Editing Skylab (section)

== Computers ==
[[File:Nasasoftwareforskylabcycle.jpg|thumb|left|Computational cycle of the Skylab computer program]]

Skylab was controlled in part by a digital computer system, and one of its main jobs was to control the pointing of the station; pointing was especially important for its solar power collection and observatory functions.<ref name="computer">{{cite web|last1=Jenkins|first1=Dennis|title=Advanced Vehicle Automation and Computers Aboard the Shuttle |url=https://history.nasa.gov/sts1/pages/computer.html |website=history.nasa.gov|publisher=NASA Printing and Design|access-date=December 31, 2017}} {{PD-notice}}</ref> The computer consisted of two actual computers, a primary and a secondary. The system ran several thousand words of code, which was also backed up on the Memory Load Unit (MLU).<ref name="computer"/> The two computers were linked to each other and various input and output items by the workshop computer interface.<ref name="IBM and Skylab">{{cite web|website=IBM Archives|title=IBM and Skylab |url=https://www-03.ibm.com/ibm/history/exhibits/space/space_skylab.html|archive-url=https://web.archive.org/web/20050119055757/http://www-03.ibm.com/ibm/history/exhibits/space/space_skylab.html|url-status=dead|archive-date=January 19, 2005|publisher=IBM|access-date=December 31, 2017 |date=January 23, 2003}}</ref> Operations could be switched from the primary to the backup, which were the same design, either automatically if errors were detected, by the Skylab crew, or from the ground.<ref name="computer"/>

The Skylab computer was a space-hardened and customized version of the TC-1 computer, a version of the [[IBM System/4 Pi]], itself based on the [[IBM System/360|System 360]] computer.<ref name="computer"/> The TC-1 had a 16,000-word memory based on ferrite memory cores, while the MLU was a read-only [[tape drive]] that contained a backup of the main computer programs.<ref name="computer"/> The tape drive would take 11 seconds to upload the backup of the software program to a main computer.<ref name="Hardware">{{cite web|work=Computers in Spaceflight: The NASA Experience |title=Chapter Three – The Skylab Computer System – Hardware |date=March 1988 |url=https://history.nasa.gov/computers/Ch3-2.html |access-date=December 31, 2017 |last1=Tomayko |first1=James E. }} {{PD-notice}}</ref> The TC-1 used 16-bit words and the [[Central processing unit|central processor]] came from the 4Pi computer.<ref name="Hardware"/> There was a 16k and an 8k version of the software program.<ref name="Software">{{cite web|work=Computers in Spaceflight: The NASA Experience |title=Chapter Three – The Skylab Computer System – Software |date=March 1988 |url=https://history.nasa.gov/computers/Ch3-3.html|access-date=December 31, 2017 |last1=Tomayko |first1=James E. }} {{PD-notice}}</ref>

The computer had a mass of 100 pounds (45.4&nbsp;kg), and consumed about ten percent of the station's [[Electric power|electrical power]].<ref name="computer"/><ref name="IBM and Skylab"/>
* Apollo Telescope Mount Digital Computer<ref name="Hardware"/>
* Attitude and Pointing Control System (APCS)<ref name="computer"/>
* Memory Load Unit (MLU).<ref name="computer"/>

After launch the computer is what the controllers on the ground communicated with to control the station's orientation.<ref name="Ch3-1">{{cite web |work=Computers in Spaceflight: The NASA Experience|title=Chapter Three – The Skylab Computer System |date=March 1988 |url=https://history.nasa.gov/computers/Ch3-1.html |access-date=17 November 2017 |last1=Tomayko |first1=James E. }} {{PD-notice}}</ref> When the sun-shield was torn off the ground staff had to balance solar heating with electrical production.<ref name="Ch3-1"/> On March 6, 1978, the computer system was re-activated by NASA to control the re-entry.<ref name="Ch3-5">{{cite web|work=Computers in Spaceflight: The NASA Experience |title=Chapter Three – The Skylab Computer System – The Reactivation Mission |date=March 1988 |url=https://history.nasa.gov/computers/Ch3-5.html|access-date=December 31, 2017 |last1=Tomayko |first1=James E. }} {{PD-notice}}</ref>

The system had a user interface that consisted of a display, ten buttons, and a three-position switch.<ref name="User Interfaces">{{cite web|work=Computers in Spaceflight: The NASA Experience |title=Chapter Three – The Skylab Computer System – User Interfaces |date=March 1988 |url=https://history.nasa.gov/computers/Ch3-4.html|access-date=December 31, 2017 |last1=Tomayko |first1=James E. }} {{PD-notice}}</ref> Because the numbers were in [[octal]] (base-8), it only had numbers zero to seven (8 keys), and the other two keys were enter and clear.<ref name="User Interfaces"/> The display could show minutes and seconds which would count down to orbital benchmarks, or it could display keystrokes when using the interface.<ref name="User Interfaces"/> The interface could be used to change the software program.<ref name="User Interfaces"/> The user interface was called the Digital Address System (DAS) and could send commands to the computer's command system. The command system could also get commands from the ground.<ref name="Software"/>

For personal computing needs Skylab crews were equipped with models of the then new hand-held electronic scientific calculator, which was used in place of slide-rules used on prior space missions as the primary personal computer. The model used was the [[HP-35|Hewlett Packard HP 35]].<ref>{{cite news |url=https://airandspace.si.edu/collection-objects/calculator-pocket-electronic-hp-35|title=Calculator, Pocket, Electronic, HP-35|date=March 14, 2016 |work=National Air and Space Museum|access-date=17 November 2017|archive-url=https://web.archive.org/web/20171118222420/https://airandspace.si.edu/collection-objects/calculator-pocket-electronic-hp-35 |archive-date=2017-11-18|url-status=dead}} [https://airandspace.si.edu/collection-objects/calculator-pocket-electronic-hp-35/nasm_A19850582000 (no picture)] {{PD-notice}}</ref> Some slide rules continued in use aboard Skylab, and a [[Slide rule|circular slide rule]] was at the workstation.<ref>{{cite web|title=Aerospace Related Slide Rules |url=http://www.sliderulemuseum.com/Aerospace.htm|website=sliderulemuseum.com|publisher=International Slide Rule Museum |access-date=December 31, 2017}}</ref>