formal definition and history, www.profibus.com for comprehensive information about the PROFIBUS Standard and benefits of its adoption, and www.profi-bus.co.uk for more information about the UK PROFIBUS Group and UK PROFIBUS Engineer Training Courses
Please forward any ideas for improving or correcting these notes to Hi-Port Telecom. Hi-Port Telecom is a Trading Style of Hi-Port Software Limited.
© Copyright Hi-Port Software Ltd 2008 revised 29 July 08