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FAQ


< How to terminate the shielding of the cable
< What is PROFIBUS-FMS?
< What is PROFIBUS-DP?
< Ident numbers, what are they, where to get them?
< What is PROFIBUS-PA?
< Hamming distance (HD), what does it mean?
< Why are terminators important?
< What is a profile? Which are available?
< What are the main advantages of PROFIBUS over other fieldbus types?
< What are master class1 and class2 different?
< What is a fail safe mode? What does a fail safe mode mean?
< Why is no signal ground/reference wire used in PROFIBUS cables?
< Is PROFIBUS an open or proprietary system?
< What is the biggest advantage to users of using PROFIBUS over competitive protocols?
< Is it possible to calculate a cost per device for a PROFIBUS System?
< What is a GSD file?
< What are the roots of PROFIBUS?
< What if the system should fail or a chip in a device fail?
< Can new devices be added to an installed PROFIBUS system in future without upgrading the system?
< Does a PROFIBUS system require skilled personnel? What is the average learning curve?
< What aftersales support services can users of PROFIBUS expect?
< What hardware and software is needed to set up a PROFIBUS system?
< What connectors, pinout and mandatory signals are used?
< Are there any particular applications/sectors of industry that particularly benefit from PROFIBUS?
< Cable specification. What to select?


How to terminate the shielding of the cable < Top

It is recommended to connect the shield on both sides low inductively with the protective ground in order to achieve optimal electromagnetic compatibility. In case of separate potentials (e.g. refinery) the shield should be connected only at one side of the bus cable to the protective ground. Preferably the connection between shield and protective ground is made via the metal cases and the screw top of D-sub connector. If this is not possible the connection can be made via pin 1 of the D-sub connector. It should be noticed that this is not the optimal solution. In such a case it is better to bare the cable shield at an appropriate point and to ground with a cable as short as possible to the metallic structure of the cabinet. This could be achieved with a ground bus bar in front of the bus connector.



What is PROFIBUS-FMS? < Top

This is the universal solution for the communication tasks at the upper level (cell level) and the Field Level of the industrial communication hierarchy. In order to carry out the extensive communication tasks with acyclic or cyclic data transfers at medium speed, the Fieldbus Message Specification (FMS) services offer a wide range of functionality and flexibility. PROFIBUS-FMS is included into the European Fieldbus Standard EN 50170.



What is PROFIBUS-DP? < Top

This is the performance optimized version of PROFIBUS, specifically dedicated to time-critical communication between automation systems and distributed peripherals. It is suitable as a replacement for the costly parallel wiring of 24 V and 4(0) to 20 mA measurement signals. PROFIBUS-DP is included into the European Fieldbus Standard EN 50170.



Ident numbers, what are they, where to get them? < Top

Every DP/PA device type has to have an individual Ident Number. This number is necessary, so that a DP-master is able to identify the types of the connected DP/PA devices without a significant protocol overhead. The master compares the Ident Number of every connected device with the Ident Number in the configuration database. User data transfer in the operation phase is only possible when the right DP-Slave is connected with the correct address. This ensures a very high protection against parameterization faults. The vendors must apply to the PPROFIBUS Support Center for an individual Ident Number for every DP/PA device type.

Please contact your local PROFIBUS organization or PROFIBUS International for further information.



What is PROFIBUS-PA? < Top

PROFIBUS-PA is the solution for process automation, connecting automation systems and decentralised field devices. PROFIBUS-PA is based on PROFIBUS-DP (acc. to EN 50170) and permits a transparent communication from general purpose automation to process automation. The Profibus-PA profile defines the behaviour of the field devices and ensures full interoperability and interchangeability of the field devices from different manufacturers. Profibus-PA operates either with intrinsic safe transmission technology (acc. to IEC 1158-2) or standard transmission technology (acc. to RS485). PA fulfils the special requirements of the process automation industry e.g. chemical or petrochemical applications.



Hamming distance (HD), what does it mean? < Top

The hamming distance is a messure for how secure a protocol is against misinterpretation of a packet with errors as a different legal packet. HD=4 tells us that at least 4 bits has to be wrong, and still match the checksum calculations in order to be mistaken as another valid packet.



