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5,701 | In 1978, Key Tronic Corporation introduced keyboards with capacitive-based switches, one of the first keyboard technologies not to use self-contained switches. There was simply a sponge pad with a conductive-coated Mylar plastic sheet on the switch plunger, and two half-moon trace patterns on the printed circuit board ... |
5,702 | Meanwhile, IBM made their own keyboards, using their own patented technology: Keys on older IBM keyboards were made with a "buckling spring" mechanism, in which a coil spring under the key buckles under pressure from the user's finger, triggering a hammer that presses two plastic sheets with conductive traces together... |
5,703 | The first electronic keyboards had a typewriter key travel distance of 0.187 inches , keytops were a half-inch high, and keyboards were about two inches thick. Over time, less key travel was accepted in the market, finally landing on 0.110 inches . Coincident with this, Key Tronic was the first company to introduce a... |
5,704 | Keytops are an important element of keyboards. In the beginning, keyboard keytops had a "dish shape" on top, like typewriters before them. Keyboard key legends must be extremely durable over tens of millions of depressions, since they are subjected to extreme mechanical wear from fingers and fingernails, and subject to... |
5,705 | Initially, sublimation printing, where a special ink is printed onto the keycap surface and the application of heat causes the ink molecules to penetrate and commingle with the plastic modules, had a problem because finger oils caused the molecules to disperse, but then a necessarily very hard clear coating was applied... |
5,706 | Three final mechanical technologies brought keyboards to where they are today, driving the cost well under $10: |
5,707 | "Monoblock" keyboard designs were developed where individual switch housings were eliminated and a one-piece "monoblock" housing used instead. This was possible because of molding techniques that could provide very tight tolerances for the switch-plunger holes and guides across the width of the keyboard so that the key... |
5,708 | The use of contact-switch membrane sheets under the monoblock. This technology came from flat-panel switch membranes, where the switch contacts are printed inside of a top and bottom layer, with a spacer layer in between, so that when pressure is applied to the area above, a direct electrical contact is made. The memb... |
5,709 | Plastic materials played a very important part in the development and progress of electronic keyboards. Until "monoblocks" came along, GE's "self-lubricating" Delrin was the only plastic material for keyboard switch plungers that could withstand the beating over tens of millions of cycles of lifetime use. Greasing or o... |
5,710 | In common use, the term "mechanical keyboard" refers to a keyboard with individual mechanical key switches, each of which contains a fully encased plunger with a spring below it and metallic electrical contacts on a side. The plunger sits on the spring and the key will often close the contacts when the plunger is press... |
5,711 | Computer keyboards include control circuitry to convert key presses into key codes that the computer's electronics can understand. The key switches are connected via the printed circuit board in an electrical X-Y matrix where a voltage is provided sequentially to the Y lines and, when a key is depressed, detected sequ... |
5,712 | The first computer keyboards were for mainframe computer data terminals and used discrete electronic parts. The first keyboard microprocessor was introduced in 1972 by General Instruments, but keyboards have been using the single-chip 8048 microcontroller variant since it became available in 1978. The keyboard switch m... |
5,713 | One test for whether the computer has crashed is pressing the caps lock key. The keyboard sends the key code to the keyboard driver running in the main computer; if the main computer is operating, it commands the light to turn on. All the other indicator lights work in a similar way. The keyboard driver also tracks the... |
5,714 | Some lower-quality keyboards have multiple or false key entries due to inadequate electrical designs. These are caused by inadequate keyswitch "debouncing" or inadequate keyswitch matrix layout that don't allow multiple keys to be depressed at the same time, both circumstances which are explained below: |
5,715 | When pressing a keyboard key, the key contacts may "bounce" against each other for several milliseconds before they settle into firm contact. When released, they bounce some more until they revert to the uncontacted state. If the computer were watching for each pulse, it would see many keystrokes for what the user thou... |
