Back Midas Rome Roody Rootana
  Midas DAQ System  Not logged in ELOG logo
Entry  22 Dec 2005, Konstantin Olchanski, Info, How do I do custom event building? 
    Reply  23 Dec 2005, Stefan Ritt, Info, How do I do custom event building? 
    Reply  03 Jan 2006, John O'Donnell, Info, How do I do custom event building? 
Message ID: 239     Entry time: 22 Dec 2005     Reply to this: 241   248
Author: Konstantin Olchanski 
Topic: Info 
Subject: How do I do custom event building? 
It turns out the the standard event builder fragment matching algorithm cannot
be used in my TPC application. I have two TPC-USB interfaces, which lack any
"busy" or synchronization logic. I send the hardware trigger into both
interfaces, and if one of them misses it, the data is out of sync forever. Consider:

trigger    trig1     trig2    trig3    trig4
TPC01      serial1   serial2  serial3  serial4
TPC02      serial1  (missing) serial2  serial3

With the event builder matching only the event serial numbers, the first event
will be okey, but the second event will have trig2 data from TPC01 and trig3
data from TPC02, etc.

The problem exists even if the TPC-USB interfaces do not miss any triggers:
during begin and end of run, the interfaces are enabled one at a time, so if a
trigger arrives after the first interface was enabled, but before the second is
enabled, the data starts being out of sync (and if the same happens during the
end-of-run, the event counts from both frontends will match, but all data would
*still* be out of sync).

Obviously additional data is needed to match the fragments.

So in each frontend, I have a high-precision timestamp (gettimeofday(), usec
resolution) and I would like to have the event builder match the timestamps
instead of event serial numbers. What is the best way to do this? The mevb.c
code does not have any user callbacks for checking "do these fragments belong to
the same event?".

P.S. The event rate will be about 1/sec from cosmic ray tests and at most
10-50/sec in the M11 beam line at TRIUMF, at these low rates, the gettimeofday()
timestamps should be adequate.

ELOG V3.1.4-2e1708b5