Computers in Spaceflight: The NASA Experience
- Chapter Four -
- Computers in the Space Shuttle Avionics System -
The future of the shuttle's computers


[130] The computers in the Shuttle were candidates for change due to the rapid progress of technology coupled with the long life of each Shuttle vehicle. First to be replaced were the engine controllers. By....

Figure 4-II.
Figure 4-II. A Shuttle main engine in a ground test. The Controller can be seen mounted on the left side of the combustion chamber. (NASA photo 885338)


[132] ....the early 1980s, Marshall Space Flight Center began studying a Block II controller design because it was becoming impossible to find parts and programmers for the late 1960s components of the Block I183. The revised computer uses a Motorola 68000 32-bit microprocessor. When selected, it was clearly the state of the art. Instead of plated wire, a CMOS-type semiconductor random-access memory is used. Finally, the software is written in the high-level programming language, C. Such a computer reflects the current design and components of a ground-based, powerful digital control system. The C language is also known as an excellent tool for software systems development. In fact, the UNIX operating system is coded in it.

Aside from the processor change, the Block II's memory was increased to 64K words. Therefore, the entire controller software, including preflight routines, can be loaded at one time. Semiconductor memories have the advantages of high speed, lower power consumption, and higher density than core, but lack core memory's ability to retain data when power is shut off. Reliability of the memory in the Block II computer was assured by replicating the 64K and providing a three-tier power supply184. Both Channel A and Channel B have two sets of 64K memories, each loaded with identical software. Failure in one causes a switch-over to the other. This protects against hardware failures in the memory chips. The three tiers of power protect against losing memory. The first level of power is the standard 115-volt primary supply. If it fails, a pair of 28-volt backup supplies, one for each channel, is available from other components of the system. Last, a battery backup, standard on most earth-based computer systems, can preserve memory but not run the processor.
The significance of the evolution to Block II engine controllers is that they represent the first use of semiconductor memories and microprocessors in a life-critical component of a manned spacecraft. Honeywell scheduled delivery of a breadboard version suitable for testing in mid-1985. The new controller is physically the same length and width, so it fits the old mounting. The depth is expected to be somewhat less. When the first of these computers flies on a Shuttle, NASA will have skipped from 1968 computer technology to 1982 technology in one leap.
IBM's new version of the AP-101 (the F) incorporates some of the same advantages gained by the new technology of the engine controllers. Increasing the memory to 256K words means that the ascent, on-orbit, and descent software can be fitted into the memory all at once. (This is not likely to happen, however, because of the pressing need to improve the crew interfaces and expand existing functions.) Higher component density allows the CPU and IOP to be fitted into one box roughly the size and weight of either of their predecessors. Execution speed is now accelerated to nearly 1 million operations per second, twice the original value. In essence, NASA has finally acquired the power and capability it wanted in 1972, before the software requirements showed the inadequacy of the original AP-101.
[133] As in the engine controllers, the memory in the AP-101F is made of semiconductors. Power can be applied to the memory even when the central processor is shut down so as to keep the stored programs from disappearing. A commercially available error detection and correcting chip is included to constantly scan the memory and correct single bit errors. These precautions help eliminate the disadvantage of volatility while still preserving the size, power, and weight advantages of using semiconductors over core memories 185.

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