HRC Event Time Tag Investigation

The following is taken from a report on an investigation of the HRC event time-tagging error by Jack Gomes.

1. Summary

We found that an incorrect signal on the HRC motherboard was connected to the time tag circuit, the effect of which is to cause each telemetered event to have a time tag which is associated with the previous event, whether that previous event was telemetered or not.

2. Details

As shown in the diagram below, the HRC starts processing each event that exceeds the amplitude of the event trigger level at time t1. Time t1 is derived from the zero crossing of the "FAST" signal, which occurs at a fixed time (~2us) after the onset of the event waveform. A signal called "GS", which is used to initiate many event processing functions, begins at this time. One of the functions of "GS" was to latch the content of the Time Counter into the Event Time Buffer. At time t2 (10us after the onset of "GS"), a signal called "CLR" transfers the Event Time Buffer contents to the TLM Event Buffer for events that meet the criteria to be telemetered. We found that a signal called T/HRES was connected to the Event Time Buffer instead of "GS". T/HRES is an inverted version of "GS", delayed by 0 to 1us. Since the time buffers transfer their data on the positive-going edge of their clock waveforms, this error delays the clocking of the Event Time Buffer until t3, which is after the Tlm Event Buffer transfer time, thus the time tag telemetered with each event actually belongs to the previous event. It is important to note that that previous event may or may not be telemetered. Events which are not telemetered are those excluded by the "validity" criteria (upper level discriminator, center blank, etc.) or those that are excluded because the event fifo is full, or the event processor is busy.

Timing signals

Mike Juda
Last modified: Mon Mar 26 09:28:25 EST 2001