Seven-segment display

A seven-segment display is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot matrix displays.

A typical 7-segment LED display component, with decimal point in a wide DIP-10 package

Seven-segment displays are widely used in digital clocks, electronic meters, basic calculators, and other electronic devices that display numerical information.[1]

History
A multiplexed 4-digit, seven-segment display with only 12 pins

Seven-segment representation of figures can be found in patents as early as 1903 (in U.S. Patent 1,126,641), 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 (U.S. Patent 974,943). In 1910, a seven-segment display illuminated by incandescent bulbs was used on a power-plant boiler room signal panel.[2] They were also used to show the dialed telephone number to operators during the transition from manual to automatic telephone dialing.[3] They did not achieve widespread use until the advent of LEDs in the 1970s.

Filament seven-segment display

Some early seven-segment displays used incandescent filaments (filament seven segment displays) while making use of an evacuated bulb, like most nixie tubes and early vacuum fluorescent displays.[4] They are also known as numitrons. [5] A variation (minitrons) made use of an evacuated (vacuum) potted box. They are filament segment displays that are housed in DIP packages like modern LED segment displays. They may have up to 16 segments.[6][7][8] 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.[9]

Early LED seven-segment displays had all their components built onto a single die. This made the digits very small. Some included mangnifing lenses onto the design in an attempt to make the digits more legible. [10][11]

They are 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.

For many applications, dot-matrix LCDs have largely superseded LED displays in general, though even in LCDs 7-segment displays are very common. Unlike LEDs, the shapes of elements in an LCD panel are arbitrary since they are formed on the display by photolithography. In contrast, the shapes of LED segments tend to be simple rectangles, reflecting the fact that they have to be physically moulded to shape, which makes it difficult to form more complex shapes than the segments of 7-segment displays. However, the high common recognition factor of 7-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.

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.

Using a restricted range of letters that look like (upside-down) digits, seven-segment displays 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 (Panaplex), vacuum fluorescent (VFD), incandescent filaments (Numitron), and others. For gasoline price totems and other large signs, vane displays made up of electromagnetically flipped light-reflecting segments (or "vanes") are still commonly used. An alternative 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 Numitron that used incandescent filaments arranged into a seven-segment display.[12] In USSR, the first electonic calculator "Vega" which produced from 1964, contains 20 decimal digits with 7-segments electroluminescent display.[13]

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.[14] Hence a 7 segment plus decimal point package will only require nine pins, though commercial products typically contain more pins, and/or spaces where pins would go, in order to match standard IC sockets. Integrated displays also exist, with single or multiple digits. Some of these integrated displays incorporate their own internal decoder, though most do not: each individual LED is brought out to a connecting pin as described.

Multiple-digit LED displays as used in pocket calculators and similar devices used multiplexed displays to reduce the number of I/O pins required to control the display. For example, all the anodes of the A segments of each digit position would be connected together and to a driver circuit pin, while the cathodes of all segments for each digit would be connected. To operate any particular segment of any digit, the controlling integrated circuit would turn on the cathode driver for the selected digit, and the anode drivers for the desired segments; then after a short blanking interval the next digit would be selected and new segments lit, in a sequential fashion. In this manner an eight digit display with seven segments and a decimal point would require only 8 cathode drivers and 8 anode drivers, instead of sixty-four drivers and IC pins.[15] 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.

Although to a naked eye all digits of an LED display appear lit, the implementation of a typical multiplexed display described above means that in reality only a single digit is lit at any given time.

A single byte can encode the full state of a 7-segment-display. The most popular bit encodings are gfedcba and abcdefg, where each letter represents a particular segment in the display. In the gfedcba representation, a byte value of 0x06 would (in a common anode circuit) turn on segments 'c' and 'b', which would display a '1'.

Characters
16x8-grid showing the 128 states of a seven-segment display

The seven elements of the display can be lit in different combinations to represent the Arabic numerals. Often the seven segments are arranged in an oblique (slanted) arrangement, which aids readability. In most applications, the seven 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 in an effort to further enhance readability.

The seven segments are arranged as a rectangle of two vertical segments on each side with one horizontal segment on the top, middle, and bottom. Additionally, the seventh segment bisects the rectangle horizontally.

The segments of a 7-segment display are referred to by the letters A to G, where the optional decimal point (an "eighth segment", referred to as DP) is used for the display of non-integer numbers.[16][14]

Decimal

The numerical digits 0 to 9 are the most common characters displayed on seven-segment displays. The most common patterns used for each of these is:[17]

The numerals 6 and 9 may be represented by two different glyphs on seven-segment displays, with or without a 'tail',[18] and the numeral 7 also has two versions, with or without segment F:[19]

Hexadecimal

Four binary bits are needed to specify the numbers 0-9, but can also specify 10-15, so usually decoders with 4 bit inputs can also display Hexadecimal (Hex) digits. Today, a combination of uppercase and lowercase letters is commonly used for A–F; this is done to obtain a unique, unambiguous shape for each hexadecimal digit (otherwise, a capital 'D' would look identical to a '0' and a capital 'B' would look identical to an '8').[20][21][22][23] Also the digit '6' must be displayed with the top bar lit to avoid ambiguity with the letter 'b'.[18]

The following lookup table may be useful for writing code to drive a 7-segment display.

