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Hall D monitoring and controls strategy/philosophy



Hall D Electronics:

Hi,

A few thoughts on our controls strategy, that may not add up to a full-fledged philosophy:

Monitoring capabilities are included in hardware designs for a number of reasons.  They 
may be needed during hardware testing and checkout phases, they may be needed to debug 
problems, and they may be used for day-to-day monitoring.   Also, the monitoring may need 
to be done only during data taking (trigger scalers, etc), or may need to be done with 
some frequency 24 hours a day.

Low volume data that is only needed during data taking can be read out via VME in 
front-end DAQ crates without affecting high-speed data taking.  High-volume monitoring 
data will likely interfere with the DAQ system if it is read out via VME, so an alternate 
path may be required unless a scheme for pausing data taking is feasible (we do this for 
scaler readout in CLAS).

Note...dual paths for readout of monitoring data may make sense in some cases, e.g. via 
both VME and I2C, CAN, or some other bus.

Monitoring data that is needed 24 hours a day, at whatever frequency, should not be read 
out via the DAQ system.  The DAQ duty cycle, averaged over a year, may be as low as 33%. 
And even if the readout frequency is low, data from front-end crates needed 24 hours a day 
is best not read out via VME, as front-end cpu's may not be booted or configured all the 
time (e.g. a crate may be on, the cpu dead, and a board heating up...if the only readout 
is via VME then there is no way to detect this problem).

This is why I prefer an alternate path, other than VME, when possible, especially for 
front-end DAQ crates.

				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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