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: 241     Entry time: 23 Dec 2005     In reply to: 239
Author: Stefan Ritt 
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:
> Hardware
> 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.

Well, I would say: this is a very poor design of an experiment. Before curing the
problems in software, I first would consider a redesign of the data readout scheme with
a global hardware trigger and a hardware busy.

> 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 do you do if the frontend clock drifts away? I have seen drifts of up to 10 sec/day
on some PCs, so your required accuracy of 1/50 s would be violated after 3 minutes. You
would have to synchronize your clocks constantly. If your synchronization algorithm
determines a clock is out of sync and adjusts it, and the delta t is more than 1/50 sec,
you are screwed.

So all together I conclude that this proposed synchronization scheme is pretty dangerous
and could ruin the whole experiment.

> 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?".

Pierre can answer that.

- Stefan
ELOG V3.1.4-2e1708b5