Frontend Event Selection with an MBD Using Q

A problem common to many complex experiments in Nuclear Physics is the need to provide for event selection at a level beyond that readily available in a fast hardware trigger. This may be desirable as a means of reducing the amount of unwanted data going to tape, or be needed to reduce system deadtime, so as not to miss an infrequent good event. The latter criterion is particularly important at low duty factor accelerators such as LAMPF, where instantaneous trigger rates may be quite high. The need for such an event selection mechanism has arisen in conjunction with the installation of a polarimeter in the focal plane of the High Resolution Spectrometer (HRS) at LAMPF. It has been met using a combination of buffered CAMAC electronics and an enhance1 ment to the LAMPF standard Q data acquisition system. The enhancement to Q allows the experimeter to specify at runtime, a set of simple tests to be performed on each event as it is processed by the MBD, and before it is passed to the PDP-11 for taping and further analysis. The experimenter may specify constraints, either gates or anti-gates, on individual data words or on the sums and differences of any number of data words. In addition he may specify that one of every N events is to be passed along without testing, so that a random sample of all events may be examined. Events are flagged as to whether they are a "tested event" or a "one-of-N" event.