The Galileo Energetic Particles Detector

Galileo EPD Handbook

Chapter 1. Instrument Summary

Telemetry and Operation Changes (continued)

Rate Channel Telemetry

As discussed earlier, we must somehow compress the new rate channel data into the existing CMS telemetry structure.  This includes not only changing the order in which we read the existing accumulator channels, but also adding the two previously unused accumulators.  Both the normal CMS rate channels and the multiplexed singles rates will be affected.

Luckily, our existing data system telemetry software was written to allow this sort of operation in flight.  The software runs the accumulator servicing via a table stored in memory.  To change the servicing sequence, we must merely generate a different table in memory, and have the program jump there instead of using the former table address.

The singles rates are a little more complicated, due to the fact that different channels were sent down depending on which side of the telescope was being processed.  The present commutation scheme is:

The singles/background flag (S/B) toggles in value every other logical record (4/3 seconds).  If we are in either the prime or unprime modes, the format "J' " or "J" would be selected, respectively, thus resulting in a pattern of ..L,J',L,J',L.. or ..L,J,L,J,L.. .In the alternating mode, the sequence would be ..L,J,J',L,J,J'.. .Therefore, we are in the "L" format every other 4/3 seconds, or every third 4/3 seconds, depending on whether we are alternating the J/J' flag.

If we hope to operate the two sides simultaneously, we must insure that a singles/background format contains information on both sensors, since we will not be toggling the J/J' flag. In addition, we must also fit in the data from those two new accumulator channels.  What we propose is that there only be two singles/background formats defined: L and C (LEMMS and CMS). The C format will replace both the J and J' formats; both the J and J' table address pointers will point to the C table.  The new singles/background table will appear as follows:

The two extra accumulators would be included in the telemetry stream by multiplexing four rate channels into two telemetry slots.  Rather than using the J/J' flag to do the multiplexing, however, we will use the S/B flag.  This means that every "full" CMS rate channel will be included in both the L and the C formats.  The remaining four channels will be included in only one of the formats; thus they will not be cleared each logical record as the singles/background rates are. This results in lower time resolution, but continuous coverage for these four channels.

Based upon their expected low count rate, we have decided to multiplex the CH5 and TH1 channels into the slot previously used only for the CH5 channel. The A singles rate, AS, previously received full coverage, despite being a singles/background rate, since it appeared in each of the L, J' and J formats.  It will now be multiplexed, via the S/B flag, with the TAC singles rate.  In effect, we are moving the CH5, TH1 and TAC channels into the S/B commutation algorithm.

Calibrator Operation

The EPD inflight calibrator circuit is used to provide test stimuli to various analog channels in both CMS and LEMMS. As presently designed, the calibrator does not exercise the TOF electronics.  In the proposed design, the TOF will be an essential channel, rather than an additional channel, so it will be added to the calibrator system.  Likewise, the Jc analog channel will now be carrying Kt information, so the Kt discriminators will also have to be calibrated.  Table 10 shows the proposed servicing routine for the calibrator system.

It is difficult to estimate exactly where the discriminators will be set in the new design, so the proper choice of attenuation decades is difficult. Therefore, the discriminator/calibration slot assignments are tentative (although we are assigning some channels to more than one decade where possible).

TABLE 10. NEW CALIBRATION CHANNEL ASSIGNMENTS

Next: Command Changes

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