Banzuzi, K

Czyzew, T

Foryt, P

Hojda, M

Kudla, M J

McCartney, B

Nakielski, T

Pietarinen, E

Pozniak, Krzysztof T

Romaniuk, Ryszard

Wrochna, G

CERN Document Server

OPTICAL LINK DEVELOPMENTS FOR THE CMS RPCK. Banzuzi (email: Kukka.Banzuzi@helsinki.fi), B. McCartney and E. Pietarinen, Helsinki Instituteof PhysicsT. Czyzew, M. Hojda, P. Foryt, T. Nakielski , K. Pozniak and R. Romaniuk, Warsaw University ofTechnologyM. Kudla and G. Wrochna, Warsaw UniversityAbstractThe rather general data link component testing andevaluation work done at the Helsinki Institute of Physicshas become more and more focused on providing aworking solution for the CMS RPC fibre optic trigger anddata links. A new test board, LINX, has been designed tofill the specific needs of RPC links prototyping, includingimplementation of a data compression scheme. In thispaper the LINX board is described in some detail, and alink solution based on commercial components availableat the moment is proposed. Gamma and proton irradiationtest results for some of these components are alsopresented.1.  INTRODUCTIONThe CMS RPC muon detector system has almost200,000 channels, but a very low occupancy.  Significantsavings on the number of high speed data links neededare possible using data compression and multiplexing.As illustated below in Fig. 1, the readout system isdivided into the following components: Frontend ReadoutBoard (FEB), which performs the digitisation of  the data,Link Board (LB) containing synchronisation and datacompression functions and the optical link. In the controlroom data from one link has to be split to severaldestinations to be forwarded to the Level-1 trigger by aTrigger Board (TB) or to the DAQ by a readout board(RB). At this end, too, the design will be modular,consisting of a separate link boards (TLB and RLB) withthe decompressor and demultiplexer circuit DEMUX andthe dedicated processor boards TB and RB.The chambers will be served with different numbersof links according to their occupancy. Up to threechambers are connected to one link in the barrel, whereasin the endcap some relatively high occupancy chambershave two links per chamber. The first plan was to use onecompressor circuit per chamber, but now it seems it maybe possible to combine the data from up to threechambers in one LMUX, meaning that we would onlyneed one kind of link board. This would add anothermultiplexing stage between the FEBs and the LBs, oralternatively we would have to accept a veryFig. 1. Block diagram of the CMS RPC optical links	 	 		  	        		         ! !  " ! # $  %&  ! ' () *+TriggerBoardReadoutBoardTo theLevel-1 triggerTo the DAQhigh pin count for the LMUX (3×96 inputs).Presently it is envisaged to mount the link boards ontwo different locations on the muon chambers: the firstfour layers will be served with LBs in the space betweenmuon stations MB/x/2 and MB/x/3, and the two outerlayers with LBs on top of MB/x/4. Short LVDS links areneeded to realise the connection between the frontendboard and the link board. The synchronisation unit hasonly recently been moved away from the FEB to the LB,preliminary tests show the jitter levels introduced by thisconnection to be acceptable [1]. A detailed configurationfor the endcap has not been proposed yet.Fig.2. The link boards will be mounted on the detector intwo different locations for each sector2.  CURRENT DESIGN BASELINE FORTHE RPC LINKS2.1 Data Compression SchemeThe data compression scheme to be used is describedin [2], only a short summary is presented here. The RPCchamber consisting of 96 strips is divided into 12 or 8strip partitions. Only the non-empty partitions are sent,together with a partition number and a delay value. Twoadditional bits are added at the multiplexing stage ofLMUX to denote the chamber number if data from morethan one chamber are sent through one link. Flag bits areset if there is a cluster in between two partitions or if thedata transfer is aborted because the maximum delay valuehas been reached. Very reliable operation is reached  witha 24-bit data word, which would also leave one extra bitfor error coding purposes, for example. With a partitionsize of 12, it would be possible to send the full raw datafrom the chamber in eight bunch crossings. Themaximum delay value could then be set to around fourbunch crossings (bx) for normal operation.2.2 Components ChosenThe most suitable components available at themoment for our application are Gigabit