5,716 | Some low-quality keyboards also suffer problems with rollover . Early "solid-state" keyswitch keyboards did not have this problem because the keyswitches are electrically isolated from each other, and early "direct-contact" keyswitch keyboards avoided this problem by having isolation diodes for every keyswitch. These e... |
5,717 | As direct-contact membrane keyboards became popular, the available rollover of keys was optimized by analyzing the most common key sequences and placing these keys so that they do not potentially produce phantom keys in the electrical key matrix , so that blocking a third key usually isn't a problem. But lower-quality ... |
5,718 | There are several ways of connecting a keyboard to a system unit using cables, including the standard AT connector commonly found on motherboards, which was eventually replaced by the PS/2 and the USB connection. Prior to the iMac line of systems, Apple used the proprietary Apple Desktop Bus for its keyboard connector... |
5,719 | Wireless keyboards have become popular. A wireless keyboard must have a transmitter built in, and a receiver connected to the computer's keyboard port; it communicates either by radio frequency or infrared signals. A wireless keyboard may use industry standard Bluetooth radio communication, in which case the receive... |
5,720 | Optical character recognition is preferable to rekeying for converting existing text that is already written down but not in machine-readable format . In other words, to convert the text from an image to editable text , a person could re-type it, or a computer could look at the image and deduce what each character is.... |
5,721 | Speech recognition converts speech into machine-readable text . This technology has also reached an advanced state and is implemented in various software products. For certain uses speech recognition is starting to replace the keyboard. However, the lack of privacy when issuing voice commands and dictation makes this... |
5,722 | Pointing devices can be used to enter text or characters in contexts where using a physical keyboard would be inappropriate or impossible. These accessories typically present characters on a display, in a layout that provides fast access to the more frequently used characters or character combinations. Popular examples... |
5,723 | Unencrypted wireless Bluetooth keyboards are known to be vulnerable to signal theft by placing a covert listening device in the same room as the keyboard to sniff and record Bluetooth packets for the purpose of logging keys typed by the user. Microsoft wireless keyboards 2011 and earlier are documented to have this vul... |
5,724 | Keystroke logging is a method of capturing and recording user keystrokes. While it is used legally to measure employee productivity on certain clerical tasks, or by law enforcement agencies to find out about illegal activities, it is also used by hackers for various illegal or malicious acts. Hackers use keyloggers as... |
5,725 | Keystroke logging can be achieved by both hardware and software means. Hardware key loggers are attached to the keyboard cable or installed inside standard keyboards. Software keyloggers work on the target computer's operating system and gain unauthorized access to the hardware, hook into the keyboard with functions pr... |
5,726 | Anti-spyware applications are able to detect many keyloggers and cleanse them. Responsible vendors of monitoring software support detection by anti-spyware programs, thus preventing abuse of the software. Enabling a firewall does not stop keyloggers per se, but can possibly prevent transmission of the logged material o... |
5,727 | Keyboards are also known to emit electromagnetic signatures that can be detected using special spying equipment to reconstruct the keys pressed on the keyboard. Neal O'Farrell, executive director of the Identity Theft Council, revealed to InformationWeek that "More than 25 years ago, a couple of former spooks showed me... |
5,728 | The use of any keyboard may cause serious injury to hands, wrists, arms, neck or back. The risks of injuries can be reduced by taking frequent short breaks to get up and walk around a couple of times every hour. As well, users should vary tasks throughout the day, to avoid overuse of the hands and wrists. When inputti... |
5,729 | Some adaptive technology ranging from special keyboards, mouse replacements and pen tablet interfaces to speech recognition software can reduce the risk of injury. Pause software reminds the user to pause frequently. Switching to a much more ergonomic mouse, such as a vertical mouse or joystick mouse may provide relief... |
5,730 | By using a touchpad or a stylus pen with a graphic tablet, in place of a mouse, one can lessen the repetitive strain on the arms and hands. |
5,731 | A logic gate is a device that performs a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has, for instance, zero rise time and unlimited fan-out, or it may refer to a non-idea... |
