Recommendations for the Use of Fieldbuses
at CERN in the LHC Era
G. Baribaud, ECP/CO , R. Barillere, ECP/CO, A. Bland, SL/CO, D. Brahy, LHC/IAS, H. Burckhart,
PPE/OPD, R. Brun SL/RF, E. Carlier SL/BT , W. Heubers PPE/HC, L. Jirden ECP/ESS, F. Perriollat,
PS/CO, D. Swoboda, EST/PB.
CERN, 1211 Geneva 23, Switzerland.

Abstract
Throughout the LHC project fieldbuses will be required
to connect and control equipment and devices of various
types. It will be necessary to integrate both industrial
equipment and custom designed devices. In order toincrease efficiency of resources a CERN working group
recommends restricting the choice of fieldbuses, to be usedin the years to come at CERN and by the various institutes
collaborating in the LHC project , to three : CAN bus,
PROFIBUS and WORLDFIP. After the selection phase,the three fieldbuses are now in the acceptance phase.
1 Objective
Fieldbuses will be used extensively to control andmonitor equipment in the LHC experiments, in the LHC
machine, in up-grading the control systems of othermachines and in various general purpose installations. A
single fieldbus may not satisfy all the users, however,
CERN cannot support all the fieldbuses likely to beavailable on the market. A plethora of fieldbuses areproposed by various standardisation bodies and byindustry. In order to reduce design time, installation costand later maintenance effort, the number of fieldbuses
used in the integration of equipment must be reduced to aminimum.
For this purpose the CERN Controls Board at its
meeting of September 13, 1995 created the Working
Group on Fieldbuses in controls . The objective of thisworking group was to define a policy for fieldbuses to be
used in the years to come at CERN and by the various
institutes collaborating to the LHC project. The domain of
application should cover both machines and experiments
requirements. The working group on fieldbuses comprises
12 members representing all the controls activities at
CERN
2 Investigation procedure
2.1 Notion of fieldbus
The term fieldbus refers to data transmission systems
used to federate or integrate rather simple equipment to be
remotely monitored and controlled. Fieldbuses should not
be confused with local area networks (LANs) although insome cases their domains of application may overlap. Bothare used in process control architecture.
Fieldbuses can be hierarchically divided into three majorgroups: a full-size fieldbus ( the closest to LAN ), device

bus and sensor bus.
Device buses and sensor buses are used at low level toconnect intelligent devices or intelligent sensors
respectively to programmable logic controllers (PLCs) or
to front-end equipment . At this level the communication isusually periodic and deterministic especially if close loop
control systems are involved. At thehigher level
fieldbuses transport data between the PLCs or front-end
equipment and the supervisory station or various types ofengineering stations. At this level the transmission is more
on-request oriented and could alternatively use LANs.
2.2 Scope of application
The investigation of needs was rather wide . The needsof accelerators and experiments must be considered forboth commercial and custom made devices. As a large partof the equipment for the LHC experiment will be provided
by industry and national laboratories, standardisation mustbe general enough to be accepted by the majority of theusers. It is not at all obvious whether these quite stringent
constraints can all be met at once by the recommendationsof the Working Group. It also needs to be seen how muchincentive can be created in order to have therecommendations widely accepted. In any case a project of
the size of LHC cannot be started without clear guidelines
in this field.
So far four main types of application are identified:I) Industrial controls with a limited number of parameters (say 10 to 1000) which do not change often andpossibly including closed loop feedback with a PLC.
Reliability, robustness and also redundancy are themain requirements for these applications.
ii) Laboratory equipment for complex control procedures
involving workstations over distances up to 100 meters.
Flexibility and modularity are needed. The data rate
may be very high.
iii) Sensor readout and actuator control with a very large
number of parameters (more than 100,000) up to 500
metres. Very low price per node, mechanicalrobustness, easy addition/removal, degraded operationand interchangeability are requested.
iv) Home built electronics to be installed inside thedetectors. The main problems are: limited space, lowpower dissipation, high radiation level, harshelectromagnetic environment and a very large numberof elements (more than 100,000).
2.3 Selection criteria
A technology-minded working group tends to selectproducts purely for technical reasons. This was common
practice at CERN in the past but the economic situation haschanged. Fieldbuses are part of a fast evolving technology
where industry nowadays plays a leading role.
CERN has invested much effort in the past in thedevelopment of fieldbus related equipment (e.g. Mil 1553
B bus, etc.). It is now agreed that a CERN specific system
can no longer be promoted. Even though the perfect matchcannot be found, a CERN specification will not be
considered.
We will promote standards and products which arecommercially available and are likely to be supported byindustry in the long term and at the same time fit ourneeds and are compatible with our constraints. Well
documented open systems based on recognised standards
should be preferred. Proprietary systems are to be avoided.
In order to better know the availability of products aquestionnaire was prepared to help in the discussion withvendors. A similar exercise was carried out at DESY.