Ethernet basedserialiser/deserialiser chips which include data encodingfor DC balance and which can be run at a variable bitrate, such as the AMCC S2061. In particular, we need tooperate at a multiple of the LHC bunch crossing rate40.08 MHz, and we plan to use a PLL component tomultiply this by three. The bit rate of the link would thenbe 1.2 Gbit/s, or 120 Mbyte/s of data.Commercial transceivers based on VCSELs and PINdiodes will be used for the electric-optic conversion andvice versa. They work with multimode fibre, and arerelatively inexpensive even though they are bidirectionaland thus include extra components not needed.Two alternatives are being investigated for thesplitter: Channel Link and regular LVDS linedrivers/receivers at 120 MHz. The latter would mean thatmore cables are needed, but the overall solution would bemore elegant as we would not have to include yet anothermultiplexing stage.In case the commercial components we plan to use donot prove to be radiation tolerant enough, the submicronlink being developed at CERN [3] would directly fit intoour design. The CMS ECAL G-link [4], on the otherhand, only allows a 16-bit word to be sent during a bx. Amore efficient and more complicated data compressionscheme would be needed to cope with this slower speed,as with the current scheme, a 3-bit data partition wouldhave to be used, giving a 32 bx buffer for sending rawdata. One such compression scheme is proposed in [5].Another solution would be to try to clock the ECAL chipoutside of its specifications at 60 MHz and send a 24-bitword in 1.5 bx, which would be very likely to causesynchronisation problems.3. LINX PROTOTYPING ENVIRONMENT3.1 Design GoalsThe LINX test board has been designed as a higherspeed replacement for a similar board described in anearlier paper [6]. The board can be used to study thecharacteristics of different link components and toevaluate their suitability for use in CMS data links. It canalso be used in radiation tolerance tests to monitor, forexample, single event effects during irradiation. Specialattention has been paid to providing a programmable testbench for the CMS RPC fibre optic trigger and data links.Together with some additional hardware, the board canbe used to replace the missing components of the RPCreadout system during the development phase, and whenthe final link boards have been manufactured, it can serveas a programmable debugging device to facilitate theirmaintenance. All aspects of the CMS RPC data transfercan be studied with a modular test bench built from LINXboards and simple adaptor/driver daughter cards. 3.2 SpecificationsLINX is a PCI board that can also be used in standalone mode through either an RS-232 or a USB interface.RPCsLink BoardsThe key component on the board is Xilinx Virtex FPGA,containing from 50,000 to 800,000 logic gates andoffering operating speeds well in excess of 100 MHz.Different size FPGAs having the same footprint will bemounted on the boards to test the functionality of theVHDL code for LMUX and LDEMUX and to find theoptimum FPGA/ASIC size for their implementation. Inthe PQF package chosen for the board, the componentshave over 120 configurable pins in total.Fig. 3. LINX  boardThe LINX includes an on-board fibre optic interfacebased on the AMCC S2061 serial backplane device and acommercial short wavelength optoelectronic transceivercomponent (VCSEL+PIN diode). The DSP is usedmainly for interfacing purposes. There are also numerousconnectors where daughter cards or other externalinputs/outputs can be connected. Several differentdaughter cards were designed for the old version of theboard, and these can also be used with the new board.Daughter cards exist for Gigabit Ethernet transceiversfrom different manufacturers and the HP G-link, amongothers.3.3 Test ProgramInitial synthesis results show that the RPC datacompression and decompression algorithms can almostcertainly be fitted into the Virtex FPGAs in their fullversions and that there will also be some space left foradditional test logic. Essentially all the features plannedfor the RPC data link can be realised on the LINX board,and we should have a proof of principle for the mostimportant features of the data transmission by the end ofthe year.For a realistic setup, the system clock can be replacedwith a 40.08 MHz input from a TTCrx board via a lemoconnector, and RPC-like test data can be generated in theDSP or a host computer and stored into a look-up table onthe