); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Hi nuts,
I'm working with Pablo at CERN on the General Machine Timing (GMT) system, and since this subject has generated quite a lot of questions I thought I'd give you some more context information: - CERN is a big (in the fact the world's biggest) complex of particle accelerators. The main end product is proton collisions at the LHC at an energy of 7 TeV, but that's not the only one. A typical trip of a proton would begin in Linac 2 (linear accelerator) then pass to the Booster (small synchrotron) then to the PS (a bigger synchrotron) then to the SPS (yet bigger) and then to the LHC (the biggest one). But if LHC is happy and collisions are ongoing, we can send the Linac 2 protons down another path. For instance they could go to the Antiproton decelerator from the PS, where they would hit a target producing anti-protons, which would be decelerated almost to a stand-still, and then mixed with positrons (anti-electrons) to create anti-hydrogen, i.e. anti-matter. All this is clearly explained in Dan Brown's 'Angels & Daemons' ;) - So CERN really looks like a factory with several production lines for different types of particles. Orchestrating all this time-multiplexed traffic of particles is a card (in fact a set of cards) called Central Timing Generator, which drives (mainly for legacy reasons) a set of fiber then multi-drop RS-422 networks with messages sent at very precise times. On the receiving end, VME, PCI and PMC cards can listen to these messages and be pre-programmed to react to a given message by a certain action. These actions can be: generating a pulse in the front panel to synchronize external hardware or generate a bus interrupt to synchronize real-time task running in different computers all around the complex. These modules also contain complicated counters with many modes of operation, and they can time-tag events precisely using CERN-made HPTDC (High Performance Time to Digital Converter) chips. The time-tagging precision is constrained by our timing distribution network rather than by the HPTDC performance, which can go down into the 10s of ps region. - For LHC, due to begin operation in 2008, we took the safest route and installed the same timing system. This made managing injection from the SPS straight forward but it brought some other problems. One of them is that we are requested, as Pablo said, to guarantee that every timing receiver board (around 500 of them in the LHC) will receive the timing messages at the same time, within 1 us. Our timing network does not have two-way calibration capability, so we end up walking around with a battery-powered Cs4000 from Symmetricom and calibrating all the outputs. The problem with this is that cabling people can change the fiber routing without asking us during the winter shutdown (CERN is a big place) and there is no way for us to know. - While I agree that a 1us change in cabling would be a major change, we do have places at CERN which use the same timing system and would greatly benefit from a 1ns two-way calibrated scheme. One of them is the SPS extraction towards Gran Sasso: SPS protons are extracted towards a target which generates muons, which after being stopped by some meters of concrete, only leave a trace of neutrinos which happily make it to the Gran Sasso National Laboratory in Italy, after a 732 km trip through the crust of the Earth. We have a simple GPS time transfer scheme with our friends in Gran Sasso to cross-correlate neutrino spills with events seen in Gran Sasso and discriminate any events from cosmic rays, but our GPS station is in the CERN Control Center (CCC), 3 km away from the SPS extraction line where we need to do the time-tagging. We will be testing a two-way scheme based on fiber and circulators at each end this autumn for the Gran Sasso problem. Any experience we gain from that will directly apply to the broader problem of re-inventing our timing system. - To answer Bruce's question on LHC timing distribution: there are 8 surface buildings spaced at regular intervals in the 27 km circumference. Fiber takes timing messages from the CCC to each of these buildings in a start configuration. Then we go down around 100 meters to the tunnel, where we use shielded twisted pair for compatibility with the legacy standard we chose to maintain. - This year we have been assigned in the Hardware and Timing team to take responsibility for real-time field busses as well, so we figured we could try to put a timing renovation project in the same basket and try to look for some synergies. For instance, we could come up with a very deterministic field-bus which could also be used as a timing system. Two-way calibration would be a requirement, and we could also do with a bit more bandwidth than at present (500 kb/s). Since we have complete freedom to choose a physical layer, we wanted to first have a hard look at Ethernet, which is probably the only physical layer I'd bet would still exist in 20 years. There was some vague feeling that choosing Ethernet would enable us to interface more easily to the rest of the world, but under close scrutiny we arrived at the same conclusion as Magnus and Jack: Ethernet brings us quite a lot of complications, and we could interface to the outside in other ways. The debate is ongoing at CERN. Ethernet's biggest point for is that it could merge two cables in one: data and timing. Cabling at CERN is a major source of expenditure, so any way to go from a two-cable to a one-cable solution is interesting for us. Before moving on, we need to understand exactly what Ethernet can give us and what the price to pay would be. This is why we are so interested in Sam Stein's paper. - In any event, it has never been contemplated to mix timing with the technical network supported by IT. First of all I am not sure they'd let us do that. Secondly, I have a hard time imagining how we could use any kind of worst-case analysis. The solution we are contemplating is offering our users a field-bus base on Ethernet that can also serve as a timing network through appropriate usage. But it is clear that each user should build its own dedicated network. So we're still in the brainstorming phase, we are very happy to listen to the very knowledgeable people in this list, it's really a treat. Thank you very much! Javier _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
