This file: http://hallaweb.jlab.org/equipment/daq/qweak_helicity.html
With thanks to Riad Suleiman, Scott Higgins, and Roger Flood.
Last updated July 27, 2010R. Michaels email@example.com
As you can see in the above document there are several "modes" of operation, and several "patterns", as well as a choice of "delay". Typically, Qweak will run in free-running mode at nominally 1 kHz with Quartet pattern and a delay of 8 windows. However, Hall A must make itself insensitive to the choices. The software which checks our data should look at the EPICS variables which tell the state of the Helicity Board. The things to check are:
Logically, if the full sequence of helicities are measured during the run, with no gaps, then it is easy to reconstruct the helicity for any given event. However, if there are gaps (due to deadtime or a "burp" in the DAQ), then one may use the calibrated clocks to fill in the gaps. In addition, due to the delayed nature of the helicity, the helicity-gated scaler data must be read with zero deadtime at the full rate of the helicity reversal.
Here is a diagram of the Electronics Schematic for the Hall A HRS. ( same in GIF same in JPG. )
We've done this before (during G0 era) but there are some new aspects: a higher frequency, the possibility of variable frequency, a different (new) predictor algorithm, and the possiblity for several combinations of mode and pattern. This means we need new versions of both the online scaler software and the THaHelicity class used by Podd.