Virtex, where it can be cycled at the required speed.Another crucial task to be carried out with a setupbased on LINX boards is the single event effect (SEE)testing of link components and FPGAs. Fig. 4 shows anRPC readout chain realised with two LINX boards (a) anda radiation test setup with one board (b). In addition tofibre optic link components,  Channel Link and otherLVDS components will be evaluated.a)b)Fig.4a) Test readout chain for the CMS RPCb) LINX SEE test setup3.4 Status of the VHDL DevelopmentThe VHDL code for the LMUX and LDEMUX isvirtually complete, only some of the test functions stillremain to be implemented.Simulation results show that the decompression couldbe realised even with a Xilinx Spartan series 40k FPGA,whereas the multiplexer part of the compressor seems torequire the highest speed grade Virtex. These results areworst case estimates, since they were obtained for adesign which did not use any optimisation techniquesspecific for the Virtex components.The choice between ASIC and FPGA should be madesoon. A decisive factor for the transmitter will be theradiation tolerance of the Virtex FPGAs. At the receivingend, FPGAs will almost certainly be used.4.  IRRADIATION TEST RESULTS4.1  Gamma Irradiation TestsGigabit Ethernet and Fibre Channel components fromseveral different manufacturers (see Table 1 for details)have been irradiated to see whether they would stand thedose present at the CMS muon detectors and in theVirtexXCV300MotorolaDSP56301AMCCS2061VCSEL+PINtransceiverPCIUSBJTAGRS-232RS-232 in ter face for  monitoring SE Us in the link com ponentsT xVirtex  BERTD SPRxVA CUUM CHAMBERJTAG readback to observe possible changes in  the Virtex configuration CO NTROL ROOM-one component irradiated  at a time-proton energy f rom  20M eV to 90 MeVCurrent  monitor to detect latchupT TCrx    test b oardLV D S cab le to  an other L IN X 2 bo ardTTCvx40 M hz clockThe LIN X2 system clock can be replaced with input from a TTC receiverVirtex LDEMU X+ BERTVirtex  LM UX + BERTD SPD SPGeneration of the test data, com munication w ith the host system via RS-232 /P CI3x 120 MHzS2 06 1S2 06 1Data input f rom other  sources directly  into the Virtex  has  also  been provided fo r T xsplit terR xcavern. The work was carried out at the 60Co irradiationfacility at the Department of Radio Chemistry, Universityof Helsinki. The dosimetry was based on doses measuredfor water at certain distances from the source. Since thesamples were placed close to the source, the amount ofscattered radiation present on them could be minimised,and the silicon equivalent accuracy was around 20%. Thiswas considered to be enough for these first tests.      Table 1. Irradiated  componentsPart Number DescriptionTQ9501-MC Triquint Fibre Channel TxVSC7135QN Vitesse GE transceiverCXB1589R Sony GE transceiverVSC7115QJ Vitesse GE TxHDMP-1022/HDMP-1024 HP G-link Tx/Rx chipsetS2061B AMCC GE compatible  transc.S2046B/S2047B AMCC GE Tx/Rx chipsetVCSEL TransceiversHoneywell HFM2450Siemens V23826-K305-C73HP HFBR-5305FThe estimated worst case dose at the RPC detector is100Gy, and this was used as the basic dose in the tests. Asignificant number of components was irradiated to200Gy and some up to 300Gy. Components weremounted on small circuit boards and were powered up.On the transceivers with optoparts attached, a fixed datapattern was fed to the transmitter side to modulate thelaser. Functionality of the components was checkedimmediately after the irradition where possible. Moredetailed measurements were carried out with the help ofan oscilloscope within few hours of the irradiation, afterwhich many of the samples were put through anaccelerated aging test at 100°C for one week.No significant changes were observed in the operatingcurrent or functionality of any of the components neitherimmediately after the irradiation nor after the acceleratedaging test. The biggest change in operating current,∆I=70mA, was observed for the HP G-link components,which received a dose of 200Gy and were put through theaging test. No other changes were observed in theiroperation.4.2 Proton Irradiation TestsThe sensitivity to displacement damage of theoptoelectronic and bipolar components was investigatedwith protons. The proton irradiations were carried outwith 75 MeV protons from a multicusp ion source at theUniversity of Jyväskylä K130-cyclotron. The beamcurrent was 180 pA, and the samples were all irradiatedto a total dose (fluence) of 5×1012 ncm-2. The beamcurrent was measured by a Faraday cup in the beam line.Fluence was measured by a set of four collimated CsI(Tl)scintillators with PIN diode readout, situated in front ofthe beam entrance to the vacuum chamber where thedevices under test were placed. A more detaileddescription of the irradiation facility can be found in [7].As with the gamma irradiation studies, no change wasobserved in the functioning of the components, or theiroperating current. The current was measured before andafter the irradiation and one month later, when theradioactivity of the samples had gone down to a moreacceptable level.   