[Olympus] rates in TOFs

[Alexander Winnebeck]

Hi Alexander,

at the moment you can just figure out which PMT has fired by reading the TDC. There is no other equipment, which knows about the Discriminator outputs.
But as far as I know we can not just read individual channels of the Fastbus ADC, but only the whole module. *Maybe Philipp or Christian should comment on this!*
We also need to know how much we can speed up the readout without harming the operation of the Fastbus sequencer.

Cheers,

Alexander

On Aug 24, 2011, at 9:58 AM, Alexander Kiselev wrote:

>  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
>> 
>> Massachusetts Institute of Technology (MIT)
>> 77 Massachusetts Avenue
>> Bldg 26 / Rm 441
>> 02139 Cambridge, MA, USA
>> Tel: +1-617-253-3680
>> 
>> DESY
>> Bldg 66 / Rm 002
>> Notkestrasse 85
>> 22603 Hamburg / Germany
>> Tel: +49-40-8998-6402
>> 
>> 

--
Dr. Alexander Winnebeck
winnebeck at mit.edu

Massachusetts Institute of Technology (MIT)
77 Massachusetts Avenue
Bldg 26 / Rm 441
02139 Cambridge, MA, USA
Tel: +1-617-253-3680

DESY
Bldg 66 / Rm 002
Notkestrasse 85
22603 Hamburg / Germany
Tel: +49-40-8998-6402