The individual segments of a seven-segment display
Hexadecimal encodings for displaying the digits 0 to F[20][21]
Digit Display gfedcba abcdefg a b c d e f g
0 0x3F 0x7E on on on on on on
1 0x06 0x30 on on
2 0x5B 0x6D on on on on on
3 0x4F 0x79 on on on on on
4 0x66 0x33 on on on on
5 0x6D 0x5B on on on on on
6 0x7D 0x5F on on on on on on
7 0x07 0x70 on on on
8 0x7F 0x7F on on on on on on on
9 0x6F 0x7B on on on on on on
A 0x77 0x77 on on on on on on
b 0x7C 0x1F on on on on on
C 0x39 0x4E on on on on
d 0x5E 0x3D on on on on on
E 0x79 0x4F on on on on on
F 0x71 0x47 on on on on

The above modern numerical scheme wasn't always followed in the past, and various other schemes could be found as well:

  • The Texas Instruments seven-segment display decoder chips 7446/7447/7448/7449 and 74246/74247/74248/74249 and the Siemens FLH551-7448/555-8448 chips used truncated versions of "2", "3", "4", "5" and "6" for digits A–E. Digit F (1111 binary) was blank.[18][24][25]
  • Soviet programmable calculators like the Б3–34 used the symbols "−", "L", "C", "Г", "E", and " " (space), allowing the error message EГГ0Г to be displayed.
  • Many earlier chips provided logic designed for only for 0–9 and higher numbers produced whatever pattern resulted. The National Semiconductor MM74C912 displayed "o" for A and B, "−" for C, D and E, and blank for F. The CD4511 just displayed blanks.
modern[20][21][22][23][26]
TI,[18] Siemens[24][25]
Elektronika
National Semiconductor

Latin alphabet

About half the letters of Latin alphabet can be reasonably implemented using seven segments. Though not every letter is available, it is possible to create many useful words. By choosing better synonyms, it is possible to work around many shortcomings of seven-segment alphabet encodings. Some letters ('O', 'I', 'S') look identical to numbers, though use of lower-case 'o' and 'i', or putting 'I' on the left as shown here, could be used instead. In addition, seven-segment displays can be used to show various other letters of the Latin, Cyrillic and Greek alphabets including punctuation, but few representations are unambiguous and intuitive at the same time.[27] When all letters need to be displayed on a device, sixteen-segment and dot matrix displays are better choices than seven-segment displays.

Latin alphabet encodings
CaseABCDEFGHIJKLMNOPQRSTUVWXYZ
Upper
Lower

Short messages giving status information (e.g. "no dISC" on a CD player) are also commonly represented on 7-segment displays. In the case of such messages it is not necessary for every letter to be unambiguous, merely for the words as a whole to be readable.

Examples:

, , , , ,
, , , , , ,
, , , , ,

Punctuation

Seven segments are capable of displaying some punctuation glyph characters. The hex value for each Unicode character is shown, of which the lower 8-bits of most of these exist as ASCII characters too.

Punctuation encodings
GlyphDisplayUnicodeName(s)
_ 0x005FUnderscore, Low dash, Low line
- 0x002DHyphen-minus, Minus, Negative, Hyphen, Dash
= 0x003DEquals, Double hyphen
≡ 0x2261Triple bar, Hamburger button
° 0x00B0Degree, Superscript zero
" 0x0022Double quote, Double prime
' 0x0027Apostrophe, Single quote, Prime
[ 0x005BLeft square bracket (conflicts - same as uppercase C in 7seg)
] 0x005DRight square bracket
0x0020Space, Blank, All segments off

See also
7, 9, 14, 16 segment displays shown side by side.

There are also fourteen and sixteen segment displays (for full alphanumerics); however, these have mostly been replaced by dot matrix displays. Twenty-two segment displays capable of displaying the full ASCII character set[28] were briefly available in the early 1980s, but did not prove popular.

