Williams tube

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Williams-Kilburn tube from an IBM 701 at the Computer History Museum, in Mountain View, California Williams tube.agr.jpg
Williams–Kilburn tube from an IBM 701 at the Computer History Museum, in Mountain View, California
Memory pattern on SWAC Williams tube CRT SWAC 003.jpg
Memory pattern on SWAC Williams tube CRT

The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory. [1] [2] It was the first random-access digital storage device, and was used successfully in several early computers. [3]

Sir Frederic Calland Williams,, known as F.C. Williams or Freddie Williams, was an English engineer, a pioneer in radar and computer technology.

Tom Kilburn British electrical engineer

Tom Kilburn was an English mathematician and computer scientist. Over the course of a productive 30-year career, he was involved in the development of five computers of great historical significance. With Freddie Williams he worked on the Williams–Kilburn tube and the world's first electronic stored-program computer, the Manchester Baby, while working at the University of Manchester. His work propelled Manchester and Britain into the forefront of the emerging field of computer science.

Computer memory physical device used to store information for immediate use in a digital electronic device

In computing, memory refers to a device that is used to store information for immediate use in a computer or related computer hardware device. It typically refers to semiconductor memory, specifically metal-oxide-semiconductor (MOS) memory, where data is stored within MOSFET memory cells on a silicon integrated circuit chip. The term "memory" is often 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.

Contents

The Williams tube works by displaying a grid of dots on a cathode ray tube (CRT). Due to the way CRTs work, this creates a small charge of static electricity over each dot. The charge at the location of each of the dots is read by a thin metal sheet just in front of the display. Since the display faded over time, it was periodically refreshed. It cycles faster than earlier acoustic delay line memory, at the speed of the electrons inside the vacuum tube, rather than at the speed of sound. However, the system was adversely affected by any nearby electrical fields, and required constant alignment to keep operational. Williams–Kilburn tubes were used primarily on high-speed computer designs.

Static electricity imbalance of electric charges within or on the surface of a material

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.

Delay line memory is a form of computer memory, now obsolete, that was used on some of the earliest digital computers. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory, but as opposed to modern random-access memory, delay line memory was sequential-access.

The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 metres per second, or a kilometre in 2.9 s or a mile in 4.7 s. It depends strongly on temperature, but also varies by several metres per second, depending on which gases exist in the medium through which a soundwave is propagating.

Williams and Kilburn applied for British patents on 11 December 1946, [4] and 2 October 1947, [5] followed by United States patent applications on 10 December 1947, [6] and 16 May 1949. [7]

Working principle

The Williams tube depends on an effect called secondary emission that occurs on cathode ray tubes (CRTs). When the electron beam strikes the phosphor that forms the display surface, it normally causes it to light up; however, if the beam energy is above a given threshold (depending on the phosphor mix) it also causes electrons to be struck out of the phosphor. These electrons travel a short distance before being attracted back to the CRT surface and falling on it a short distance away. The overall effect is to cause a slight positive charge in the immediate region of the beam where there is a deficit of electrons, and a slight negative charge around the dot where those electrons land. The resulting charge well remains on the surface of the tube for a fraction of a second while the electrons flow back to their original locations. [1] The lifetime depends on the electrical resistance of the phosphor and the size of the well.

Secondary emission a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles

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.

Phosphor substance exhibiting luminescence

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.

Electron subatomic particle with negative electric charge

The electron is a subatomic particle, symbol
e
or
β
, 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. Being fermions, 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.

The process of creating the charge well is used as the write operation in a computer memory, storing a single binary digit, or bit. A collection of dots or spaces, often one horizontal row on the display, represents a computer word. There is a relationship between the size and spacing of the dots and their lifetime, as well as the ability to reject crosstalk with adjacent dots. This places an upper limit on the memory density, and each Williams tube could typically store about 1024 to 2560 bits of data. Because the electron beam is essentially inertia-free and can be moved anywhere on the display, the computer can access any location, making it a random access memory. Typically, the computer would load the address as an X and Y pair into the driver circuitry and then trigger a time base generator that would sweep the selected locations, reading from or writing to the internal registers, normally implemented as flip-flops.

The bit is a basic unit of information in information theory, computing, and digital communications. The name is a portmanteau of binary digit.

Areal density is a measure of the quantity of information bits that can be stored on a given length of track, area of surface, or in a given volume of a computer storage medium. Generally, higher density is more desirable, for it allows more data to be stored in the same physical space. Density therefore has a direct relationship to storage capacity of a given medium. Density also generally affects the performance within a particular medium, as well as price.

A time base generators, or timebase, is a special type of function generator, an electronic circuit that generates a varying voltage to produce a particular waveform. Time base generators produce very high frequency sawtooth waves specifically designed to deflect the beam in cathode ray tube (CRT) smoothly across the face of the tube and then return it to its starting position.

