ACIS Users Guide, Rev. A

2.0 Overview

2.1 Instrument Hardware Overview

Figure 1 illustrates the overall hardware architecture of ACIS.

Figure 1. Hardware Architecture

• RCTU - Remote Command and Telemetry Unit

  This provides the command and telemetry interface between ACIS and the spacecraft.

• PSMC - Power Supply and Mechanism Controller

  This provides power to the ACIS Digital Processor Assembly (DPA), which contains the Back End Processors and Front End Processors, and to the Digital Electronics Assembly (DEA), which contains the CCD Video Controller Boards and two interface boards. It is also responsible for controlling the ACIS doors, vent valves, etc. (not shown).

• BEP - Back End Processor

  ACIS has two redundant Back End Processors, responsible for the overall control of the instrument, including command and data handling. Although both BEPs are usually powered at the same time, only one BEP is active at a time. Each BEP is powered by one side of the PSMC DPA Power Supply.

• FEP - Front End Processors

  ACIS has 6 Front End Processors which are responsible for acquiring digitized image data from the CCD Video Controllers, and for detecting candidate X-ray events from these images. For a normal science observation, data from each CCD used for the observation is processed by any one of the 6 FEPs (NOTE: it is possible, however, to have more than one FEP process data from the same CCD). FEPs 0 - 2 are powered by A-side of the PSMC DPA Power Supply, and FEPs 3 - 5 are powered by the B-side of the supply. In order to process data from 6 CCDs simultaneously, both sides of the PSMC DPA Power Supply must be on.

• DEA 11th Board

  This is the primary interface board for the Detector Electronics Assembly, It contains power switching logic for the DEA Video Boards, a Focal Plane temperature controller, and some housekeeping logic. These components are powered by the primary side of the PSMC DEA Power Supply. This board also contains a set of relays used to switch power on the video boards to the current PSMC DEA Supply. Unlike the DPA, only one side of the PMSC DEA Power Supply may be on at a time.

• DEA 12th Board

  This board is a backup interface board for the DEA. It is identical to the 11th board except that it does not have the power-switching relays, nor does it have the housekeeping logic. This board is powered by the redundant side of the PSMC DEA Power Supply.

• Video Boards

  ACIS has 10 DEA Video Boards. Each board is hard-wired to a particular CCD (see diagram for the mapping). Each video board contains a sequencer (loaded by the BEP) which is used to transfer charge within the CCDs. Each also contains digitization circuits to convert the analog CCD data into a digital form, which is then fed to the FEPs.

The ACIS Mongoose processors are all configured by hardware to use "little-endian" byte ordering. For example, the 32-bit value 0x12345678 is stored as bytes in RAM as follows:

By convention, bits within a word are numbered with the least-significant bit as bit number 0. This is consistent with the "MIPS Programmers Reference Guide."

Unless otherwise specified, all signed values use two's complement representation.

2.2 Instrument Software Overview

The ACIS Instrument Software runs on two types of processors within the system: the Back End Processor (BEP) and the set of Front End Processors (FEP). The BEP loads its software either from EEPROM or from the uplink command channel. Once up and running, the BEP can enable power to the FEPs, and load software into the FEPs using a shared-memory interface.

The BEPs run a single-processor preemptive, multi-tasking kernel which allows the BEP to be performing more than one task at a time. For example, a BEP can be processing science data, while acquiring DEA housekeeping data, while performing a memory dump (BTW: This is a very useful feature when trying to diagnose certain classes of problems). The telemetry produced by the BEP is organized into data packets, which appear in the Science Data portion of the telemetry stream. Each packet is preceded by a 32-bit synchronization pattern, and contains a length field indicating the number of words in the packet. This allows telemetry produced by the several tasks running on the BEP to be merged into the telemetry stream (for example, memory dump telemetry packets may be mixed in with science event data packets).

The following lists the BEP's tasks, grouped according to their priority (highest priority, 51, is first, and lowest priority listed, 55, is last. Tasks with the lowest priority number have the highest run-time priority):

Each FEP, on the other hand, runs a single main thread, with a single interrupt handler to count the arrival of images produced by the DEA. The FEPs are only commanded by the software running on the active BEP. This single thread performs the high-performance portion of the science data processing, and polls its shared memory interface for requests from the BEP. In general, the FEPs are only asked to do one thing at a time by the BEPs.

2.3 Commands and Telemetry

ACIS is commanded via its Remote Command and Telemetry Unit (RCTU). This unit provides various types of commands and telemetry. The ACIS hardware is commanded via high-level pulse commands to the ACIS Power Supply and Mechanism Controller (PSMC) and 16-bit serial digital commands to the ACIS hardware serial port. Commands to the ACIS software are contained within a series of 16-bit serial command words, known as command packets, which are addressed to the ACIS software serial port. For the purposes of this document, "hardware commands" will refer to either high-level pulse commands addressed to the PSMC, or serial digital commands addressed to the ACIS hardware serial port. "Software commands" will always refer to command packets addressed to the ACIS software serial port.

The ACIS hardware provides engineering telemetry which is read by the spacecraft and placed in fixed places within the spacecraft telemetry stream. The ACIS software produces telemetry as a stream of 32-bit words, packed and placed into the spacecraft telemetry stream allocated for ACIS science telemetry. These packets may or may not be separated by a variable number of fill bytes (0xb7), placed into the stream by the ACIS hardware when a packet is not being transferred.

For details on the content and format of the ACIS hardware commands and telemetry, refer to the ACIS Instrument Protocol and Command List (MIT-CSR 36-01410). For details on the content and format of the ACIS software command and telemetry, refer to the ACIS IP&CL Software Structure Definitions (MIT-CSR 36-53204.0204).