5,732 | The primary way of building logic gates uses diodes or transistors acting as electronic switches. Today, most logic gates are made from MOSFETs . They can also be constructed using vacuum tubes, electromagnetic relays with relay logic, fluidic logic, pneumatic logic, optics, molecules, acoustics, or even mechanical or ... |
5,733 | Logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic. Logic circuits include such devices as multiplexers, registers, arithm... |
5,734 | Compound logic gates AND-OR-Invert and OR-AND-Invert are often employed in circuit design because their construction using MOSFETs is simpler and more efficient than the sum of the individual gates. |
5,735 | The binary number system was refined by Gottfried Wilhelm Leibniz , influenced by the ancient I Ching's binary system. Leibniz established that using the binary system combined the principles of arithmetic and logic. |
5,736 | In an 1886 letter, Charles Sanders Peirce described how logical operations could be carried out by electrical switching circuits. Early electro-mechanical computers were constructed from switches and relay logic rather than the later innovations of vacuum tubes or transistors . Ludwig Wittgenstein introduced a version... |
5,737 | From 1934 to 1936, NEC engineer Akira Nakashima, Claude Shannon and Victor Shestakov introduced switching circuit theory in a series of papers showing that two-valued Boolean algebra, which they discovered independently, can describe the operation of switching circuits. Using this property of electrical switches to imp... |
5,738 | Metal–oxide–semiconductor devices in the forms of PMOS and NMOS were demonstrated by Bell Labs engineers Mohamed M. Atalla and Dawon Kahng in 1960. Both types were later combined and adapted into complementary MOS logic by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963. |
5,739 | There are two sets of symbols for elementary logic gates in common use, both defined in ANSI/IEEE Std 91-1984 and its supplement ANSI/IEEE Std 91a-1991. The "distinctive shape" set, based on traditional schematics, is used for simple drawings and derives from United States Military Standard MIL-STD-806 of the 1950s and... |
5,740 | The mutual goal of IEEE Std 91-1984 and IEC 617-12 was to provide a uniform method of describing the complex logic functions of digital circuits with schematic symbols. These functions were more complex than simple AND and OR gates. They could be medium-scale circuits such as a 4-bit counter to a large-scale circuit su... |
5,741 | IEC 617-12 and its renumbered successor IEC 60617-12 do not explicitly show the "distinctive shape" symbols, but do not prohibit them. These are, however, shown in ANSI/IEEE Std 91 with this note: "The distinctive-shape symbol is, according to IEC Publication 617, Part 12, not preferred, but is not considered to be in... |
5,742 | In the 1980s, schematics were the predominant method to design both circuit boards and custom ICs known as gate arrays. Today custom ICs and the field-programmable gate array are typically designed with Hardware Description Languages such as Verilog or VHDL. |
5,743 | By use of De Morgan's laws, an AND function is identical to an OR function with negated inputs and outputs. Likewise, an OR function is identical to an AND function with negated inputs and outputs. A NAND gate is equivalent to an OR gate with negated inputs, and a NOR gate is equivalent to an AND gate with negated inpu... |
5,744 | This leads to an alternative set of symbols for basic gates that use the opposite core symbol but with the inputs and outputs negated. Use of these alternative symbols can make logic circuit diagrams much clearer and help to show accidental connection of an active high output to an active low input or vice versa. Any ... |
5,745 | A De Morgan symbol can show more clearly a gate's primary logical purpose and the polarity of its nodes that are considered in the "signaled" state. Consider the simplified case where a two-input NAND gate is used to drive a motor when either of its inputs are brought low by a switch. The "signaled" state occurs when... |
5,746 | De Morgan's theorem is most commonly used to implement logic gates as combinations of only NAND gates, or as combinations of only NOR gates, for economic reasons. |
5,747 | Output comparison of various logic gates: |
5,748 | Charles Sanders Peirce showed that NOR gates alone can be used to reproduce the functions of all the other logic gates, but his work on it was unpublished until 1933. The first published proof was by Henry M. Sheffer in 1913, so the NAND logical operation is sometimes called Sheffer stroke; the logical NOR is sometim... |
5,749 | Logic gates can also be used to hold a state, allowing data storage. A storage element can be constructed by connecting several gates in a "latch" circuit. Latching circuitry is used in static random-access memory. More complicated designs that use clock signals and that change only on a rising or falling edge of the c... |
