[Halld-online] question about scalers in the EPICS archiver

[Hovanes Egiyan]

Hi Richard,In other halls this kind of data is writen to data files usind CODA DAQ.I believe we also have that capability.Hovanes


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-------- Original message --------
From: Richard Jones <richard.t.jones at uconn.edu> 
Date: 1/11/18  1:39 PM  (GMT-05:00) 
To: Hovanes Egiyan <hovanes.egiyan at gmail.com> 
Cc: halld-online at jlab.org 
Subject: Re: [Halld-online] question about scalers in the EPICS archiver 

Hello Hovanes and all,
This entails a serious loss of precision for long-running tallies. Essentially it means that all we have in EPICS are one-second snapshots every once in a while, between which the values can wander by 3% without any record of having done so. This makes them useless for any kind of normalization, which is what I was hoping to get from them. Suppose I wanted to get the total number of counts accummulated by a scaler over a several hour period, is there any way to get that information from anywhere in our online system right now?
-Richard
On Thu, Jan 11, 2018 at 1:28 PM, Hovanes Egiyan <hovanes.egiyan at gmail.com> wrote:


  
  
    Richard, 

    

    The discriminator scalers are latched every second and the units are
    Hz. Sergey Furletov 

    runs the server that reads the values out into shared memory, so I
    do not know the details. I believe 

    that the value is non-integral because it gets normalized to a clock
    to get rates more precisely in Hz. So 

    the square root of the value in the DB for the scaler should be a
    good estimate of the statistical error. 

    

    Archiving of the scalers in MYA (or other PVs) has a deadband (for
    this particular variable it is 

    3*sqrt(value)+0.03*value), which means that if the next reading
    (which is indicated by a change in 

    the value of the EPICS variable) is within that deadband of the
    latest recorded value then it does not get 

    recorded. So, if you stripchart an EPICS variable live, you will see
    a lot more frequent value changes 

    compared to what you see when you look at the archive chart of the
    same variable. 

    This is done to reduce the update rate in the DB, otherwise it would
    cost a lot more money to keep all 

    variables in the DB without any deadbands. The deadbnads are
    adjustable though, I can request smaller 

    deadbands for a small subset of variables. 

    

    Hovanes 

    

    

    
    

    On 01/11/2018 12:41 PM, Richard Jones
      wrote:

    
    
      Hello Hovanes and all,
        

        
        I am accessing the GlueX scalers in the EPICS archive using
          the MYA interface, and have run into a question. I am looking
          at a variable like TAGM:T:8:scaler_t1
            whose name suggests that it is the discriminator scaler for
            TAGM column 8. When I look at the numbers in that variable,
            they are obviously not scalers. They are non-integer values
            in the range of 3000-8000 for the runs that I am looking at.
            Are they rates? In what units? kHZ? What is the sampling
            interval? This matters because of statistical error. I see
            entries in there sometimes separated by tens of seconds
            during a run. In the time between the samples, were the
            scalers free-running so that I have the full statistical
            information in the one reading at the end of the interval,
            or are all of the statistics during the seconds in between
            the records in the DB lost?
        

          
        -Richard
            J.
      
      

      
      

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