File $Source: /home/juda/www/hrc_flight/RCS/nil_mode.html,v $
Version $Revision: 1.7 $
Last Update $Date: 1999/05/10 18:08:08 $
HRC CCB Status Approved

OP03 Procedures op03:

HRC Command Sequences

Select HRC Next-in-line Mode Telemetry


This sequence switches telemetry format of the spacecraft to format 2 (ACIS observing mode telemetry allocation). In addition to making the CTU change telemetry formats a serial digital command must be issued to the HRC to cause it to switch the telemetry synchronization signal it uses from the science header pulse to the major frame pulse (see the HRC serial digital telemetry schematic). The input for serial digital telemetry switches between two FIFO buffers: one with detector rate and housekeeping data ("secondary science" data) and one with detector event data ("primary science" data). When in OBS mode the science header pulse is used to synchronize the reading of 72 bytes of serial digital telemetry from the secondary science FIFO buffer and then switches to reading from the primary science FIFO buffer. In NIL mode the major frame pulse provides this synchronization. The HRC must be commanded to use the appropriate signal for the selected telemetry format.

Note: This particular example assumes that the HRC next-in-line format we are switching to is format 2 (ACIS observing mode). A similar sequence must be performed when switching to any format where the HRC has the next-in-line telemetry allocation (all formats but #1?). In the sequence below this is done by a change of the second command to the approprate mnemonic for the desired format (e.g. CSELFMT4) and changing the telemetry verifier to the corresponding value (e.g. CCSDSTMF = FMT4 for the previous command example).

The serial digital command to the HRC to switch the synchronization signal is sent first; followed by the command to the CTU to change formats. Serial digital commands to the HRC to reset and then re-enable the serial digital telemetry FIFOs are then issued; this flushes the buffers and restarts them.


The HRC serial digital telemetry will not make sense unless both the CTU and HRC are switched together. It may take as many as three full major frames of telemetry (3×32.8 sec) after the sequence is executed before the HRC serial digital telemetry is correctly formatted.

Flow Diagram

Delta-Time Command Mnemonic Description Cmd Field Mnemonic Data Parameter Telemetry Verifier Expected Value Notes
00:00:01.000 2NXILASL Select timing for HRC next-in-line mode telemetry     2OBNLASL NEXT sd1a01
00:00:00.500 CSELFMT2 CTU telemetry mode select format 2     CCSDSTMF FMT2  
00:00:01.000 2FIFOAOF Reset event data fifo     2FIFOAVR RSET sd4500
00:00:01.000 2FIFOAON Enable event data FIFO     2FIFOAVR ENAB sd4501

$Log: nil_mode.html,v $

Revision 1.7 1999/05/10 18:08:08 jhc

corrected initial time delay to 1 sec

Revision 1.6 1998/09/11 20:51:55 juda

changed "Caution" of 1 major frame to synch to as many as 3 - minor grammar corrections to description section.

Revision 1.5 1998/06/08 17:52:08 juda

corrected spelling error "Rest" to "Reset"

Revision 1.4 1998/05/13 19:07:07 juda

Added "File" entry to version table

Revision 1.3 1998/05/07 12:01:24 juda

corrected description text for 2NXILASL mnemonic - this is next-in-line timing

Revision 1.2 1998/05/06 19:30:28 juda

Marked as approved

Revision 1.1 1998/05/05 13:04:31 juda

Initial revision

Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Mail Stop 70
Cambridge, MA 02138, USA
Ph.: (617) 495-7062
Fax: (617) 495-7356