5,750 | These logic circuits are used in computer memory. They vary in performance, based on factors of speed, complexity, and reliability of storage, and many different types of designs are used based on the application. |
5,751 | A functionally complete logic system may be composed of relays, valves , or transistors. The simplest family of logic gates uses bipolar transistors, and is called resistor–transistor logic . Unlike simple diode logic gates , RTL gates can be cascaded indefinitely to produce more complex logic functions. RTL gates were... |
5,752 | As integrated circuits became more complex, bipolar transistors were replaced with smaller field-effect transistors ; see PMOS and NMOS. To reduce power consumption still further, most contemporary chip implementations of digital systems now use CMOS logic. CMOS uses complementary MOSFET devices to achieve a high spe... |
5,753 | For small-scale logic, designers now use prefabricated logic gates from families of devices such as the TTL 7400 series by Texas Instruments, the CMOS 4000 series by RCA, and their more recent descendants. Increasingly, these fixed-function logic gates are being replaced by programmable logic devices, which allow desig... |
5,754 | There are several logic families with different characteristics such as: RDL , RTL , DTL , TTL and CMOS. There are also sub-variants, e.g. standard CMOS logic vs. advanced types using still CMOS technology, but with some optimizations for avoiding loss of speed due to slower PMOS transistors. |
5,755 | Electronic logic gates differ significantly from their relay-and-switch equivalents. They are much faster, consume much less power, and are much smaller . Also, there is a fundamental structural difference. The switch circuit creates a continuous metallic path for current to flow between its input and its output. The ... |
5,756 | Another important advantage of standardized integrated circuit logic families, such as the 7400 and 4000 families, is that they can be cascaded. This means that the output of one gate can be wired to the inputs of one or several other gates, and so on. Systems with varying degrees of complexity can be built without gre... |
5,757 | The output of one gate can only drive a finite number of inputs to other gates, a number called the 'fan-out limit'. Also, there is always a delay, called the 'propagation delay', from a change in input of a gate to the corresponding change in its output. When gates are cascaded, the total propagation delay is approxim... |
5,758 | Logic built with FeFET transistors can retain their state to speed recovery in case of a power loss. |
5,759 | A three-state logic gate is a type of logic gate that can have three different outputs: high , low and high-impedance . The high-impedance state plays no role in the logic, which is strictly binary. These devices are used on buses of the CPU to allow multiple chips to send data. A group of three-states driving a line ... |
5,760 | In electronics, a high output would mean the output is sourcing current from the positive power terminal . A low output would mean the output is sinking current to the negative power terminal . High impedance would mean that the output is effectively disconnected from the circuit. |
5,761 | Non-electronic implementations are varied, though few of them are used in practical applications. Many early electromechanical digital computers, such as the Harvard Mark I, were built from relay logic gates, using electro-mechanical relays. Logic gates can be made using pneumatic devices, such as the Sorteberg relay o... |
5,762 | In principle any method that leads to a gate that is functionally complete can be used to make any kind of digital logic circuit. Note that the use of 3-state logic for bus systems is not needed, and can be replaced by digital multiplexers, which can be built using only simple logic gates . |
5,763 | In computer architecture, a bus is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components and software, including communication protocols. |
5,764 | Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel electrical busbar. Modern computer buses can use both parallel and bit serial connections, and can be wired in either a mult... |
5,765 | Computer systems generally consist of three main parts: |
5,766 | An early computer might contain a hand-wired CPU of vacuum tubes, a magnetic drum for main memory, and a punch tape and printer for reading and writing data respectively. A modern system might have a multi-core CPU, DDR4 SDRAM for memory, a solid-state drive for secondary storage, a graphics card and LCD as a display s... |
5,767 | In most traditional computer architectures, the CPU and main memory tend to be tightly coupled. A microprocessor conventionally is a single chip which has a number of electrical connections on its pins that can be used to select an "address" in the main memory and another set of pins to read and write the data stored a... |
