The instrument consisted of a 1.5 inch diameter x 0.75 inch thick CsI(Tl) scintillator. The data were stored in 3 independent memories, with integrations of 250 ms, 32 ms, and 4 ms. Half of each memory circulated real-time data, while the other half stored data only if a trigger occurred, thus precursor information was kept. Nested time histories for 128 s with 250 ms resolution, 16 s with 32 ms resolution, and 2 s with 4 ms resolution were recorded. A trigger could occur by the count rate exceeding any of 3 time-based rate comparisons with thresholds commandable from the ground. The lower energy threshold could be adjusted from 60 to 110 keV in 8 steps, while the upper threshold was 1500 keV. The instrument operated nearly continuously, shutting down only for solar occultation and an occasional incidental power shutoff.