Charpy, A.

Frühauf, J.

Heinz, A.

Hoffmann, J.

Johansson, H. T.

Koch, K.

Kurz, N.

Pietri, S.

Simon, H.

GSI Repository

Time and trigger distribution for NUSTAR DAQ systems ∗A. Charpy1, J. Fru¨hauf 2, A. Heinz1, J. Hoffmann2, H.T. Johansson1, K. Koch2, N. Kurz2, S. Pietri2,H. Simon2, the EDAQ working group2, and the FAIR@GSI division21Chalmers Univ. of Technology, Go¨teborg, Sweden; 2GSI, Darmstadt, GermanyNUSTAR experiments will be based on a complex smor-gasbord of devices and detectors of many different typeswith wildly varying speeds. Merging the data meaning-fully together requires proper timing distribution and trig-ger interconnections. With detectors spread over differentexperimental areas, sometimes hundreds of meters apart,and often-changing experimental set-ups, a flexible inte-gration approach is the key to success.Time distributionNUSTAR DAQ systems need distributed timing signalsfor two purposes:• Time-of-flight measurements. The most demandingof these measurements (< 10 ps at short distances)are driving the precision requirements.• Inter-system event synchronisation. The event-wisedata from different detectors, operated with commontriggers but in separate dead-time domains, can bemerged based on time. Requirement: a few 10 ns.The FAIR infrastructure caters for these needs by twodevelopments for the accelerator systems: BuTiS [1] andWhite Rabbit [2]. The Bunch Timing System distributesa stabilised frequency using optical fibres to distributionboxes that can be located in each experimental area. Thesereference signals can then be further sent to front-endboards that perform high-precision time measurements.The reference signal is effectively used to drive the front-end clock. Measurements have shown an precision betterthan 20 ps between systems separated by 2 km of opticalfibre. For absolute timing, BuTiS-derived timing requirescooperation with an external reference to label the clockcycles, e.g. White Rabbit.White Rabbit is a synchronous Ethernet protocol, i.e. theclocks of all switches and interface cards are synchronisedto one device. It has demonstrated synchronisation betterthan 100 ps/km [3]; fully sufficient for all but the most de-manding measurements. In addition to synchronising time,White Rabbit acts as 1 Gbps Ethernet connection.For systems with lower timing requirements (∼10 ns),which do not need a network connection, or when the over-head of aligning the local clock cycles with synchronousEthernet is too constraining, a light-weight serial time dis-tribution protocol is available [4]. Sender and receivers do∗Work supported by FAIR@GSI PSP codes: 1.2.2.4., 1.2.5.1.4. and2.4.3.1.not need to operate with the same clock frequency; the pro-tocol is also uni-directional and medium independent.Trigger distributionSpatially distributed NUSTAR experiments require avery flexible scheme for connecting systems, and their trig-gers. The trigger logic of an experiment needs to commu-nicate with participating detector systems, to• receive trigger signals for coincidences.• send master start and trigger decisions to front-endand read-out systems.• receive dead-time information for use as veto signal.These signals must be reasonably fast, with at most a fewµs latency. (With analog delay lines this would have beena few 100 ns.) Thus, transport over any packet-switchednetwork is not feasible, and hardware signals must be used.Manual changing of the hardware connections is very timeconsuming, especially so in the generally non-accessibleSuperFRS tunnel. A staggering number of direct peer-to-peer connections would be needed with many detectors. In-stead, remotely controlled FPGA-based switch-boxes areplaced to allow the maximum flexibility. This approachworks directly with trigger signals and dead-times, as thosecarry binary information.For an MBS-compatible TRIVA-style trigger a few bitsof information must be distributed (trigger number, eventcount, reset signal). For the systems to work correctly insync, it is also necessary that the master listens to the dead-time of each involved slave. In order to avoid a dedicatedelectrical bus, connecting all systems in a hardwired dead-time domain, a uni-directional serial trigger protocol dis-tributing triggers from the master module can be used, asdemonstrated in a prototype environment [4].Directing data-flow from arbitrary front-ends via eventbuilders and time sorters to online analysis and storage isstraight-forward using switched Ethernet connections.References[1] P. Moritz and B. Zipfel, GSI Scientific Report 2011, p. 478.[2] J. Serrano et al., “The White Rabbit Project”, Int. Conf. onAccelerator and Large Experimental Physics Control System,Oct. 2009. Also http://www.ohwr.org/projects/white-rabbit.[3] D. Beck, private communication.[4] H.T. Johansson et al, “TRLO II – friendly FPGA trigger con-trol”, this report.GSI SCIENTIFIC REPORT 2013 FG-S-FRS-14doi:10.15120/GR-2014-1-FG-S-FRS-14 353

Time and trigger distribution for NUSTAR DAQ systems

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Time and trigger distribution for NUSTAR DAQ systems

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