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


4.4 Testing and Checkout

Preparation of test procedures and construction of test facilities should begin early in the program. The test specifications should clearly distinguish between the tests required at various stages of design, qualification, production, installation, and flight. Feasibility tests of components and circuits may be conducted prior to completion of a breadboard computer. Engineering prototype computers may be used for hardware logical and electrical verification and for software verification in hybrid simulations. Full engineering qualification testing should be conducted using an early production model, and every production model should be subjected to an acceptance test which is as thorough as possible without overstressing the computers. Finally, the computer system and its interfaces should be tested in the vehicle to give sufficient confidence that no significant degradation has occurred since acceptance. It may be advantageous at any stage of testing to test the computer with another flight computer or a ground-based computer.

Generally, worst-case conditions should be used for testing. For qualification, the components should be tested to failure to realistically establish their actual limits. Where test specifications are not given, MIL Standard 883 contains suggested test methods and procedures for microelectronics. The computer should be tested while operating under environmental conditions including temperature cycling, vibration, vacuum, and radiation. If a failure occurs during a test, a logic proof should verify the failure point, repair should be effected, and elimination of the failure should be proven by demonstration. The test should be recycled to the previous checkpoint unless a major repair was required, in which case, the entire test cycle should be repeated. Special tests should be devised to verify each of the computer's functions, such as guidance, control, etc. Interfaces should be represented as realistically as possible, including waveforms, impedances, noise, voltage variations, etc.

A functional test program should be initiated early in the development phase. The availability of such a program should be reflected in the design to maximize ease of checkout. For acceptance testing, this program should exercise all the I/O units and the entire memory, while operating under environmental conditions. If possible, the test conditions to be withstood should be specified, rather than arbitrary. Special purpose hardware subsystems should be considered for monitoring and diagnosing the rest of the computer, so that acceptance testing can be largely carried out by the special purpose part. If this procedure is adopted, only the special purpose equipment need be validated.

The ease of checkout during acceptance and qualification tests and, particularly, during prelaunch activities should be considered during the design of the computer. Prelaunch checkout should be as complete a checkout of the computer and its interfaces as can be conducted without removing the computer from the vehicle or disconnecting interfaces for any operation. The external equipment required and the ability of the computer to aid the checkout procedure should be considered. It is necessary to specify the electrical capabilities of the signals and circuits in the computer when operating with ground-support equipment cables and external loads, and, whenever possible, to specify the operating routines for checking various parts of the computer system using ground-based computers prior to launch. Ground-based checkout systems should have such specific features as: checks for proper operation of arithmetic, control, memory, and I/O sections; verification of any index or extension registers and of testing and jump instructions; testing of multiplexers and A/D converters; and testing of the system outputs including any D/A output devices and discretes delivered to telemetry or control devices. If a redundant system is used, each channel should be verified prior to launch. If necessary, external indicators should be used to verify the proper operation of clocks and power supplies in the system through the external harness. Provision should be made for varying all primary and secondary voltages for onboard margin testing. Self-test programs for onboard testing should include parity checks and internal coding checks on transmission. Use of an integrated testing concept in which certain prelaunch and inflight tests are carried out in a common manner by essentially the same automatic equipment is desirable, both for economic reasons and to maintain continuity of testing.


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