5,768 | It is possible to allow peripherals to communicate with memory in the same fashion, attaching adapters, either on the motherboard or in the form of expansion cards, directly to the system bus. This is commonly accomplished through some sort of standardized electrical connector, several of these forming the expansion bu... |
5,769 | As the number of potential peripherals grew, using an expansion card for every peripheral became increasingly untenable. This has led to the introduction of bus systems designed specifically to support multiple peripherals. Common examples are the SATA ports in modern computers, which allow a number of hard drives to b... |
5,770 | In modern systems the performance difference between the CPU and main memory has grown so great that increasing amounts of high-speed memory is built directly into the CPU, known as a cache. In such systems, CPUs communicate using high-performance buses that operate at speeds much greater than memory, and communicate w... |
5,771 | Given these changes, the classical terms "system", "expansion" and "peripheral" no longer have the same connotations. Other common categorization systems are based on the bus's primary role, connecting devices internally or externally, PCI vs. SCSI for instance. However, many common modern bus systems can be used for b... |
5,772 | The internal bus, also known as internal data bus, memory bus, system bus or front-side bus, connects all the internal components of a computer, such as CPU and memory, to the motherboard. Internal data buses are also referred to as local buses, because they are intended to connect to local devices. This bus is typical... |
5,773 | The external bus, or expansion bus, is made up of the electronic pathways that connect the different external devices, such as printer etc., to the computer. |
5,774 | An address bus is a bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus . The width of the address bus determines the amount of memory a system can address. For example, a system with a 3... |
5,775 | Early processors used a wire for each bit of the address width. For example, a 16-bit address bus had 16 physical wires making up the bus. As the buses became wider and lengthier, this approach became expensive in terms of the number of chip pins and board traces. Beginning with the Mostek 4096 DRAM, address multiplexi... |
5,776 | Typically two additional pins in the control bus—a row-address strobe and the column-address strobe -- are used to tell the DRAM whether the address bus is currently sending the first half of the memory address or the second half. |
5,777 | Accessing an individual byte frequently requires reading or writing the full bus width at once. In these instances the least significant bits of the address bus may not even be implemented - it is instead the responsibility of the controlling device to isolate the individual byte required from the complete word transm... |
5,778 | Historically, there were also some examples of computers which were only able to address words -- word machines. |
5,779 | The memory bus is the bus which connects the main memory to the memory controller in computer systems. Originally, general-purpose buses like VMEbus and the S-100 bus were used, but to reduce latency, modern memory buses are designed to connect directly to DRAM chips, and thus are designed by chip standards bodies suc... |
5,780 | Buses can be parallel buses, which carry data words in parallel on multiple wires, or serial buses, which carry data in bit-serial form. The addition of extra power and control connections, differential drivers, and data connections in each direction usually means that most serial buses have more conductors than the mi... |
5,781 | Network connections such as Ethernet are not generally regarded as buses, although the difference is largely conceptual rather than practical. An attribute generally used to characterize a bus is that power is provided by the bus for the connected hardware. This emphasizes the busbar origins of bus architecture as supp... |
5,782 | However, this distinction—that power is provided by the bus—is not the case in many avionic systems, where data connections such as ARINC 429, ARINC 629, MIL-STD-1553B , and EFABus are commonly referred to as “data buses” or, sometimes, "databuses". Such avionic data buses are usually characterized by having sever... |
5,783 | The simplest system bus has completely separate input data lines, output data lines, and address lines.
To reduce cost, most microcomputers have a bidirectional data bus, re-using the same wires for input and output at different times. |
5,784 | Some processors use a dedicated wire for each bit of the address bus, data bus, and the control bus.
For example, the 64-pin STEbus is composed of 8 physical wires dedicated to the 8-bit data bus, 20 physical wires dedicated to the 20-bit address bus, 21 physical wires dedicated to the control bus, and 15 physical wire... |
5,785 | Bus multiplexing requires fewer wires, which reduces costs in many early microprocessors and DRAM chips.
