[Halld-online] Asynchronous scaler readout in Hall D

[Elliott Wolin]

To all concerned with scalers in the Hall D Online,

Hovanes, Dave and I met to consider many details of dealing with 
scalers. We propose the guidelines below. Note that these refer to 
normal data taking, not to code development/debugging efforts.  Further, 
the discussion below refers to asynchronous readout of scalers, not to 
how scalers are read out and inserted into the data stream.


- the preferred method for reading scalers asynchronously (i.e. from 
registers) is to use a single process that writes to shared memory.  
This allows us to control resource usage on the ROC's and manage ROC 
processes.

- this process is independent of the DAQ system. It should be the only 
processes that reads scalers from the registers, even the ROC code 
should not do this.

- any other process that needs scaler data can get it from the shared 
memory. This includes an EPICS IOC process and perhaps other processes.

- current thinking is to use an EPICS IOC to serve asynchronous scalers 
out to monitoring processes or to servers that bundle and then 
redistribute scaler and other data.

- One question that arose was whether all crates should be part of the 
DAQ system. In particular, should the discriminator crates include a TI 
board, accept triggers, be part of the CODA DAQ configuration, etc?  In 
favor is the simplicity of putting discriminator scalers in the data
stream and the fact that in this way it is possible to read scalers out 
of all crates at the same time.  The downside is that more crates will 
have to participate in the DAQ system and that there are other ways to 
get discriminator scalers into the data stream.


Comments, questions, concerns...?

Thanks,

-- 

				Sincerely,
					Elliott


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  Those raised in a morally relative or neutral environment will hold
		    no truths to be self-evident.
				

Elliott Wolin
Staff Physicist, Jefferson Lab
12000 Jefferson Ave
Suite 8 MS 12A1
Newport News, VA 23606
757-269-7365

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