|Computer memory types|
|Early stage NVRAM|
The Selectron was an early form of digital computer memory developed by Jan A. Rajchman and his group at the Radio Corporation of America (RCA) under the direction of Vladimir K. Zworykin. It was a vacuum tube that stored digital data as electrostatic charges using technology similar to the Williams tube storage device. The team was never able to produce a commercially viable form of Selectron before magnetic-core memory became almost universal, and it remains practically unknown today.
In computing, memory refers to the computer hardware integrated circuits that store information for immediate use in a computer; it is synonymous with the term "primary storage". Computer memory operates at a high speed, for example random-access memory (RAM), as a distinction from storage that provides slow-to-access information but offers higher capacities. If needed, contents of the computer memory can be transferred to secondary storage; a very common way of doing this is through a memory management technique called "virtual memory". An archaic synonym for memory is store.
Jan Aleksander Rajchman was a Polish electrical engineer and computer pioneer.
The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a wholly owned subsidiary of General Electric (GE); however, in 1932, RCA became an independent company after GE was required to divest its ownership as part of the settlement of a government antitrust suit.
Development of Selectron started in 1946 at the behest of John von Neumann of the Institute for Advanced Study,who was in the midst of designing the IAS machine and was looking for a new form of high-speed memory.
John von Neumann was a Hungarian-American mathematician, physicist, computer scientist, and polymath. Von Neumann was generally regarded as the foremost mathematician of his time and said to be "the last representative of the great mathematicians"; a genius who was comfortable integrating both pure and applied sciences.
The Institute for Advanced Study (IAS) in Princeton, New Jersey, in the United States, is an independent, postdoctoral research center for theoretical research and intellectual inquiry founded in 1930 by American educator Abraham Flexner, together with philanthropists Louis Bamberger and Caroline Bamberger Fuld.
The IAS machine was the first electronic computer to be built at the Institute for Advanced Study (IAS) in Princeton, New Jersey. It is sometimes called the von Neumann machine, since the paper describing its design was edited by John von Neumann, a mathematics professor at both Princeton University and IAS. The computer was built from late 1945 until 1951 under his direction. The general organization is called Von Neumann architecture, even though it was both conceived and implemented by others. The computer is in the collection of the Smithsonian National Museum of American History but is not currently on display.
Their original design concept had a capacity of 4096 bits, with a planned production of 200 by the end of 1946. They found the device to be much more difficult to build than expected, and they were still not available by the middle of 1948. As development dragged on, the IAS machine was forced to switch to Williams tubes for storage, and the primary customer for Selectron disappeared.
RCA continued work on the concept, re-designing it for a smaller 256-bit capacity. The 256-bit Selectron was projected to cost about $500 each when in full production. While they were more reliable and faster than the Williams tube, the cost and the lack of availability meant they were used only in one computer: the RAND Corporation's JOHNNIAC.
The JOHNNIAC was an early computer built by the RAND Corporation that was based on the von Neumann architecture that had been pioneered on the IAS machine. It was named in honor of von Neumann, short for John v. NeumannNumerical Integrator and Automatic Computer. JOHNNIAC is arguably the longest-lived early computer, being used almost continuously from 1953 for over 13 years before finally being shut down on February 11, 1966, logging over 50,000 operational hours.
Both the Selectron and the Williams tube were superseded in the market by the compact and cost-effective magnetic-core memory, in the early 1950s.
The Williams tube was an example of a general class of cathode ray tube (CRT) devices known as storage tubes.
Storage tubes are a class of cathode-ray tubes (CRTs) that are designed to hold an image for a long period of time, typically as long as power is supplied to the tube.
The primary function of a conventional CRT is to display an image by lighting phosphor using a beam of electrons fired at it from an electron gun at the back of the tube. The target point of the beam is steered around the front of the tube though the use of deflection magnets or electrostatic plates.