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2.4 Technical requirements
The technical requirements of the accelerators and theexperiments have many common points. However there aresome exceptions like the fact that the LHC machine needs
to cover long distances whereas the experiments' needs are
locally confined.
The main points to be considered are:
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Open system to avoid proprietary systems.
Industry standards which have a proven acceptanceby European industry.
Wide range of commercial products available (fromchip sets up to complex nodes, including softwareand development systems).
Cheap connections (including cabling, connectors,
interface electronics, software).
Capacity for reliable remote hardware resets andremote diagnostic facilities in non accessible areas
specially in the experiments where radiation levels
may be very high.
Robustness
Determinism.This notion is of great theoreticalimportance in the design of fieldbuses and LANs.
The limits of response time must be clearly stated. In
the experiments we usually do not have problems
however in the LHC machine this point is ofparamount importance to control the magnet currents
for example.
Bus mastership
The notion of multimastership is considered to be ofinterest in some cases, but polling may also be ofinterest in some cases. The capability for each node
to be able to transmit a message to any other node ofa bus is the important feature.
Redundancy
This feature is more and more required in magnet
and cryogenic control systems. Fault tolerant systems
will be needed in some limited access environment.
Coverage
The distance normally required should fit an•••experimental hall or an experimental cavern, 100 up
to a maximum of 500 metres should suffice inpractice. Within the large machines this distancemust be extended up to 1000 metres. There are somecases in the LHC machine environment where
distances of 30 km must be serviced (e.g. SPS/LEP
reboot and configuration links).
Throughput, bandwidthInstead of expressing the throughput in terms of bits
per second, it is most appropriate to express thethroughput in term of actions required per second
regardless of the implementation of the physicallayers and of the driving software.
Galvanic isolation
This feature is an essential part of high qualitytransmission specially for lengths over 100 mmetresand for heavy duty equipment (e.g. kicker magnets)
creating harsh environmental conditions.
Radiation hardness
At the sensor or actuator level in some LHC detector
environment the radiation dose may reach up to 500
Gy integrated over 10 years, the neutron flux may be
as high as 10**13 per square centimetre.
Magnetic field environment
Some equipment will have to operate in magnetic
fields reaching 1kG which poses serious problems forcoils, transformers and DC-DC converters.
Low power dissipation
In some detectors the power dissipated by theelectronic devices must be kept to a very low level.
Time scale
The system will have to be operated for at least 10
years and accessibility may be limited in time.

2.5 Cost considerations
The machines and experiments are getting more complex
and are increasing in size while, on the other hand, staff
and budgets are reduced. The cost considerations are
extremely important especially as fieldbus equipment will
be used in large quantities. It is important to consider the
global costs involved, i.e. prices for hardware and software
as well as manpower for configuration, installation andmaintenance. In this context it is clear that well provenindustrial solutions should be considered first.
Furthermore, due to cost considerations it is necessary tocater for simple low cost solutions for trivial and large
scale applications in addition to the solutions needed formore complex applications.
2.6 ESONE workshop[1]In order to better evaluate the state of the art in industry,a first workshop on fieldbuses in collaboration with
ESONE took place at CERN on 18 and 18 March 1996.
During this workshop presentations from industrial firms
proposing either proprietary systems (InterBus-S,
Lonworks, PDV-BUS) or open systems (CAN bus,
PROFIBUS, WORLDFIP) took place. Other national
laboratories (DESY, SACLAY, KFA, GSI, NIKEF andEPFL) also reported on their experience and plans. The

