Support Processing

1553 Bus Management

CRISP communicates with the C&DH system over a 1553B bus. The DPU acts as a remote terminal (RT) with the active C&DH system acting as the bus controller (BC). The traffic on the 1553B bus is periodic, repeating every one second major frame. The major frame is divided into 25 minor frames, numbered 0 to 24. Individual messages are distinguished by their transmit or receive subaddress. In addition to the messages described in Reference 3, CRISP handles the messages shown in the following table.

Four times per minor frame, the current MIMU gyro angles are sent to CRISP. CRISP immediately forwards these to the TPU. At the end of the major frame an attitude correction and the commanded attitude are sent to CRISP from G&C. These are also forwarded to the TPU. At the beginning of the major frame CRISP provides its estimate of the current attitude and time to closest approach. The formats of these messages are shown in the following tables.

Inter-Integrated Circuit (I2C) Bus Management

A collection of environmental data is collected by the DPU. There are analogs including voltages and currents read via one of the DPU's two Inter-Integrated Circuit (I2C) buses. A second I2C bus is used to collect temperatures. The data collected is included in the status subpacket and image headers. Each analog is also monitored when it is read (see below). Collection and monitoring occur once per second.

Alarms and Monitoring

Alarms report problems found by the DPU software. Each alarm is described by an ID, two values, and a flag. The ID identifies the problem that has occurred and the accompanying values offer additional information. The flag indicates whether the alarm was caused by a transient or a persistent condition. See Appendix 3 for a list of alarms.

The alarms are divided into two groups: one for reporting internal software problems and another for reporting out-of-limit conditions for monitored data. Software problems are all reported as transient alarms. When the problem occurs, the alarm is generated and the software recovers from the problem as best it can.

Values read from the I2C buses are monitored by the DPU The monitoring is event driven: a monitor cycle is performed on each item as it becomes available. For example, each I2C analog is monitored when it is read. The monitoring algorithm is described in more detail in Reference 3.

The monitored data is summarized in the following table. The monitor class is encoded as S= shutdown, N=nop, and R=redo. The reported alarm Ids are for low and high excursions; similarly there are low and high response macro Ids.

Fault Avoidance and Recovery

Many of the DPU's hardware control registers contain triple redundancy and voting logic for each bit. The DPU software periodically rewrites these registers to correct single bit errors.

The DPU has a watchdog timer. If the watchdog timer is not tickled from time to time, the processor is reset. The watchdog timeout is 2.95 seconds. The watchdog does not run until it has been enabled; once enabled, it can never by disabled except by processor reset, watchdog or otherwise. The DPU monitors all periodic processes every second. If they are all running, the watchdog is tickled. Some aperiodic processes do not participate.

Closest Approach

The DPU will automatically select and run a macro to orchestrate data collection during closest approach. The tracker estimates the time to closest approach, the miss distance between the spacecraft and the comet, and the angle from the spacecraft to the comet. The TPU uses a set of thresholds to categorize the comet angle into either "inbound", "inbound imaging", "closest approach", or "outbound imaging". The TPU also categorizes the miss distance as either "very near", "near", "nominal", "far", "very far", "too far #1", "too far #2", or "too far #3". Once the DPU is commanded to monitor the closest approach data (CRS_CA_START), it watches the angle until "inbound imaging" is detected. Then the DPU starts the closest approach imaging macro. The miss distance category is used to select the macro. The macro chosen depends on whether "good" targeting or "bad" targeting was selected (an argument to CRS_CA_START). In "good" targeting mode, miss distances from "very near" to "very far" cause the corresponding "very near" to "very far" macro to be run; miss distances that are "too far" cause the "very far" macro to be run. In "bad" targeting mode, the "too far" miss distances will cause corresponding "bad targeting" macros to be run. Once one of the imaging macros has been selected, no other closest approach macro will be started. The DPU continues to monitor the comet angle. Once the "closest approach" stage is reached, a closest approach macro will be run.

The DPU will automatically run a post-encounter macro. The macro is started once the comet angle reaches "outbound imaging". The macro will be allowed to run to completion. Then the DPU closest approach monitor enters an idle state; another command (CRS_CA_START) is required before it will start monitoring the comet angle again. The closest approach monitor can be commanded to the idle state at any time (CRS_CA_RESET). The comet angle and miss distance thresholds are all TPU parameters.

Return to CRISP Software User's Guide. Report problems to John Hayes.