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The SC4020 was a valve-based system that generated output based on a set of commands that it received on an IBM 7-track magnetic tape. Probably the major use of the two IBM decks on Atlas was the generation of output tapes destined for the SC4020.
In this picture, the operator is mounting a magnetic tape for processing. In operation, the second bay would be closed. In the photograph, the charactron tube is rotated forward in its maintenance position. Normally it would be flush underneath the camera above.
SC4020 commands included:
- Graphical commands to draw lines or plot characters on the tube face of the charactron tube.
- Above the tube (housed in the second bay from the left) wasa semi-silvered mirror that split the beam so that half deflected to the hardcopy photographic paper camera seen at the left and the other half going straight up to the 16mm or 35 mm camera situated above the tube.
- In normal operation, the camera shutter was open all the time. The image on the charactron tube was not refreshed but was visible for sufficient time to generate an image on the film. In consequence, there is to limit on the amount of information that can be stored on a frame of film apart from photographic ones.
- Commands on the tape advanced the film in either camera and could also back up the magnetic tape and interpret a sequence again.
- A plate could be placed between the tube and the camera so that standard backgrounds could be added at no cost.