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3.0 Requirements

3.1 External Interface Requirements

3.1.1 User Interfaces

The ACIS Science Instrument has no direct in-flight human user interfaces. All human interaction with ACIS is managed by the AXAF Science Center (ASC) and the Operations Control Center (OCC).

3.1.2 Hardware Interfaces

The detailed ACIS hardware to software interfaces are described in the Digital Processor Assembly (DPA) Hardware Specification and System Description. Figure 12 and Figure 13 illustrate the key interfaces to the DPA's software.

FIGURE 12. Back End Processor Software Context Diagram

FIGURE 13. Front End Processor Software Context Diagram

3.1.3 External Software Interfaces

ACIS uses a commercial off-the-shelf real-time executive to perform task control functions and provide some resource management services. ACIS is using:

Nucleus RTX system , Product # NU920102, from Accelerated Technology, Inc.

The interface requirements for this product are defined by the Nucleus RTX Reference Manual, NU910701 and the Nucleus RTX Internals Manual, NU910702.

ACIS is planning on using the following capabilities of Nucleus RTX:

A detailed list of each used function is provided in the ACIS SIS Detailed Design Specification.

3.1.4 Communications Interfaces Serial Commands

Serial commands to ACIS shall be delivered via the RCTU's Serial Digital Command Channel. All ACIS Science Instrument Software commands are organized into command packets. Command packets consist of a series of 16-bit serial digital command words. Table 2 describes the overall command packet format:

TABLE 2. Serial Command Packet Format (Continued)

Req    Word #            Field Name          Description                                            
 2-1   0                 Packet Word Length  This describes the number of 16-bit words in the       
                                             command packet, including this field. This field can   
                                             range from 3 to 256.                                   
 2-2   1                 Packet Identifier   This identifies the command packet in a sequence.      
                                             This field can range from 0 to 65,535.                 
 2-3   2                 Command Opcode      This identifies the operation to be performed by       
                                             ACIS. This field can contain up to 65,536 values,      
                                             but only a small subset of possible values are recog   
                                             nized by the ACIS Science Instrument Software.         
 2-4   3..(Packet Word   Command Data        This area contains any data associated with the        
       Length - 1)                           command                                                

The information content of the Command Opcode and Command Data areas of a command packet are described later in this document as part of the system features. The exact bit-for-bit layout of this area will be defined later, during the design process, and will be provided as part of the Design Specifications and Operations Manual. Discrete Commands and Flags

Discrete commands to ACIS Science Instrument Software shall be provided using ACIS hardware status registers. Table 3 lists these signals.

TABLE 3. Discrete Commands and Flags (Continued) (Continued)

Req    Name                    Source              Description                                    
 3-1   Reset                   Ground Command      This signal resets the ACIS Back End           
                                                   Processor. There is no explicit software       
                                                   monitoring of this signal. This signal will    
                                                   cause ACIS to begin executing its soft         
                                                   ware contained in its Boot ROM.                
 3-2   Boot Modifier Flag      Ground Command      When asserted, this signal instructs the       
                                                   ACIS Boot ROM software to load code            
                                                   and data via the Serial Command Chan           
                                                   nel (Load from Uplink), and to execute         
                                                   the loaded code.                               
 3-3   Science Header Synch    S/C Telemetry       This signal indicates the start of a Science   
                               Clocks              Header Telemetry Frame.                        
 3-4   Radiation Monitor Flag  S/C Radiation Moni  This signal indicates that the S/C Radia       
                               tor                 tion Monitor has detected a level greater      
                                                   than its current threshold.                    
------------------------------------------------------------------------------------------------ Spacecraft Telemetry Formats

The AXAF-I telemetry definition consists of several formats. ACIS serial telemetry data appears in two of these formats. Format 1 is normally used when HRC is the main observing instrument, and ACIS is "next-in-line." In this configuration, ACIS telemetry is taken at 512bps and stored in the engineering sections of the Format 1 telemetry.

Format 2 is normally used when ACIS is the main observing instrument. In this configuration, ACIS serial telemetry is taken at 24Kbps and is stored in the Science Data portions of the Format 2 telemetry. Table 4 illustrates the overall layout of Format 2's Science Frame, as described by the RCTU User's Guide. Sixteen Science Frames make up a Telemetry Major Frame (not shown). The shaded areas contain data formatted and supplied by the ACIS Science Instrument Software, where each contiguous science data region in the Science Frame is numbered from 0 to 63.

TABLE 4. Science Frame - Format 2 (for reference only)

                                                                    Repeat 7 times                      
Minor   CCSDS   Engineering  Science   ACIS Time-       Science     Engineering  Science   Engineering  
Frame   VCDU                 Hdr       stamp            Data                     Data                   
0       Header               (Minor    See              Section[0]               Sec                    
                             Frame 0   Section                           tions                  
                             Only)                                               [1..7]                 
Bytes   6       26           24        4                68          32           96        1            
Minor   CCSDS   Engineering  Science Data                           Engineering  Science   Engineering  
Frame   VCDU                 Section [8]                                         Data                   
1       Header                                                                   Sec                    
Bytes   6       26           96                                     32           96        1            
.       .                                                                                               
.       .                                                                                               
.       .                                                                                               
Minor   CCSDS   Engineering  Science Data                           Engineering  Science   Engineering  
Frame   VCDU                 Section [56]                                        Data                   
7       Header                                                                   Sec                    
Bytes   6       26           96                                     32           96        1            

When created by the ACIS Science Instrument Software, the science data sections are treated as contiguous.