  • Nine-segment display
  • Fourteen-segment display
  • Sixteen-segment display
  • Dot matrix display
  • Nixie tube display
  • Vacuum fluorescent display

References
  1. "Seven Segment Displays". Archived from the original on 2012-04-04.
  2. Rogers, Warren O. (1910-02-01). "Power Plant Signalling System". Power and the Engineer. 32 (5): 204–206. Archived from the original on 2014-03-31. Retrieved 2016-10-06.
  3. Clark, E. H. (December 1929). "Evolution of the Call-Indicator System" (PDF). Bell Laboratories Record. 8 (5): 171–173.
  4. "Archived copy". Archived from the original on 2016-06-11. Retrieved 2020-04-16.CS1 maint: archived copy as title (link)
  5. "IEE Apollo DA-2110 Numitron Tube (DA2110, RCA DR-2110) - Industrial Alchemy". www.industrialalchemy.org.
  6. "Wamco KW-105AL Alphanumeric Minitron Display - Industrial Alchemy". www.industrialalchemy.org.
  7. "Numitron Tube Tutorial". 2011-12-21. Archived from the original on 2018-09-25. Retrieved 2020-04-14.
  8. "Archived copy". Archived from the original on 2018-02-17. Retrieved 2020-04-14.CS1 maint: archived copy as title (link)
  9. "Alco MSM-5A Mosaic Indicator Incandescent Display - Industrial Alchemy". www.industrialalchemy.org.
  10. "Archived copy". Archived from the original on 2018-08-31. Retrieved 2020-04-16.CS1 maint: archived copy as title (link)
  11. "Archived copy". Archived from the original on 2018-03-04. Retrieved 2020-04-16.CS1 maint: archived copy as title (link)
  12. "Advert for RCA NUMITRON Display Devices". Electronic Design. Hayden. 22 (12): 163. 1974-06-07. Archived from the original on 2014-03-31. Retrieved 2012-06-22.
  13. "Museum of Soviet Calculators - VEGA". web.archive.org. 2010-09-29.
  14. Elektrotechnik Tabellen Kommunikationselektronik (3rd ed.). Braunschweig, Germany: Westermann Verlag. 1999. p. 110. ISBN 3142250379.
  15. e.g. DCR 1050m Archived 31 March 2014 at the Wayback Machine
  16. "Seven Segment Displays". Archived from the original on 2012-01-05. Retrieved 2012-11-14.
  17. Nührmann, Dieter (1981). Written at Achim, Bremen, Germany. Werkbuch Elektronik (in German) (3 ed.). Munich, Germany: Franzis-Verlag GmbH. p. 695. ISBN 3-7723-6543-4.
  18. BCD-to-Seven-Segment Decoders/Drivers: SN54246/SN54247/SN54LS247, SN54LS248 SN74246/SN74247/SN74LS247/SN74LS248 (PDF), Texas Instruments, March 1988 [March 1974], SDLS083, archived (PDF) from the original on 2017-03-29, retrieved 2017-03-30, […] They can be used interchangeable in present or future designs to offer designers a choice between two indicator fonts. The '46A, '47A, 'LS47, and 'LS48 compose the 6 and the 9 without tails and the '246, '247, 'LS247, and 'LS248 compose the 6 and the 0 with tails. Composition of all other characters, including display patterns for BCD inputs above nine, is identical. […] Display patterns for BCD input counts above 9 are unique symbols to authenticate input conditions. […]
  19. For example the fx-50F calculator from Casio and other models from the same manufacturer.
  20. "Driving 7-Segment Displays". Maxim Integrated. 2004. Archived from the original on 2017-03-20. Retrieved 2017-03-20.
  21. electronic hexadecimal calculator/converter SR-22 (PDF) (Revision A ed.). Texas Instruments Incorporated. 1974. p. 7. 1304-389 Rev A. Archived (PDF) from the original on 2017-03-20. Retrieved 2017-03-20.
  22. electronic calculator – TI programmer (PDF). Texas Instruments Incorporated. 1977. p. 7. Archived (PDF) from the original on 2017-03-28. Retrieved 2017-03-28.
  23. electronic calculator – TI LCD programmer (PDF). Texas Instruments Incorporated. 1981. p. 8. Archived (PDF) from the original on 2017-03-28. Retrieved 2017-03-28.
  24. Beuth, Klaus; Beuth, Annette (1990). Digitaltechnik. Elektronik (in German). 4 (7 ed.). Würzburg, Germany: Vogel Buchverlag. pp. 301–303. ISBN 3-8023-0584-1.
  25. Datenblatt FLH551-7448, FLH555-8448, 74248 (in German). Siemens.
  26. "Application Note 3210 – Quick-Start: Driving 7-Segment Displays with the MAX6954" (PDF) (Application note) (3 ed.). Maxim Integrated. March 2008 [2004-06-25]. Archived (PDF) from the original on 2017-03-20. Retrieved 2013-05-06.
  27. Downie, Neil A. (2003). Ink Sandwiches, Electric Worms and 37 Other Experiments for Saturday Science. Johns Hopkins University Press. p. 271.
  28. "DL-3422 4-digit 22-segment alphanumeric Intelligent Display™ preliminary data sheet". Internet Archive. Litronix 1982 Optoelectronics Catalog. p. 82. Retrieved 2016-09-03.
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