The Galileo Energetic Particles Detector

Galileo EPD Handbook

Chapter 1. Instrument Summary

Telemetry and Operation Changes (continued)

Rate Channel ID Codes

In the CMS system, an event can fall into either the "main array," or the single parameter channels. There are 13 channels defined in the main array of the old design; 10 in Jab x K space and 3 in Jc x K space.  Each of these channels was assigned a rate ID code by the Rate Logic circuitry.  The PHA uses this code to decide if the event is worth processing (only channels with an ID code are eligible for consideration). It also uses the code to decide what priority level the event belongs to, what analog multiplexer channels to activate, and what detector group the event came from.

We are eliminating only 3 main array channels (Jc channels), but we would like to add 13 new ones.  It would be nice, therefore, if we could just add additional ID codes (and bits) as necessary.  The hardware that uses these codes, however, is not easily changed.  The decisions are made in a lookup table stored in a CDP1834 CMOS ROM on the PHA, and unfortunately this ROM cannot be reprogrammed or replaced.  We will therefore have a fixed set of decision criteria to work with.

This limitation is a difficult problem for us, since the new rate channel assignment philosophy will vary greatly from that used previously.  In the old design, only 13 of the possible 16 ID codes are assigned; the PHA ignores all events tagged with the other 3 codes.  Since this is programmed into the ROM, we will still be limited to 13 active codes in the new design.  The solution to this problem is to have several channels represented by the same rate ID code (discontinue the 1:1 mapping).  This would mean that when an event is tagged with a given ID code, the event could have occurred in one of two or three channels; such a scheme will not appreciably degrade our science data since we have the high resolution TOF, E, and delta E data also.

The only area where this solution fails is the selection of the correct analog mux position for any given event.  We will have to disconnect the ROM-generated signal (J SELECT), and replace it with a new logic definition (i.e., select the Jab channel whenever K0 is active).

Priority Assignments

In the CMS priority system, each priority is given equal attention by the PHA.  Within any given priority, however, the events are processed on a first come, first serve basis.  It is therefore desirable to group rate channels together which are expected to see approximately the counting rates.  The priority assignments were made by trying to maintain the present ID code and priority assignments on the prime side, and matching appropriate unprime channels with them.

Table 8 shows the proposed CMS rate ID and priority assignments.

TABLE 8. NEW CMS RATE ID AND PRIORITY ASSIGNMENT

Singles channels:  CA1, CM1, CH1
Subcommutated detector rate channels:  JAS, JBS, KS, KTS, STARTS, TACS
Eliminated channels:  CE1, CE2, CE3, CP1, CP2, CP3, CA0, CM0, CH0, CA2, CM2, CH2

TABLE 9. CMS RATE CHANNEL REPLACEMENTS

Remaining singles channels: CA1, CM1, CH1
Subcommutated detector rate channels: JAS, JBS, KS, KTS, STARTS, TACS
Eliminated channels: CE1, CE2, CE3, CP1, CP2, CP3, CA0, CM0, CH0, CA2, CM2, CH2, LS, JCS

Continue: Rate Channel Telemetry

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