The spacecraft-generated "Science Hdr" portion of each Minor Frame 0 contains the timecode corresponding to the start of the Science Frame. Serial Telemetry

ACIS outputs serial telemetry information using a DMA device connected to the RCTU's serial telemetry channel. ACIS serial telemetry shall be sent asynchronously to the telemetry frame clocks. When ACIS is the main observing instrument, the telemetry data are sent at a 24Kbps rate, and will appear in the Science Data sections of Format 2. When ACIS is "next-in-line", the telemetry data will normally be sent at 512bps and appear as part of Format 1's Engineering Data.

The ACIS Science Instrument Software shall format its telemetry information into telemetry packets. Each telemetry packet will contain a series of 32-bit words. Although each packet is an integer number of 32-bit words, the hardware byte-aligns the words within the telemetry frame (i.e. a packet may start on a byte-boundary within a telemetry frame). Table 5 illustrates the overall format of ACIS telemetry packets.

TABLE 5. Serial Telemetry Packet Format (Continued)

Req    Word #       Field Name             Description                                                 
 5-1   Word 0       Start of ACIS Telem    This item starts each packet and is used to locate pack     
                    etry Packet Indicator  ets within the Science Frame telemetry stream. This         
                                           value must be different from any hardware generated or      
                                           spacecraft generated fill patterns and unlikely to appear   
                                           within the science data. The value of this field in hexa    
                                           decimal is 0x0736f4166 ("soAf").                            
 5-2   Word 1       Packet Data Count      This item describes the total number of 32-bit values       
       Bits 0..9                           contained in the packet, including the "Start of Packet     
                                           indicator." The minimum length of an ACIS telemetry         
                                           packet is 2 long words (8 bytes). The maximum length        
                                           of an ACIS telemetry packet is 1023 long words (i.e.        
                                           4092 bytes).                                                
 5-3   Word 1       Format Tag             This item identifies the format of the data contained       
       Bits 10..15                         within the packet. This field supports up to 64 formats.    
 5-4   Word 1       Sequence Number        This field identifies the packet emitted by the ACIS Sci    
       Bits 16..31                         ence Instrument Software. This field wraps once every       
                                           65,536 packets. When combined with the Science              
                                           Frame time-stamp, this value uniquely identifies each       
                                           telemetry packet produced by the ACIS Science Instru        
                                           ment Software.This field allows the ground to deter         
                                           mine the number of ACIS packets lost due to a               
                                           telemetry drop-out condition.                               
 5-5   Words 2 -    Data                   This area contains the bulk of the packet's data. The       
       (Packet                             content and format of this data is identified by the For    
       Data                                mat Tag field described above.                              
       Count - 1)                                                                                      

The information content of the Format Tag and Data areas of a telemetry packet are described later in this document as part of the system features. The exact formats of these areas will be defined later, during the design process, and will be provided as part of the Design Specifications and Operations Manual. In general, these formats shall provide enough redundant information to allow data sets to be interpreted in situations where one or more previous data set(s) has been corrupted or lost. Discrete Telemetry

Discrete telemetry from ACIS Science Instrument Software shall be provided using 4 output bits, writable using an ACIS hardware register. These four signals will be sampled relative to each "Science Header Pulse" by the RCTU and included as part of the ACIS engineering telemetry stream. ACIS Science Run Timestamps

"The first 32 bits of the Science Data written by ACIS into Section[0] of Minor Frame[0] (i.e. the first 32 bits following the receipt of the Science Frame Pulse) shall be the value of a timestamp counter specially sampled and inserted into the serial output stream by ACIS hardware. Its value represents the number of ACIS 100 KHz pixel clock cycles since it was last reset or wrapped. This counter will wrap about once per 12 hours. ACIS shall use this same counter to timestamp the start of its Science Runs (see Section 3.1.2 on page 32). This, coupled with the spacecraft supplied timestamp information in the Science Header of each frame, specifies the start time of a run (assuming that the spacecraft timestamp information can be related to universal time). All science data sent by ACIS will be indexed relative to the start of the run, using pixel-clock driven counters (such as exposure counts, or data set counts)."

FIGURE 14. ACIS Science Time-stamping Timeline

The ACIS timestamp inserted by the hardware into the telemetry stream relates the counter with the Science Frame. Given any Science Frame number produced within about 12 hours after the start of the science run (reference frame #), its corresponding ACIS inserted timestamp (reference timestamp) and the number of 100KHz ticks per Science Frame (ticks/frame), the Science Frame number corresponding to a given ACIS science data timestamp is as follows: If the data timestamp is less than the reference timestamp:

(EQ 1) Frame # = reference frame - (reference timestamp - data timestamp)/ticks per frame If the data timestamp is greater than the reference timestamp, then the 32-bit timestamp counter must have wrapped once since the data timestamp was sampled (assuming the reference frame is within an hour after the start time):

(EQ 2) Frame # = reference frame - (232 - (data timestamp - reference timestamp))/ticks per frame

NOTE: If the first packet of a run takes longer than 12 hours to appear in telemetry, one must use more robust algorithm that relies on a-priori ground knowledge of the how the instrument has been run prior to the start of the current run).

The computed Frame Number is then used to identify which science header contains the S/C clock corresponding to the time during which the run was started (NOTE: Operations may instead choose to relate the Frame Number directly to Universal Time). Refer to Appendix D - for some guidelines on processing ACIS time-stamps.


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