Why are terminators important? < Top

Termination of a bus line is done to prevent signal reflections on the PROFIBUS cable. Wrong or missing termination of the line results in lower efficiency due to transmission errors. Worst case is that the communication link is lost. In addition to traditional termination, the PROFIBUS termination also provides a defined idle level on the cable.



What is a profile? Which are available? < Top

Profiles are commitments about used non-mandantory services and bus parameters for specific areas of applications. It helps to minimize the implementation efforts to implement PROFIBUS functionallty into appropriate field devices.

Profiles are available for



What are the main advantages of PROFIBUS over other fieldbus types? < Top

This is a very subjective issue, as the faced challenge is setting the importance of the different aspects leading to a choice. But - provided that the technical solution is satisfactory for the task, it all boils down to cost/benefit comparisons.



What are master class1 and class2 different? < Top

A class 1 master can communicate activly only with it's configured slaves and is able to communicate in a passive way with a class 2 master.

The class 2 master is the 'supervisory' master. He can communicate with other class 1 masters, theire slaves and his own slaves for configuration, diagnostic and data/parameter exchange purpose.



What is a fail safe mode? What does a fail safe mode mean? < Top

This mode defines the status of IO signals of IO modules in the case of a bus or system failure. It is common that all output signals are going to the zero voltage level in a fail safe situation.



Why is no signal ground/reference wire used in PROFIBUS cables? < Top

To ensure easy handling an additional signal ground/reference wire is not used by PROFIBUS. It is recommended to isolate the interface circuit from the local ground (e.g. by opto couplers). This reduces a possible common mode voltage between transceivers to a minimum.



Is PROFIBUS an open or proprietary system? < Top

PROFIBUS is completely Open. It was originally standardised in Germany in 1989 as DIN 19245 and in July 1996 as EN 50 170. The EN 50 170 specification is available through any of the national standards bodies of CENELEC / IEC and the PROFIBUS Specification can be supplied by any of the Regional PROFIBUS Associations.



What is the biggest advantage to users of using PROFIBUS over competitive protocols? < Top

PROFIBUS provides users and manufacturers with the best possible chance of a future proof bus system that copes with the broad needs of factory and process automation. The market acceptance of the technology and the sheer momentum of current product developments proves that in addition to the EN 50170 approval, PROFIBUS is the de-facto standard bus of choice throughout Europe, based on technology that allows for future device types and applications too.



Is it possible to calculate a cost per device for a PROFIBUS System? < Top

Not generally, but to indicate the base technology costs a PROFIBUS ASIC (Application Specific Integrated Circuit) costs from around $20 for a simple slave which completely handles the PROFIBUS Protocol to just $12 for an intelligent slave and $30 for a master ASIC, which both require a microprocessor. In addition most field devices are already using digital technology internally, and introducing a digital interface removes the need for expensive digital to analog converter circuitry. So PROFIBUS connectivity should not add significantly to standard field device cost.



What is a GSD file? < Top

A GSD file is used to identify a PROFIBUS-DP/PA device. (Master or Slave). It contains data making it possible to have manufacturer independent configuration tools. Typical information in a GSD file is Vendor information, Baudrates supported, Timing information, Options/features suppported and Available i/o signals. A GSD file must be available for every DP/PA slave.



What are the roots of PROFIBUS? < Top

by Armin Steinhoff.

The base of the specification of the PROFIBUS standard was a research project (1987-1990) with the following members:

and five German research institutes

There was also a minor sponsorship of the German government. The result of this project was the first Draft of the DIN 19245, the PROFIBUS standard, part 1 and 2.

Part 3, PROFIBUS-DP was defiened 1993 by the following working group:

Yes, 10 years is a long time .... hope this helps a little bit to remember!



What if the system should fail or a chip in a device fail? < Top

PROFIBUS has been designed to allow configurations where redundant cabling is possible and this takes account of wire breaks. Also node failures can be configured to be ignored or to trigger a stop in the master, in this case unaffected nodes can continue operation. When a failure is identified the master will immediately resend the telegram and you can configure the number of times a re-try is attempted. Information relating to the failure is generally available on a node, module within a node and a channel specific basis. Certification testing of devices through an authorised test laboratory ensures that failures conform to what is expected of them.