questionnaire on fieldbuses was used to get comparable
results from the various manufacturers.
2.7 Other standardisation efforts
There are currently many fieldbus standardisation efforts
under way around the world:
FIELDBUS FOUNDATION[2]The Fieldbus Foundation consortium is contributing to
the IEC/ISA efforts to define an international standard.
This seems a far reaching goal and many years will be
required before an agreement is reached and practicalproducts are made available. However, we cannot wait for
future possible standardisations, so we have adopted apragmatic approach and consider only existing and provenproducts.
CENELEC[3]The European Fieldbus Standard EN 50170 was ratified
in Brussels on March 15, 1996 under the auspices of
CENELEC, a European standards organisation. The
standard concerns three fieldbuses: PROFIBUS (the FMS
and DP layers , the PA layer will be integrated later),WORLDFIP and P-NET. According to European
Standardisation rules EN 50170 is accepted in 18
participating countries of the European Union, even in
those that voted against it. The existing national standards(DIN 19 245 for PROFIBUS in Germany, UTE 4660 forWORLDFIP in France and DSF 21906 for P-NET in
Denmark) was withdrawn by June 30, 1997 and replaced
by EN 50170.
NOAH[4]
The goal of the NOAH project under the auspices of
CENELEC is to define the necessary elements to allow theindustrial use of distributed application across P-NET,PROFIBUS and WORLDFIP fieldbuses (fieldbus standard
EN 50170).
3 Recommendations
3.1 The proposed buses
The Working Group recommends the following threefieldbuses: CAN bus, PROFIBUS and WORLDFIP.
PROFIBUS and WORLDFIP should ultimately be madecompatible at the application level through the CENELEC
standardisation effort. We don't know yet whether CAN
will also benefit from this standardisation effort.
As far as the applications are concerned CAN bus should
mainly be used at the device and sensor levels while
PROFIBUS and WORLDFIP should mainly be used formore complex control tasks although they can also be used
at a lower level. We will now discuss their relative merits.
The main characteristics of the three recommended
fieldbuses have been listed to the best of our knowledge
for comparative purposes[5].
3.2 CAN Bus (Controller Area Network)[6]This industrial bus was primarily intended for the
automotive market, it meets high requirements for the
reliability of data transmission. It was developed by the
BOSCH company in Germany, it can be used as an open
system without the need to pay any licence fees. It ismainly based on the ISO 11898 norms. It complies withthe following standards: ISO/TC22,SC3/WG1, N427EISO.
It was also standardised by the SAE (American Society ofAutomotive Engineers).
It is now used in many non-automotive industrial
applications (e.g. controls of production lines and machine
tools, medical apparatus, textile machines, mobile andstationary agricultural and nautical machinery etc.). It
enjoys wide acceptance by industry and researchlaboratories. Chips are available and are mass produced sothey are cheap and reliable but not radiation resistant. Dueto the requirements of the automotive market they will beavailable for a long period of time. For the time being only
the two lower ISO layers (physical and data-link) aredefined. Many manufacturers are proposing layers on topof the standardised ones ( e.g. DeviceNet) although it isclear that the latter must be considered proprietary.
This bus is already in use in ATLAS[7] and CMS plan touse it soon. It is used for real time control applications.
3.3 PROFIBUS (PROcess FIeld BUS)[8]PROFIBUS is the result of a project funded by theGerman Federal Ministry of Research and Technology[9].
It can be used at three levels: FMS, DP or PA to better
match the application. It is well developed and supported
by large companies such as SIEMENS. It complies with
the DIN 19245 Standard, part 1 and 2. It already has a wide
acceptance in European and American industry.
PROFIBUS can work in multimaster or in master /slavemode. PROFIBUS FMS is suitable for communication
between intelligent stations (e.g. PLCs or PCs).
PROFIBUS DP is optimised for fast and efficient datatransmission between controllers and the decentralised
periphery (e.g. sensors). Baud rates can be selected from
9.6 kBits/s up to 12 MBits/s. PA can be used for
application in intrinsically safe areas.
Presently PROFIBUS has more than 1,000,000 installed
nodes, 1000 products are offered by 650 vendors in 17
countries.
PROFIBUS can be found in many industrial control
applications at CERN.
3.4 WORLDFIP(Factory Instrumentation Protocol) [10]WORLDFIP complies with the UTE-C-46-601 throughC-46-607 French norms available from AFNOR. It is asystem based on a centralised access method where one
Master continuously distributes the access right (token) tothe different stations. WORLDFIP provides data transfer
services for cyclic variables, event variables (acyclic) andmessages in a send-and-request series of application
functions (a subset of MMS[11]). WORLDFIP supports
three standardised transmission rates: 2.5 MBits/s for fast
real time operations over short distances using twisted paircables or optical fibres, 1MBit/s the standard operation