The only noticeable change occurredfor the HP G-link components, for which the operatingcurrent increased from 880 mA to 920 mA after theirradiation and to 940 mA one month after the irradiation.Altogether the results from both kinds of irradiationtest look promising: total dose or displacement damageeffects are not likely to be a problem on the CMS muondetectors for the components we have chosen. In a waythe results were also disappointing, since we were notable to exclude any of the components with these simpletests. It has to be noted, though, that the sample sizeswere rather small (2-3 pieces of each component for thegamma irradiation and 1 piece of each component for theproton irradiation).In addition to the link components, one VirtexXCV300 FPGA was also irradiated with protons to seewhether it would suffer any permanent damage. We havenot yet analysed the results.The next step is to do single event effect tests. Fortechnical reasons, it has been decided to perform thelatchup and SEU tests with protons, which at energiesabove 20 MeV behave in a very similar way to neutrons,with possibly a neutron irradiation in a reactor to checkfor the effects of low-energy neutrons. Another reasonthat supports the use of protons rather than neutrons isthat preliminary calculations show charged hadrons todominate the SEU rate even in the CMS cavern, wherethey form only 1% of the total hadron flux [8].The plan is to perform latchup measurements fordifferent proton energies to establish the LET of thecomponents, and SEU tests for the serdes componentsand Virtex FPGAs with the help of the LINX board. TheFPGA configurations needed for these tests are underpreparation.5. CONCLUSIONSThe full RPC data transmission chain is to be built thisyear with LINX boards. This highly modular setup is verysuitable for testing the data compression and multiplexingscheme, signal splitting at the receiver end and theoverall integrity of the proposed solution.In the first irradiation tests we were unable to excludeany of the candidate components on the basis of a poorradiation tolerance. It is to be stressed, however, that theresults are very preliminary and single event effect testsare still to be done. The sample size was also ratherlimited. More tests, concentrating on the components wehave chosen for our baseline design, will be carried outsoon.References[1] A. Ranieri. Private communication.[2] M. Gorski, I. M. Kudla and K. Pozniak. High speeddata transmission and compression for the cms rpcmuon trigger. Proceedings from the 3rd Workshopon Electronics for LHC Experiments, 1997, London.[3] A. Marchioro. Proposal for a Gb/s link for LHCexperiments, CMS workshop on optical links,January 1998.[4] P. Denes, ECAL optical links, CMS workshop onoptical links, January 1998.[5] K. Banzuzi and E. Pietarinen. A Data CompressionAlgorithm for CMS RPC Detector Readout withFibre Optic Links, HIP Preprint Series HIP-1997-75/EXP.[6] E. Pietarinen. A Fibre Optic Link Evaluation and TestSystem LINX, HIP Preprint Series HIP-1997-76/EXP[7] A. Virtanen, J. Hyvönen, K. Ranttila, I. Rekikoski andJ. Tuppurainen. Heavy ion and proton test site atJYFL-accelerator laboratory. Department of Physics,University of Jyväskylä, preprint 16/98.[8] M. Huhtinen. RD49 Workshop presentation, June1999.AcknowledgementsWe would like to thank Mika Huhtinen, Pertti Aarnio,Pierre Jarron and Federico Faccio for their help withplanning the irradiation tests, Prof. Timo Jaakkola andLaboratory Manager Martti Hakanen of the University ofHelsinki Radiochemistry Department, and Prof. JuhaÄystö and Dr Ari Virtanen of the University of JyväskyläAccelerator Laboratory, for the permission to use theirfacilities and their help.

Optical link developments for the CMS RPC

Optical link developments for the CMS RPC

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