2.1.3 Man/Machine Interface
For manned space vehicles, online communication between the computer and the crew is required. Thus, the computer must possess the capability to request action or information, display real-time status and other data, and accept and execute manual commands. Because man is accustomed to alphabetic and decimal characters whereas computers operate with binary numbers, all information to be communicated must be converted to the proper form before it is intelligible. Several implications of the man/machine interface on the computer design are discussed in reference 13. The major effect on the computer design has been to increase both the memory and input/output requirements. Although the programs required to implement the man/machine interface are long, their duty cycle is low so they do not consume much of the computer's time (ref. 14).
The Apollo guidance computer uses a peak rate of about 3,600 operations per second to interpret an input or handle an output in less than 0.1 sec, which appears as virtually an instantaneous response to the operator (ref. 7). The interface between the crew and the AGC consists of a display and keyboard assembly (DSKY) which has function keys for verb, noun, clear, standby, keyboard release, enter, and reset; numeric keys for zero through nine; and plus and minus signs. Three two-digit electroluminescent (EL) displays are used to indicate the verb, noun and program number that the computer is currently using, a three five-digit-plus-sign EL displays are used to show input and output data. Discrete outputs turn on status lamps to indicate a variety of events including uplink activity, gimbal lock, and operator error.
The data entry and display assembly used for communication with the abort electronics assembly (AEA) computer in the Apollo lunar module is similar to the DSKY but less elaborate. It also comprises a keyboard and EL address and data displays (ref. 15). A push-button keyboard was used for manual data insertion to the Gemini computer, but EL devices were not considered because the circuitry required was too complex, and the display intensity would have been insufficient in bright sunlight. Instead, electromechanical decimal wheel devices were used (ref. 16).
No alphabetic input or output has been used with the Gemini or Apollo computers, although the use of symbolic coding would have reduced training burdens as well as simplifying control and interpretation problems. The main drawback in the use of alphabetic or symbolic coding at the time of the Apollo design was the lack of reliable, solid-state display translators. However, the convenience of alphabetic information will undoubtedly lead to the use of symbolic codes in future spaceborne computers. The application of symbolic coding for spacecraft computer control and display has been investigated at the NASA Ames Research Center (ARC) in a simulation environment with considerable success (ref. 17).
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