The flying-spot store was an optical digital memory used in early
stored program control
Stored program control (SPC) is a telecommunications technology for telephone exchanges. Its characteristic is that the switching system is controlled by a computer program stored in a memory in the switching system. SPC was the enabling technolog ...
components of
electronic switching systems.
The flying-spot store used a
photographic plate
Photographic plates preceded photographic film as a capture medium in photography, and were still used in some communities up until the late 20th century. The light-sensitive emulsion of silver salts was coated on a glass plate, typically thinn ...
as the store of binary data. Each ''spot'' on the plate was an opaque (logical 0) or transparent (logical 1) area that stored one
bit. Spots were read and written with an optical mechanism with an access time of ca. one microsecond. The optical system consisted of a
cathode ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...
that functioned as a
flying-spot scanner, the source of light beams produced on its face and focused onto the photographic plates with a system of lenses. A
photomultiplier detected the light beam. The light beam could be split into several components to read multiple plates simultaneously, permitting the formation of a group of bits for each location.
A flying-spot store was the main permanent program and data memory for the first electronic central office installed on a trial basis in
Morris, Illinois in 1960. The memory was organized as a set of nine photographic plates and permitted reading two spots per memory address, yielding a ''word'' size of 18 bits. For temporary data, the system used a barrier-grid electrostatic
storage tube.
See also
*
1ESS switch
The Number One Electronic Switching System (1ESS) was the first large-scale stored program control (SPC) telephone exchange or electronic switching system in the Bell System. It was manufactured by Western Electric and first placed into servi ...
References
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category:Computer memory
Cathode ray tube