Seven-segment display

Seven-segment representation of figures can be found in patents as early as 1903 (in ), when Carl Kinsley invented a method of telegraphically transmitting letters and numbers and having them printed on tape in a segmented format. In 1908, F. W. Wood invented an 8-segment display, which displayed the number 4 using a diagonal bar (). In 1910, a seven-segment display illuminated by incandescent bulbs was used on a power-plant boiler room signal panel. They were also used to show the dialed telephone number to operators during the transition from manual to automatic telephone dialing. They did not achieve widespread use until the advent of LEDs in the 1970s. Some early seven-segment displays used incandescent filaments in an evacuated bulb; they are also known as numitrons. A variation (minitrons) made use of an evacuated potted box. Minitrons are filament segment displays that are housed in DIP (dual in-line package) packages like modern LED segment displays. They may have up to 16 segments. There were also segment displays that used small incandescent light bulbs instead of LEDs or incandescent filaments. These worked similarly to modern LED segment displays. Vacuum fluorescent display versions were also used in the 1970s. Many early (c. 1970s) LED seven-segment displays had each digit built on a single die. This made the digits very small. Some included magnifying lenses in the design to try to make the digits more legible. Other designs used 1 or 2 dies for every segment of the display. Liquid-crystal display (LCD) largely superseded LED seven-segment displays. The shapes of elements in an LCD panel are arbitrary since they are formed on the display by photolithography, while LED segments tend to be simple rectangles, because they have to be physically moulded to shape, which makes it difficult to form more complex shapes. However, the easy recognition of seven-segment displays, and the comparatively high visual contrast obtained by such displays relative to dot-matrix digits, makes seven-segment multiple-digit LCD screens very common on basic calculators. Seven-segment displays have been recommended against for critical information (i.e. information that is dangerous to health or safety if misread) due to misreading if upside down or if segments are "stuck" on or off. Backlit dot-matrix LCDs have largely superseded seven-segment displays on such equipment. The seven-segment display has inspired type designers to produce typefaces reminiscent of that display (but more legible), such as New Alphabet, "DB LCD Temp", "ION B", etc. The seven-segment pattern is sometimes used in posters or tags, where the user either applies color to pre-printed segments, or applies color through a seven-segment digit template, to compose figures such as product prices or telephone numbers. Seven-segment displays, which use a restricted range of letters that look like (upside-down) digits, are commonly used by school children to form words and phrases using a technique known as "calculator spelling." ==Implementations==

Seven-segment displays may use a liquid-crystal display (LCD), a light-emitting diode (LED) for each segment, an electrochromic display, or other light-generating or -controlling techniques such as cold cathode gas discharge (neon) (Panaplex), vacuum fluorescent (VFD), incandescent filaments (Numitron), and others. For gasoline price totems and other large signs, electromechanical seven-segment displays made up of electromagnetically flipped light-reflecting segments are still commonly used. A precursor to the 7-segment display in the 1950s through the 1970s was the cold-cathode, neon-lamp-like nixie tube. Starting in 1970, RCA sold a display device known as the '''' that used incandescent filaments arranged into a seven-segment display. In USSR, the first electronic calculator "Vega", which was produced from 1964, contains 20 decimal digits with seven-segment electroluminescent display. In a simple LED package, typically all of the cathodes (negative terminals) or all of the anodes (positive terminals) of the segment LEDs are connected and brought out to a common pin; this is referred to as a "common cathode" or "common anode" device. Often in pocket calculators the digit drive lines would be used to scan the keyboard as well, providing further savings; however, pressing multiple keys at once would produce odd results on the multiplexed display. ==Characters==

The seven segments are arranged as a rectangle, with two vertical segments on each side and one horizontal segment each at the top, middle, and bottom. Often the rectangle is oblique (slanted), which may aid readability. In most applications, the segments are of nearly uniform shape and size (usually elongated hexagons, though trapezoids and rectangles can also be used); though in the case of adding machines, the vertical segments are longer and more oddly shaped at the ends, to try to make them more easily readable. The seven elements of the display can be lit in different combinations to represent each of the Arabic numerals. The individual segments are referred to by the letters "a" to "g", and an optional decimal point (an "eighth segment", referred to as DP) is sometimes used for the display of non-integer numbers. A single byte can encode the full state of a seven-segment display, including the decimal point. The most popular bit encodings are gfedcba and abcdefg. In the gfedcba representation, a byte value of 0x06 would turn on segments "c" and "b", which would display a "1" (). block, to replicate early computer fonts that included seven-segment versions of the digits. The official reference shows the less-common four-segment "7" (). Hexadecimal The binary-coded decimal (BCD) 0 to 9 digit values require four binary bits to hold their values. Since four bits (24) can hold 16 values, this means hexadecimal (hex) digits can be represented by four bits too. Due to the limited number of segments in seven-segment displays, the hexadecimal digits B and D are displayed as lowercase letters to avoid confusion with 8 () and 0 () respectively. The digit "6" must also be displayed with the topmost segment as to avoid ambiguity with the letter "b" (). : Early decoder ICs often produced unintuitive patterns or duplicates of digits for 10-15, as they were designed to use as few gates as possible and only required to produce 0-9. Letters Many letters of the Latin alphabet can be reasonably implemented on a seven-segment display. Though not every letter is available, it is possible to create many useful words. By careful choice of words, one can sometimes work around unavailable letters. Uppercase letters "B", "I", "S", "Z", and "D" & "O" conflict with the common seven-segment representation of digits "8", "1", "5", "2", and "0" (, , , , ) respectively, and the lowercase letter "g" with digit "9" (). Upper case () could be put on the left (as lower-case L is shown here) but this is not often done. Lowercase 'b' and 'q' are identical to the alternate numerical digits '6' and '9' (, ). : The following are some English word examples seen on actual electronic equipment (first line appeared on some CD players): :, , , , , :, , , , , , :, , , , , , ==See also==