Selectron tube

The Selectron is a special electron tube, which starting emerging as a volatile memory for the mid-20th century tube computers like ENIAC was developed in 1946. The development of these storage tube were designed by Jan A. Rajchman, which was in 1974 awarded the IEEE Edison Medal. The development was carried out essentially at the company Radio Corporation of America (RCA ), which coined the brand name Selectron. The Selectron, as well as the independent in the same period resulting Williams tube, never reached a marketable condition because the were replaced difficult in construction to manufacturing and error-prone in use storage tubes mid-1950s by the more robust and technologically better core memory.

Construction

As all the storage tubes is also based, in principle, Selectron on a cathode ray tube whose luminescent layer is the information storage. The single bits are written into the individual dots, which are arranged in rows and columns in the Leuchschicht by a focused electron beam to the point. Here, a charge in the light-emitting layer placed at a logic -1, at a logic 0 is missing this charge. To read out the charge states are more thin metal wires that serve as readout wire in the luminescent layer between the storage points. During read-out, an electron beam of lower intensity than the writing is selected, so that different levels of voltage pulses result from the secondary emission of electrons in the area of the luminescent layer on the read-out wire, depending on whether the memory point of the luminescent layer before an electric charge has been stored or not. The reading of a memory location is a consuming, so it must be rewritten for the preservation of the memory state after a read operation, the read value.

Each memory cell must further be continuously read out and rewritten, since the charge stored in the light-emitting layer, only a certain period of time can be maintained. This is similar to a refresh cycle for dynamic memory devices. In contrast to Williams tube short refresh times, a weak electron beam is applied at the Selectron kontunierlich and unfocused on the entire light-emitting layer, which is considerably weaker than the electron beam to read, not to cause secondary emission of the quenching effect of the light emitting layer. This random charge losses of the luminescent layer can be compensated to a certain degree and it can be the necessary refresh cycles stretch over time. In addition, the Selectron with a lower operating voltage can be operated when the Williams tube that requires a high voltage of several kilovolts for the operation.

The Selectron has also fundamental differences in design for 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 for image display, no resemblance to a display tube is visually recognizable in the Selectron. Due to the low operating voltage of the complete structure is compact in a glass flask of about 15 cm in height and place the electrode system is constructed as shown in the figure shown on the right. In the middle of the tube, in the cross-sectional view at the top, is the indirectly heated cathode, followed by the continuous downward focusing and control wires for the electron beams. Mounted on the bottom of the screen, with the tube in the glass flask in the outer region to the glass tube out are the individual points of the phosphor-coated light emitting layer, which serve as storage elements and represent the anode.