Computers in Spaceflight: The NASA Experience
- Chapter Three -
- The Skylab Computer System -
User Interfaces
[79] NASA and IBM designed the computer system to operate autonomously. One crewman reported "not much interaction" with the system at all62, but the capability was present for significant activity if needed63. The crew could enter data and actually make changes in the software through a keyboard located in the DAS on the ATM Control and Display Console.
The DAS had only 10 keys and a three-position switch. The keys were the digits 0-7 (all entries were in octal), a clear key, and an enter key. The switch could select either power bus one or two, or be off. Above the DAS was an "Orbit Phase" panel containing a digital readout of minutes and seconds to the next orbital benchmark. When the first keystroke of a five-digit command was made, the uplink DCS commands were inhibited, and the time remaining clock inputs were inhibited, so that the clock digits could be used for displaying the keystrokes. In that mode, five digits would be lit instead of four. The remaining four keystrokes were the data/command inputs64. The display of the keystrokes represented an echo. If the sequence was correct, the astronaut pressed the enter key, or else he would restart the input process. Pressing the clear key brought back the digital clock. The rather limited nature of this command system indicates that it was intended for sparing use.
Besides the DAS, one other switch on the control panel related to the computer system. In the "Attitude Control" area of the panel was a three-position switch that controlled which computer was in actual use. It could be set for automatic (and usually was), in which case the redundancy management software would take care of matters. Alternately, the crew could purposely select either the primary or secondary computer. If either of these was selected, then automatic changeover was inhibited65. The switch gave the crew protection from....

Figure 3-5.
Figure 3-5. Dr. Edward Gibson at the Apollo Telescope Mount Control console. The interface to the digital computer is at lower left, on the panel immediately above the coil of cable. (NASA photo 4-60352)
[81] .....failure of the redundancy management software. Incidentally, the switch was not a common three-position toggle switch but, instead, required the crew to pull out and rotate the post. This protected the crew from accidental switching.

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