Reading the memory took place via a secondary effect caused by the writing operation. During the short period when the write takes place, the redistribution of charges in the phosphor creates an electrical current that induces voltage in any nearby conductors. This is read by placing a thin metal sheet just in front of the display side of the CRT. During a read operation, the beam writes to the selected bit locations on the display. Those locations that were previously written to are already depleted of electrons, so no current flows, and no voltage appears on the plate. This allows the computer to determine there was a "1" in that location. If the location had not been written to previously, the write process will create a well and a pulse will be read on the plate, indicating a "0". [1]

Reading a memory location creates a charge well whether or not one was previously there, destroying the original contents of that location, and so any read has to be followed by a write to reinstate the original data. In some systems this was accomplished using a second electron gun inside the CRT that could write to one location while the other was reading the next. Since the display would fade over time, the entire display had to be periodically refreshed using the same basic method. However, as the data is read and then immediately written, this operation can be carried out by external circuitry while the central processing unit (CPU) was busy carrying out other operations. This refresh operation is similar to the memory refresh cycles of DRAM in modern systems.

Since the refresh process caused the same pattern to continually reappear on the display, there was a need to be able to erase previously written values. This was normally accomplished by writing to the display just beside the original location. The electrons released by this new write would fall into the previously written well, filling it back in. The original systems produced this effect by writing a small dash, which was easy to accomplish without changing the master timers and simply producing the write current for a slightly longer period. The resulting pattern was a series of dots and dashes. There was a considerable amount of research on more effective erasing systems, with some systems using out-of-focus beams or complex patterns.

Some Williams tubes were made from radar-type cathode ray tubes with a phosphor coating that made the data visible, while other tubes were purpose-built without such a coating. The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators, since the face of the tube was covered by the pickup plate. If a visible output was needed, a second tube connected in parallel with the storage tube, with a phosphor coating, but without a pickup plate, was used as a display device.

Development

Developed at the University of Manchester in England, it provided the medium on which the first electronically stored-memory program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June 1948. [8] In fact, rather than the Williams tube memory being designed for the Baby, the Baby was a testbed to demonstrate the reliability of the memory. [9] [10] Tom Kilburn wrote a 17-line program to calculate the highest proper factor of 218. Tradition at the university has it that this was the only program Kilburn ever wrote. [11]

Williams tubes tended to become unreliable with age, and most working installations had to be "tuned" by hand. By contrast, mercury delay line memory was slower and not truly random access, as the bits were presented serially, which complicated programming. Delay lines also needed hand tuning, but did not age as badly and enjoyed some success in early digital electronic computing despite their data rate, weight, cost, thermal and toxicity problems. However, the Manchester Mark 1, which used Williams tubes, was successfully commercialised as the Ferranti Mark 1. Some early computers in the United States also used Williams tubes, including the IAS machine (originally designed for Selectron tube memory), the UNIVAC 1103, IBM 701, IBM 702 and the Standards Western Automatic Computer (SWAC). Williams tubes were also used in the Soviet Strela-1 and in the Japan TAC (Tokyo Automatic Computer). [12]

See also

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References

Notes

  1. 1 2 3 Kilburn, Tom (1990), "From Cathode Ray Tube to Ferranti Mark I", Resurrection, The Computer Conservation Society, 1 (2), ISSN   0958-7403 , retrieved 15 March 2012
  2. Brian Napper (25 November 1998). "Williams Tube". University of Manchester. Retrieved 1 October 2016.
  3. "Early computers at Manchester University", Resurrection, The Computer Conservation Society, 1 (4), Summer 1992, ISSN   0958-7403 , retrieved 7 July 2010
  4. GB Patent 645,691
  5. GB Patent 657,591
  6. U.S. Patent 2,951,176
  7. U.S. Patent 2,777,971
  8. Napper, Brian, Computer 50: The University of Manchester Celebrates the Birth of the Modern Computer, archived from the original on 4 May 2012, retrieved 26 May 2012
  9. Williams, F.C.; Kilburn, T. (Sep 1948), "Electronic Digital Computers", Nature, 162 (4117): 487, doi:10.1038/162487a0. Reprinted in The Origins of Digital Computers
  10. Williams, F.C.; Kilburn, T.; Tootill, G.C. (Feb 1951), "Universal High-Speed Digital Computers: A Small-Scale Experimental Machine", Proc. IEE, 98 (61): 13–28, doi:10.1049/pi-2.1951.0004.
  11. Lavington 1998 , p. 11
  12. Research, United States Office of Naval (1953). A survey of automatic digital computers. Office of Naval Research, Dept. of the Navy. p. 87.

Bibliography

Further reading