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Williams tube
The Williams tube, or the Williams–Kilburn tube named 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.
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 operates faster than earlier acoustic delay-line memory, at the speed of the electrons inside the vacuum tube, rather than at the speed of sound. The system was adversely affected by nearby electrical fields, and required frequent adjustment to remain operational. Williams–Kilburn tubes were used primarily on high-speed computer designs.
Williams and Kilburn applied for British patents on 11 December 1946, and 2 October 1947, followed by United States patent applications on 10 December 1947, and 16 May 1949.
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 illuminate. 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. The lifetime depends on the electrical resistance of the phosphor and the size of the well.
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 positively charged dot is erased (filling the charge well) by drawing a second dot immediately adjacent to the one to be erased (most systems did this by drawing a short dash starting at the dot position, the extension of the dash erased the charge initially stored at the starting point). This works because the negative halo around the second dot will fill in the positive center of the first dot. A collection of dots or spaces, often one horizontal row on the display, represents a computer word. Increasing beam energy makes the dots bigger and last longer, but requires them to be further apart, since otherwise, nearby dots erase each other.
The beam energy has to be large enough to produce dots with a usable lifetime. This places an upper limit on the memory density, and each Williams tube could typically store about 256 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 memory address as an X and Y pair into the driver circuitry and then trigger a time base generator to sweep the selected locations, reading from or writing to the internal registers, normally implemented with flip-flops.
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 first 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 sheet, indicating a "0".
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Williams tube
The Williams tube, or the Williams–Kilburn tube named 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.
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 operates faster than earlier acoustic delay-line memory, at the speed of the electrons inside the vacuum tube, rather than at the speed of sound. The system was adversely affected by nearby electrical fields, and required frequent adjustment to remain operational. Williams–Kilburn tubes were used primarily on high-speed computer designs.
Williams and Kilburn applied for British patents on 11 December 1946, and 2 October 1947, followed by United States patent applications on 10 December 1947, and 16 May 1949.
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 illuminate. 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. The lifetime depends on the electrical resistance of the phosphor and the size of the well.
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 positively charged dot is erased (filling the charge well) by drawing a second dot immediately adjacent to the one to be erased (most systems did this by drawing a short dash starting at the dot position, the extension of the dash erased the charge initially stored at the starting point). This works because the negative halo around the second dot will fill in the positive center of the first dot. A collection of dots or spaces, often one horizontal row on the display, represents a computer word. Increasing beam energy makes the dots bigger and last longer, but requires them to be further apart, since otherwise, nearby dots erase each other.
The beam energy has to be large enough to produce dots with a usable lifetime. This places an upper limit on the memory density, and each Williams tube could typically store about 256 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 memory address as an X and Y pair into the driver circuitry and then trigger a time base generator to sweep the selected locations, reading from or writing to the internal registers, normally implemented with flip-flops.
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 first 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 sheet, indicating a "0".
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