One common multiplexing scheme, address multiplexing, has already been mentioned.
Another multiplexing scheme re-uses the address bus pins as the data bus pins, an approach used by conventional PCI and the 8086.
The... |
5,786 | Over time, several groups of people worked on various computer bus standards, including the IEEE Bus Architecture Standards Committee , the IEEE "Superbus" study group, the open microprocessor initiative , the open microsystems initiative , the "Gang of Nine" that developed EISA, etc. |
5,787 | Early computer buses were bundles of wire that attached computer memory and peripherals. Anecdotally termed the "digit trunk" in the early Australian CSIRAC computer, they were named after electrical power buses, or busbars. Almost always, there was one bus for memory, and one or more separate buses for peripherals. Th... |
5,788 | One of the first complications was the use of interrupts. Early computer programs performed I/O by waiting in a loop for the peripheral to become ready. This was a waste of time for programs that had other tasks to do. Also, if the program attempted to perform those other tasks, it might take too long for the program t... |
5,789 | High-end systems introduced the idea of channel controllers, which were essentially small computers dedicated to handling the input and output of a given bus. IBM introduced these on the IBM 709 in 1958, and they became a common feature of their platforms. Other high-performance vendors like Control Data Corporation im... |
5,790 | To provide modularity, memory and I/O buses can be combined into a unified system bus. In this case, a single mechanical and electrical system can be used to connect together many of the system components, or in some cases, all of them. |
5,791 | Later computer programs began to share memory common to several CPUs. Access to this memory bus had to be prioritized, as well. The simple way to prioritize interrupts or bus access was with a daisy chain. In this case signals will naturally flow through the bus in physical or logical order, eliminating the need for co... |
5,792 | Digital Equipment Corporation further reduced cost for mass-produced minicomputers, and mapped peripherals into the memory bus, so that the input and output devices appeared to be memory locations. This was implemented in the Unibus of the PDP-11 around 1969. |
5,793 | Early microcomputer bus systems were essentially a passive backplane connected directly or through buffer amplifiers to the pins of the CPU. Memory and other devices would be added to the bus using the same address and data pins as the CPU itself used, connected in parallel. Communication was controlled by the CPU, wh... |
5,794 | For instance, a disk drive controller would signal the CPU that new data was ready to be read, at which point the CPU would move the data by reading the "memory location" that corresponded to the disk drive. Almost all early microcomputers were built in this fashion, starting with the S-100 bus in the Altair 8800 compu... |
5,795 | In some instances, most notably in the IBM PC, although similar physical architecture can be employed, instructions to access peripherals and memory have not been made uniform at all, and still generate distinct CPU signals, that could be used to implement a separate I/O bus. |
5,796 | These simple bus systems had a serious drawback when used for general-purpose computers. All the equipment on the bus had to talk at the same speed, as it shared a single clock. |
5,797 | Increasing the speed of the CPU becomes harder, because the speed of all the devices must increase as well. When it is not practical or economical to have all devices as fast as the CPU, the CPU must either enter a wait state, or work at a slower clock frequency temporarily, to talk to other devices in the computer. Wh... |
5,798 | Such bus systems are also difficult to configure when constructed from common off-the-shelf equipment. Typically each added expansion card requires many jumpers in order to set memory addresses, I/O addresses, interrupt priorities, and interrupt numbers. |
5,799 | "Second generation" bus systems like NuBus addressed some of these problems. They typically separated the computer into two "worlds", the CPU and memory on one side, and the various devices on the other. A bus controller accepted data from the CPU side to be moved to the peripherals side, thus shifting the communicatio... |
5,800 | However, these newer systems shared one quality with their earlier cousins, in that everyone on the bus had to talk at the same speed. While the CPU was now isolated and could increase speed, CPUs and memory continued to increase in speed much faster than the buses they talked to. The result was that the bus speeds wer... |
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