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness, and fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphorescent materials are known for their use in radar screens and glow-in-the-dark materials, whereas fluorescent materials are common in cathode ray tube (CRT) and plasma video display screens, fluorescent lights, sensors, and white LEDs.
The electron is a subatomic particle, symbol
, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, ħ. As it is a fermion, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light. The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy.
An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode ray tubes (CRTs), used in nearly all television sets, computer displays and oscilloscopes that are not flat-panel displays. They are also used in field emission displays (FEDs), which are essentially flat-panel displays made out of rows of extremely small cathode ray tubes. They are also used in microwave linear beam vacuum tubes such as klystrons, inductive output tubes, travelling wave tubes, and gyrotrons, as well as in scientific instruments such as electron microscopes and particle accelerators. Electron guns may be classified by the type of electric field generation, by emission mechanism, by focusing, or by the number of electrodes.
Storage tubes were based on CRTs, sometimes unmodified. They relied on two normally undesirable principles of phosphor used in the tubes. One was that when electrons from the CRT's electron gun struck the phosphor to light it, some of the electrons "stuck" to the tube and caused a localized static electric charge to build up. The second was that the phosphor, like many materials, also released new electrons when struck by an electron beam, a process known as secondary emission.
Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it is able to move away by means of an electric current or electrical discharge. Static electricity is named in contrast with current electricity, which flows through wires or other conductors and transmits energy.
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and image intensifier tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.
Secondary emission had the useful feature that the rate of electron release was significantly non-linear. When a voltage was applied that crossed a certain threshold, the rate of emission increased dramatically. This caused the lit spot to rapidly decay, which also caused any stuck electrons to be released as well. Visual systems used this process to erase the display, causing any stored pattern to rapidly fade. For computer uses it was the rapid release of the stuck charge that allowed it to be used for storage.
In the Williams tube, the electron gun at the back of an otherwise typical CRT is used to deposit a series of small patterns representing a 1 or 0 on the phosphor in a grid representing memory locations. To read the display, the beam scanned the tube again, this time set to a voltage very close to that of the secondary emission threshold. The patterns were selected to bias the tube very slightly positive or negative. When the stored static electricity was added to the voltage of the beam, the total voltage either crossed the secondary emission threshold or didn't. If it crossed the threshold, a burst of electrons was released as the dot decayed. This burst was read capacitively on a metal plate placed just in front of the display side of the tube.
There were four general classes of storage tubes; the "surface redistribution type" represented by the Williams tube, the "barrier grid" system, which was unsuccessfully commercialized by RCA as the Radechon tube, the "sticking potential" type which was not used commercially, and the "holding beam" concept, of which the Selectron is a specific example.
In the most basic implementation, the holding beam tube uses three electron guns; one for writing, one for reading, and a third "holding gun" that maintains the pattern. The general operation is very similar to the Williams tube in concept. The main difference was the holding gun, which fired continually and unfocussed so it covered the entire storage area on the phosphor. This caused the phosphor to be continually charged to a selected voltage, somewhat below that of the secondary emission threshold.
Writing was accomplished by firing the writing gun at low voltage in a fashion similar to the Williams tube, adding a further voltage to the phosphor. Thus the storage pattern was the slight difference between two voltages stored on the tube, typically only a few tens of volts different.In comparison, the Williams tube used much higher voltages, producing a pattern that could only be stored for a short period before it decayed below readability.
Reading was accomplished by scanning the reading gun across the storage area. This gun was set to a voltage that would cross the secondary emission threshold for the entire display. If the scanned area held the holding gun potential a certain number of electrons would be released, if it held the writing gun potential the number would be higher. The electrons were read on a grid of fine wires placed behind the display, making the system entirely self-contained - the Williams tube's read plate was in front of the tube, and required continual mechanical adjustment to work properly.The grid also had the advantage of breaking the display into spots without requiring the tight focus of the Williams system.
