NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

3. System Design

3.2.1 Physical Features

Mechanical design of the computer shall take into account at least the following considerations: size, weight, acceleration and vibration, mechanical shock, humidity, ambient pressures and temperatures, corrosive atmospheres, and human factors. Consideration should also be given to adequate sealing, access to subunits for repair and access to connectors. Proven techniques of interconnection and packaging shall be utilized. The electrical properties of interconnections shall be controlled with respect to electromagnetic interference susceptibility, cross talk, and transmission properties. The power source and ground requirements for the system shall be compatible with spacecraft provisions. Standby or backup power is to be provided whenever needed.


3.2.2 Memory Features

The computer must have a memory subsystem capable of storing all programs including prelaunch testing programs. There shall be adequate random access, high-speed memory to hold all programs, and data which will be required simultaneously during peak load periods. The program memory shall exhibit adequate reliability with the capability of withstanding power supply transients and temporary power failures. If the memory is not fixed or 'read-only," precautions shall be taken to preclude irrecoverable loss of the programs or critical data resulting from an electrical transient or a program flaw. The memory system shall be capable of being turned off and on without losing its contents. Provision for a backup or reload capability shall be considered for storage reliability.

Consideration shall be given to ways in which expanded memory requirements can be met, such as by the addition of memory modules or a bulk memory, or by reloading the memory from external sources.


3.2.3 Processor Features

The computer shall provide an instruction repertoire commensurate with the mission demands and operating speeds which are clearly adequate for peak loads. These shall include as a minimum the following basic instructions: load, store, logic, arithmetic, input/output, conditional and unconditional branches, and shifts. Special instructions to facilitate particular functions, such as data conversion for nonbinary inputs (e.g., man/machine interface), telemetry formatting, and driving displays, are to be provided if required. Floating-point or multiple precision instructions, or instructions to facilitate these operations, should be considered in the design if needed to satisfy precision requirements and to reduce software development costs. If a sample problem is used to demonstrate the efficiency and speed of the computer for a given mission, the sample problem's instruction mix must closely compare with that to be used during the actual mission.


3.2.4 Input/Output Features

The computer shall provide a means for timely subroutine entry when critical input signals are received and for adequate buffering of data which do not require immediate servicing. Priority sequencing of responses to critical inputs shall be provided if necessary.

The interface circuitry and devices shall provide both for adequate reception and transformation to internal form of all signals from sensors, external communications from the ground or crew, and other system inputs, and for output signals of proper electrical characteristics for all actuators, telemetry, displays, and other output devices. When analog-to-digital and/or digital-to-analog conversion is necessary, the speed, precision, and accuracy of conversion shall be within necessary tolerances. The input/output electrical characteristics shall provide for adequate matching of convertors to the input or output electrical interface circuitry.

The computer and the peripheral units shall be designed in such a way that the time required to respond to an input request or to generate an output signal shall be within the physical requirements of the vehicle system. Likewise, the interface bandwidth shall be adequate to support the required data volume, including recovery modes, without unnecessarily burdening the computer.

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Last Revised: February 03, 2010
Digital Engineering Institute
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