Can new devices be added to an installed PROFIBUS system in future without upgrading the system? < Top

Any new functionality for PROFIBUS will offer backward compatibility. Quite specifically if new functions are offered it will be possible to use them on the same wire as devices which do not offer the new functions. This allows progression of the system without forcing users to upgrade unless they specifically wish to enhance the functionality of the rest of the system. An example is the latest extensions to the DP Protocol that allow acyclic transmission to slaves but leave communications with existing devices unaffected.



Does a PROFIBUS system require skilled personnel? What is the average learning curve? < Top

PROFIBUS has been designed to be simple. Assuming a knowledge of PLCs and field device technology it has been our experience that the necessary information to set up a network can be imparted in as little as half a day. There are of course training courses available from vendor members (2 days typically) and the PROFIBUS user organisation itself runs a series of practical workshops from integration to device implementation.



What aftersales support services can users of PROFIBUS expect? < Top

There are currently more than 600 companies worldwide who have committed to the PROFIBUS User Organisation, some 150 of which companies manufacture Profibus products and support them via their thousands of agents, distributors and subsidiaries worldwide. Many of these companies are vendors of both (a) master and (b) slave technology. The Regional PROFIBUS Associations also provide support in a number of countries where they can offer advice on product sourcing, implementation and application by 'phone, fax and Email. Many of these assocations run training courses and seminars. PROFIBUS is of course a core part of most vendors product offering and you would expect many of their support staff to be able to deal with PROFIBUS technical issues.



What hardware and software is needed to set up a PROFIBUS system? < Top

In addition to a PLC and the devices to be controlled

is required.



What connectors, pinout and mandatory signals are used? < Top

The PROFIBUS Standard does not specify an alternative to the 9 pin D-SUB connectors, but it is often necessary to have alternatives available.

The test specification for DP-Slaves defines:

Alternative connectors may be used. No special connector is defined. But if the device with these alternative connectors should be certified, it must have all the mandatory signals of the Profibus D-SUB connector available.

Furthermore, you should take into consideration for high speed usage, that some additional components should be used in combination with the D-SUB connector, or any other one.

These components (R, L, C) are specified into the Implementation Guideline for PROFIBUS-DP, and are explained in detail including sample circuit diagrams in the new PROFIBUS-DP book (The Rapid Way to PROFIBUS-DP). This book may be ordered from your local PROFIBUS organization.



Are there any particular applications/sectors of industry that particularly benefit from PROFIBUS? < Top

PROFIBUS has already found many applications in factory, process and building automation, for example from Automotive manufacture at General Motors across Europe, to Breweries like Bitburger and Guinness to building management like the Russian Kremlin building. PROFIBUS is also being used in applications that were never envisaged from its conception, because it has been constructed in a way that allows for many new device types to be added. However, the two key areas of industry that can most benefit from ist adoption are :

Factory automation applications, where signal wiring costs are cut significantly and many devices can be installed directly on the bus, thus removing the need for many input and output signals.

Process Automation, where cabling costs are significantly reduced; devices can be placed directly in the hazardous areas; many analogue I/O cards can be eliminated by transferring all data on one twisted pair cable. This also reduces the need for I/S barriers.



Cable specification. What to select? < Top

The PROFIBUS standard defines two variations of the bus cable. However it is recommended to use cable Type A in all new installations.

Type A is especially recommended for high transmission speeds (>500 kBaud) and permits doubling of the network distance in comparison to Type B.

Technical specification:

Impedance: 35 up to 165 Ohm at frequencies from 3 to 20 Mhz.

Cable capacity: < 30 pF per meter.

Core diameter: > 0,34 mm˛, corresponds to AWG 22.

Cable type: twisted pair cable. 1x2 or 2x2 or 1x4 lines.

Resistance: < 110 Ohm per km.

Signal attenuation: max. 9 dB over total length of line section.

Shielding: CU shielding braid or shielding braid and shielding foil

Max. Bus length: 200 m at 1500 kbit/s, up to 1,2 km at 93,75 kbit/s. Extendable by repeaters.



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