General operation was the same as the Williams system, but the holding concept had two major advantages. One was that it operated at much lower voltage differences and was thus able to safely store data for a longer period of time. The other was that the same deflection magnet drivers could be sent to several electron guns to produce a single larger device with no increase in complexity of the electronics.
The Selectron further modified the basic holding gun concept through the use of individual metal eyelets that were used to store additional charge in a more predictable and long-lasting fashion.
Unlike a CRT where the electron gun is a single point source consisting of a filament and single charged accelerator, in the Selectron the "gun" is a plate and the accelerator is a grid of wires (thus borrowing some design notes from the barrier-grid tube). Switching circuits allow voltages to be applied to the wires to turn them on or off. When the gun fires through the eyelets, it is slightly defocussed. Some of the electrons strike the eyelet and deposit a charge on it.
The original 4096-bit Selectron 10-inch-long (250 mm) by 3-inch-diameter (76 mm) vacuum tube configured as 1024 by 4 bits. It had an indirectly heated cathode running up the middle, surrounded by two separate sets of wires — one radial, one axial — forming a cylindrical grid array, and finally a dielectric storage material coating on the inside of four segments of an enclosing metal cylinder, called the signal plates. The bits were stored as discrete regions of charge on the smooth surfaces of the signal plates.was a
The two sets of orthogonal grid wires were normally "biased" slightly positive, so that the electrons from the cathode could flow through the grid and reach the dielectric. The continuous flow of electrons allowed the stored charge to be continuously regenerated by the secondary emission of electrons. To select a bit to be read from or written to, all but two adjacent wires on each of the two grids were biased negative, allowing current to flow to the dielectric at one location only.
Writing was accomplished by selecting a bit, as above, and then sending a pulse of potential, either positive or negative, to the signal plate. With a bit selected, electrons would be pulled onto (with a positive potential) or pushed from (negative potential) the dielectric. When the bias on the grid was dropped, the electrons were trapped on the dielectric as a spot of static electricity.
To read from the device, a bit location was selected and a pulse sent from the cathode. If the dielectric for that bit contained a charge, the electrons would be pushed off the dielectric and read as a brief pulse of current in the signal plate. No such pulse meant that the dielectric must not have held a charge.
The smaller capacity 256-bit (128 by 2 bits) "production" device [ citation needed ] so that the bit status could also be read by eye.was in a similar vacuum-tube envelope. It was built with two storage arrays of discrete "eyelets" on a rectangular plate, separated by a row of eight cathodes. The pin count was reduced from 44 for the 4096-bit device down to 31 pins and two coaxial signal output connectors. This version included visible green phosphors in each eyelet
The cathode-ray tube (CRT) is a vacuum tube that contains one or more electron guns and a phosphorescent screen, and is used to display images. It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. The images may represent electrical waveforms (oscilloscope), pictures, radar targets, or other phenomena. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material is not intended to have significant meaning to a visual observer.
In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory. It was the first random-access digital storage device, and was used successfully in several early computers.
A field-emission display (FED) is a flat panel display technology that uses large-area field electron emission sources to provide electrons that strike colored phosphor to produce a color image. In a general sense, an FED consists of a matrix of cathode ray tubes, each tube producing a single sub-pixel, grouped in threes to form red-green-blue (RGB) pixels. FEDs combine the advantages of CRTs, namely their high contrast levels and very fast response times, with the packaging advantages of LCD and other flat-panel technologies. They also offer the possibility of requiring less power, about half that of an LCD system.
Direct-view bistable storage tube (DVBST) was an acronym used by Tektronix to describe their line of storage tubes. These were cathode ray tubes (CRT) that stored information written to them using an analog technique inherent in the CRT and based upon the secondary emission of electrons from the phosphor screen itself. The resulting image was visible in the continuously glowing patterns on the face of the CRT.
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube, which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode consists of an evacuated glass envelope containing five electrodes in this order: a cathode heated by a filament, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.
