PET Command Words

PET responds to three four-byte command words: the first word mostly enables or sets levels on various discriminators, and has never been changed; the second word modifies the coincidence equation, and has seen a lot of action; and the third word enables event buffers, pulse-height analyzers, and calibration and other global modes, and has only been changed once, at the beginning of the mission (plus routine changes once a month to enable and disable calibration). The three main tables below show the meaning of the command word bits, which rates or event buffers they affect (for the second word only), and their nominal settings, and the last table shows the changes that have been made to the second command word. There are also a couple of tables in the discussion of command word two that give the detailed rate and event logic definitions. Except for the last (state-change) table, this information was lifted from the PET information book put together by the Caltech folks shortly after launch.

Command Word 1

The first word has always had values as shown in this table; the hexadecimal values of the four bytes are 1f ff d7 0c.

Command Word 2

Below are three tables: the first shows the logical definitions of the PET rates that can be changed by command, the second does the same for the event buffers, and the third defines the command bits in the second word that effect most of these changes. Square brackets around a term in the logic equations indicate that the term is nominally present but can be deleted, and slashes around a term indicate that the term is nominally not present but can be added. LOZMODE in the PEN rate equation is a "bit" that has always been set (there is a high-Z mode with lower gains that has never been used), and the others indicate detector and PHA thresholds and discriminators. AL and AH indicate low and high guard thresholds; P1 through P8 indicate individual detectors; "*" indicates logical complement or veto; a suffix "A" or "B", as in P1A, indicates a higher threshold; and other suffixes (like P2E, P8R*, etc.) indicate that the relevant command bit affects the term only in one equation, whereas (for example) [P1] indicates that this term is removed from most of the equations at the same time if the relevant command bit is set.

Command Word 3

The third word was changed from its nominal state at the beginning of the mission, to suppress the PENE buffer, and has been left unchanged since. The hexadecimal values of the four bytes in the nominal state in the table below are 00 ff bc 02, and the values for the "permanent" state are 00 f7 bc 02.

Changes to Command Word 2

The following table shows the values of the four bytes of the second command word at various times in the mission, including the nominal (design) state.

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new 6 August 1999, revised 7 January 2000