frequency for twisted pair cable at distance less than 500
metres and a low frequency of 31.25 kBits/s for longsegments up to 2000 metres. An additional frequency of
5Mbit/s was specified for optical cable transmission [12].
It is presently installed at SACLAY and it is planned touse it for the LHC magnets controls [13].

Some needs concerning the RF systems wereexplained though the needs for LHC in the matter are not
yet well known.
The RF experts gave their view on interoperability ofdevices and requirement of commercial development
systems for WORLDFIP.

4 Acceptance of the recommendations

4.5 Upper layers of CAN

In order to create some incentive to use the recommended
fieldbuses as widely as possible, technical, administrativeand commercial support will have to be set up at CERN .
This support should be limited to the three recommended
fieldbuses.

For the time being only the two lower layers of CAN are
standardised, we still have to define what should berecommended above these two layers. We will also define
profiles for fan trays and high voltage power supplies aswell. A special seminar will be held at CERN as soon as
possible to address these problems.

4.1 Training
According to the real needs of the users practical courses
by experts on CAN were organised at CERN. Courses on
PROFIBUS will be organised soon.
4.2 Practical developments at CERNA CAN bus driver was tested in an EPICS environmentby the controls expert of the ATLAS experiment.
A set of models showing connection capabilities of thethree fieldbuses was developed in the LHC division andpresented at the second workshop.

4.6 Expertise at CERN
The expertise on the three recommended fieldbuses will
be ensured by three different expert groups which alreadyuse the fieldbuses in question in a running project. The
experts will also ensure a technical liaison with experts
from industry.
4.7 Exchange of expertiseAn already existing user group will be used to ensure a
constant exchange of information between people having
activities linked with fieldbuses.

4.3 The second workshop
A three day workshop and an industrial exhibition
limited to the three recommended fieldbuses were held at
CERN on 16 to 18 September 1997. This workshop
permitted an important exchange of information between
users of fieldbuses at CERN and experts from industry.
The needs of CERN and the capabilities of the buses wereboth expressed. The results can be summarised as follows:[1] The three recommended fieldbuses meet a largepart of CERN requirements,[2] They are supported by industry and products arecommercially available,[3] None of them can cope with the radiation hardness
requirements of the LHC experiments(only the 1553B fieldbus presents such a feature)
therefore special actions will have to be envisaged,[4] Some problems related to high magnetic fieldenvironment still remain for the LHC experiments,
none for the LHC machine,[5] Power dissipation problems still have to beaddressed,[6] Some new actions were identified during the post
mortem meeting held after the workshop.
4.4 Special WORLDFIP seminar
Subsequently a special workshop on WORLDFIP wasorganised at CERN for potential users. The WORLDFIP
experts and CERN experts could discuss the opportunity touse WORLDFIP for the real time control system of the
LHC magnet currents which is a fundamental issue for the
LHC machine.

4.8 Purchasing
Given the large amount of modules to be purchased by
CERN and the various collaborating bodies, centralised
negotiation and purchasing procedures should be set up.
4.9 Radiation hardness
This problem must be addressed in close collaboration
with industry, LHC experiments and universities involvedin these activities.
5 Conclusion
The Working Group on Fieldbuses recommends
restricting the choice of Fieldbuses to be used in the years
to come at CERN to the following three : CAN bus,
PROFIBUS and WORLDFIP.
This proposal is mostly for pragmatic reasons. It could be
reviewed if significant development takes place in thefieldbus domain .
The necessary steps to have these recommendations
widely accepted by the community are being done and thethree fieldbuses start to enjoy a wide acceptance.
An effort is still requeired for the commercial support.
Training has already started via the CERN technicaltraining service.
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Proceedings of the ESONE Workshop on Fieldbuses
held at CERN, Geneva, Switzerland, 18 & 19 March,
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