The penetron, short for penetration tube, is a type of limited-color television used in some military applications. Unlike a conventional color television, the penetron produces a limited color gamut, typically two colors and their combination. Penetrons, and other military-only cathode ray tubes (CRTs), have been replaced by LCDs in modern designs.
Regenerative capacitor memory is a type of computer memory that uses the electrical property of capacitance to store the bits of data. Because the stored charge slowly leaks away, these memories must be periodically regenerated to prevent data loss.
Mellon optical memory was an early form of computer memory invented at the Mellon Institute in 1951. The device used a combination of photoemissive and phosphorescent materials to produce a "light loop" between two surfaces. The presence or lack of light, detected by a photocell, represented a one or zero. Although promising, the system was rendered obsolete with the introduction of magnetic-core memory in the early 1950s. It appears that the system was never used in production.
The Computron was an electron tube designed to perform the parallel addition and multiplication of digital numbers. It was conceived by Richard L. Snyder, Jr., Jan A. Rajchman, Paul Rudnick and the digital computer group at the laboratories of the Radio Corporation of America under the direction of Vladimir Zworykin. Development began in 1941 under contract OEM-sr-591 to Division 7 of the National Defense Research Committee of the United States Office of Research and Development.
The inductive output tube (IOT) or klystrode is a variety of linear-beam vacuum tube, similar to a klystron, used as a power amplifier for high frequency radio waves. It evolved in the 1980s to meet increasing efficiency requirements for high-power RF amplifiers in radio transmitters. The primary commercial use of IOTs is in UHF television transmitters, where they have mostly replaced klystrons because of their higher efficiencies and smaller size. IOTs are also used in particle accelerators. They are capable of producing power output up to about 30 kW continuous and 7 MW pulsed and gains of 20–23 dB at frequencies up to about a gigahertz.
The beam-index tube is a color television cathode ray tube (CRT) design, using phosphor stripes and active-feedback timing, rather than phosphor dots and a beam-shadowing mask as developed by RCA. Beam indexing offered much brighter pictures than shadow-mask CRTs, reducing power consumption, and as they used a single a electron gun rather than three, they were easier to build and keep in alignment.
This is a subdivision of the Oscilloscope article, discussing the various types and models of oscilloscopes in greater detail.
This article discusses the history and development of oscilloscope technology. The modern day digital oscilloscope grew out of multiple developments of analog oscilloscopes, which in turn grew out of the older oscillograph. The oscillograph started as a hand drawn chart which was later slightly automated. This then grew into galvanometer driven recorders and photographic recorders. Eventually, the cathode ray tube came along and displaced the oscillograph, eventually taking over the majority of the market when advancements such as triggers were added to them. With the lowering costs of digital circuitry, the digital oscilloscope has taken over the majority of the modern oscilloscope sales, with almost no makers offering analog cathode ray tube oscilloscopes. However, the oscillograph lives on to a degree in pen chart recorders for electrical signals.
Beam deflection tubes, sometimes known as sheet beam tubes, are vacuum tubes with an electron gun, a beam intensity control grid, a screen grid, sometimes a suppressor grid, and two electrostatic deflection electrodes on opposite sides of the electron beam, that can direct the rectangular beam to either of two anodes in the same plane. They can be used as two-quadrant, single-balanced mixers or (de)modulators with very linear qualities, their mode of operation similar to one-half of a Gilbert Cell, by applying an unbalanced signal f1 to the control grid and a balanced signal f2 to the deflection electrodes, then extracting the balanced mixing products f1 − f2 and f1 + f2 from the two anodes. Similar to a pentagrid converter, the cathode and the first two grids can be made into an oscillator. Two beam deflection tubes can be combined to form a double-balanced mixer.
A deflection yoke is a kind of magnetic lens, used in cathode ray tubes to scan the electron beam both vertically and horizontally over the whole screen.
|Wikimedia Commons has media related to Selectron tubes .|