Selectron
The Selectron is a special electron tube , which was developed beginning in 1946 as a volatile memory for tube computers such as ENIAC that were emerging in the middle of the 20th century . The development of this storage tube comes from Jan A. Rajchman , who was awarded the IEEE Edison Medal in 1974. The development was mainly carried out at the Radio Corporation of America (RCA), which also coined the Selectron brand name . The Selectron, like the Williams tube that was developed independently during the same period , never reached a market-ready state, since the storage tubes, which were difficult to produce in construction and prone to errors in use, were replaced by the more robust and technologically better core storage in the mid-1950s .
construction
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Selectron for 256 bits or 4096 bits
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Like all storage tubes , the Selectron is also based in principle on a cathode ray tube whose luminous layer is used to store information. The individual bits are written in individual points, which are arranged in lines and rows in the luminous layer, by an electron beam focused on the point. With a logic 1 a charge is introduced into the luminous layer, with a logic 0 this charge is missing. To read out the charge states, there are several thin metal wires in the area of the luminous layer between the storage points, which serve as readout wires. When reading out, an electron beam with a lower intensity is selected than when writing, so that the secondary emission of electrons in the area of the luminous layer results in voltage pulses of different levels on the readout wire, depending on whether an electrical charge was previously stored in the storage point of the luminous layer or not. Reading out a memory location is consuming, so the read value must be rewritten after a read process in order to maintain the memory status.
Furthermore, each memory cell must be continuously read out and rewritten, since the stored charge in the luminous layer can only be held for a certain period of time. This is similar to a refresh cycle for dynamic memory modules . In contrast to the Williams tube with short refresh times, a weak electron beam is applied continuously and unfocused over the entire luminous layer with the Selectron, which is significantly weaker than the electron beam for reading out, so as not to cause any secondary emission with the extinguishing effect in the luminous layer. As a result, accidental charge losses in the luminous layer are compensated to a certain extent and the necessary refresh cycles can be stretched over time. In addition, the Selectron can be operated with a lower operating voltage than the Williams tube, which requires a high voltage of a few kilovolts for operation.
The Selectron also has fundamental structural differences to the Williams tube. While the Williams tube still has the shape of a typical cathode ray tube, as it was also used in the first television sets to display images, the Selectron does not have any visual resemblance to a display tube. Due to the low operating voltage, the entire structure can be compactly accommodated in a glass bulb approx. 15 cm high and the electrode system is structured as shown in the illustration on the right. The indirectly heated cathode is located in the middle of the tube, above in the sectional view, followed by the focusing and control wires for the electron beams that go down. In the lower area of the picture, attached to the tube in the glass bulb in the outer area towards the glass tube, are the individual points of the phosphor-coated luminous layer, which serve as storage elements and represent the anode .
Web links
Individual evidence
- ↑ N. Metropolis et al. (Ed.): History of Computing in the Twentieth Century . Academic Press, 1980, ISBN 0-12-491650-3 , pp. 465 to 469 .
- ^ Jan Rajchman: The Selectron. (No longer available online.) RCA Laboratories, Princeton, archived from the original on July 7, 2003 ; Retrieved October 27, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
