3.2.5 SI Monitoring

There will be ground monitoring of the focal plane temperature, power consumption, and processor status. For each observation, CCD bias and noise level and rejected event counts will be monitored. Periodic checks will be made of CCD CTE and low energy response, as well as the HRMA focus. Much of the detailed housekeeping data is contained within the `Science Data' portion of the ACIS telemetry, and so requires extraction from the various formats in the same manner as the X-ray event data. This means that some lag for extraction and analysis will be required. (Housekeeping data, which is directly relevant to health and safety issues, is more trivially extracted.)

In order to help a maintainer develop a history of the operation of ACIS, the ACIS Science Instrument Software maintains and telemeters software housekeeping information. Whenever an observer or maintainer sends a command to the ACIS Science Instrument Software, ACIS echoes the command in a telemetry packet. This allows a maintainer to verify that all commands intended for ACIS software were received and executed. In order to allow monitoring of the overall operation of ACIS without relying on its serial science telemetry, ACIS also provides some active status information using the discrete telemetry signals, read by the hardware and placed into the Engineering portion of the telemetry stream.

The ACIS software indicates its boot and running status using the four discrete telemetry signals provided by its hardware. During boot, these signals indicate the current mode of the boot and current state. If ACIS hangs during the boot, the boot status provides an indication of the last action performed prior to the lockup. While running after a commanded reset, the ACIS software cycles the signals between two or more values, indicating the current overall state of the software. If ACIS hangs, these signals freeze into one state until the Watchdog Timer resets the system. After a watchdog reset, ACIS cycles these signals between values which indicate that ACIS locked up at some point and was reset by the Watchdog Timer.

ACIS maintains a set of system statistics representing various operating conditions, such as the number of interrupts, context switches, etc. These statistics also contain the number of occurrences of various warning conditions, such as the number of dropped exposures. These statistics are represented as an array of counters. Periodically, ACIS telemeters and resets the contents of these statistic counters.

Both the ACIS Back End and Front End software periodically set their respective Watchdog Counters. The value written into the counter determines the time-out duration of the Watchdog Timer. In the event of a crash whereby the ACIS software hangs, the Watchdog Timers will reset the hung processor. If ACIS encounters a condition from which it cannot recover, it makes a best attempt to complete the current telemetry item being written to the RCTU, then issues a telemetry message indicating that it is about to reset. Table 3.16 illustrates the contents of this error message.

**Table 3.16:** Fatal Error Telemetry Content
Item	Description
Fatal Error Code	Indicates point at which fatal error was detected.
Optional Argument	Allows more information about the error to be telemetered. Meaning depends on the Fatal Error Code.

Periodically, a maintainer will wish to monitor various DEA operating values. ACIS provides commands allowing the maintainer to select which values to monitor and to start and stop the monitoring process. Some of these housekeeping values are coupled to specific CCDs. This housekeeping run can execute concurrently with an ACIS Science Run, and is similar in nature. To perform a DEA Housekeeping Run, the maintainer commands ACIS to accept a parameter block containing a list of DEA registers to sample and then instructs ACIS to execute the run. ACIS then samples specific items by issuing queries to the DEA and storing the responses. ACIS then telemeters the set of sampled values. The maintainer allows ACIS to continue its data acquisition for the desired time period, then commands ACIS to terminate the run.

ACIS maintains one or more lists of DEA Housekeeping items to monitor. The maintainer installs this list by issuing a ``Load DEA Housekeeping Parameter Block'' command and selects which list to use for a run via the ``Block Id'' field of the list. Table 3.17 illustrates this command. The Start and Stop DEA Housekeeping Run commands are outlined in Tables 3.18 and 3.19, respectively. During a DEA Housekeeping Run, the ACIS software reports the acquired DEA register values. Table 3.20 illustrates the contents of this telemetry.

**Table 3.17:** Load DEA Housekeeping Parameter Block Command Packet
Field Name	Description
Packet Word Length	Total number of 16-bit words in command packet.
Packet Sequence #	Identifies the command packet in a series.
Command Opcode	Load_DEA_Housekeeping_Parameter_Block command opcode. Instructs ACIS software to store the List Data contained within the packet.
Block ID	Block within ACIS.
Block CRC	Checks the load.
Sampling Period	Rate at which the ACIS should sample and telemeter the acquired information.
DEA Item List	List of DEA items to sample.

**Table 3.18:** Start DEA Housekeeping Run Command
Field Name	Description
Packet Word Length	Total number of 16-bit words in the command packet.
Packet Sequence #	Identifies the command packet in a series.
Command Opcode	Start_DEA_Housekeeping_Run command opcode. Instructs ACIS software to retrieve the parameter block indicated by the ``Block ID'' below, and run the requested DEA Housekeeping operation. ACIS will continue to run the mode until commanded to stop.
Block ID	Identifies the DEA Housekeeping Parameter block to use for the run. This block may have been loaded just prior to the Start Run command, or may be part of either the pre-loaded library, or one of the blocks provided at launch.

**Table 3.19:** Stop DEA Housekeeping Run Command
Item	Description
Packet Word Length	Total number of 16-bit words in the command packet.
Packet Sequence #	Identifies the command packet in a series.
Command Opcode	Stop_DEA_Housekeeping_Run command opcode. Instructs ACIS software to stop the current run.
Block ID	DEA Housekeeping Parameter block used for this run.

The only activities which require real-time or near real-time monitoring is the `Bakeout' mode. Bakeout does not occur during normal operations, but the mode exists in case frost or condensation occurs on the ACIS instrument in sufficient quantity to impair instrument efficiency. When commanded by the ground control ACIS will enter Bakeout mode, turning on heaters to raise the instrument temperature. While the heaters are on, the DEA and DPA must not be turned on. The effect of erroneously turning on the DEA and DPA while the heaters are on will be to exceed the fuse rating on the ACIS power, resulting in failure of the experiment. Note that turning on DEA and DPA power requires two commands (an `arm' and `execute' pair), so that accidental activation is very unlikely. The ACIS team has communicated this requirement to the AXAF Operations team, who have accepted the responsibility.