[Olympus] rates in TOFs

[Alexander Kiselev]

  Hi Alexander,

> maybe let me add some more information about the fastbus readout.
> The drift chamber TDCs are zero suppressed, so the data load is 
> depending on the occupancy. During the commissioning we found a typical 
> amount of data of less than 100 bytes per detector. The TDC of the TOFs 
> is also of the same order. The most data is coming from the TOF ADCs, 
> because they are not zero suppressed. The amount of data is 524 
> bytes/event.

  may I ask once again, what would be the effort to check pattern of 
PMTs above threshold first, and read out only those which fired? 

  Regards,
    Alexander.


> The maximum rates of 1.3 kHz we could observe during the August run is 
> not a real hardware limit, but was a safe setting for the read out. 
> Philipp is now working on the optimization of it. I have no idea how 
> much speed we could gain.
> If we want to speed up with additional hardware, it would make sense to 
> put the TOF ADC into a third crate. To find another crate is not a 
> problem, as you wrote yesterday, but the SFI sequencer is the challenge. 
> I'm not aware of any at DESY or other labs I have access to.
> 
> Cheers,
> 
> Alexander  
> 
> On Aug 24, 2011, at 8:42 AM, Alexander Kiselev wrote:
> 
> >  Good mornig colleagues,
> > 
> >>  Installation of say two more crates would increase the effective readout 
> >> speed by a factor of almost 2, as we discussed today. This may be pretty 
> > 
> >  this factor of 2 is way too optimistic I admit. Let's estimate correct 
> > numbers useful for further discussion. Sorry if the below calculations
> > look trivial.
> > 
> >  Indeed numbers quoted during the Monday meeting (max readout rate 1300Hz
> > at 2% live time, "typical readout fastbus") have nothing to do with the
> > real running conditions. Let's assume ~750us average "event cycle (readout
> > time + waiting for next trigger)" for simplicity. 2% live time would mean
> > that next trigger waiting time was only ~750us*0.02=15us, so the original 
> > trigger load was ~67kHz. Clearly we will never run under conditions like 
> > this. 
> > 
> >  I'd say with so large readout times we should never go beyond ~1kHz 
> > original trigger load (~1ms next trigger waiting time in the above 
> > calculations). Assuming now ~750us average readout time for simplicity, 
> > this would correspond to the on-tape (accepted) trigger rate of ~1/1750us 
> > or ~570Hz at a ~43% dead time, obviously. 
> > 
> >  If we manage to decrease the readout time from ~750us to say ~400us, 
> > average event cycle will become 1400us for the same 1kHz trigger load, 
> > which would mean ~715Hz accepted trigger rate at a ~29% dead time. So 
> > gain would be significant, but no way close to a factor of 2.
> > 
> >  It should also be mentioned that useful (ep-elastic) trigger rate at 
> > nominal luminosity is ~15Hz in BLAST acceptance and about the same in 
> > 12-degree Lumi acceptance. Even if we increase average luminosity by 
> > a factor of 2, trigger budget will (should!) be always dominated by  
> > calibration (inclusive) triggers and random coincidences. Under these
> > circumstances with a relatively slow readout hardware it does not make 
> > sense to run at higher dead times, since this causes 1) smaller 
> > *fraction* of ep-elastic events on tape, obviously; 2) in fact smaller 
> > *rate* of ep-elastic events on tape, less obvious; 3) worse signal
> > to background ratio if random coincidences start dominating. Derivation 
> > of the "best figure of merit" running strategy in terms of DAQ rates
> > as a function of luminosity, readout time, random coincidence scaling 
> > and allowed fraction of calibration triggers is a rather straightforward 
> > excersice, which "everybody else can do" :-)
> > 
> >  Best regards,
> >    Alexander.
> 
> --
> Dr. Alexander Winnebeck
> winnebeck at mit.edu
> 
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