Electronic Displays
by Rick Furr (rfurr@vcalc.net)
(since April 22, 1997)
Last Update: August 22, 2021 -- THE CALCULATOR REFERENCE
One of the more interesting things about old calculators is how they displayed
their numbers. As easy as it seems today, in the late 60s and early 70s it was
quite hard to devise a display system for a calculator, especially a portable
one. This article will describe the construction and operation of the major
types of electronic displays both past and present.
The Cathode Ray Tube (CRT) was developed for television in the 40s. The CRT
shoots a focused electron beam from the back of the tube to the front of the
tube. The front of the tube is coated with phosphors that glow when they are
struck by the electron beam. An image is created by moving the electron beam
back and forth across the back of the screen. The beam moves in a pattern from
left to right, top to bottom and then it repeats. Each time the beam makes a
pass across the screen, it lights up phosphor dots on the inside of the glass
tube, thereby illuminating the active portions of the screen. The intensity of
the beam is modulated thus causing the screen phosphors to glow with
different intensities or to even not glow at all. The desired images to be
displayed are actually retraced between 30 to 70 times each second. This keeps
the images continually refreshed in the glowing screen phosphors without a
flicker being perceivable to the eye.
The electron beam is generated from a filament and electrically charged cathode
in the back neck of the CRT. The electron beam is first passed through a
control grid. The control grid modulates the intensity of the electron beam.
The higher the intensity the brighter the phosphor dot it strikes will glow.
Next the beam passes through an accelerating electrode, this will speed up the
electron beam. Then the beam passes through a focusing anode. This will focus
or tighten the stream of electrons. All of these elements comprise the
electron gun structure housed in the neck of the CRT.
The structure on the neck of the CRT is the yoke. The yoke contains four
electromagnets placed around the neck of the CRT in 90 degree increments. By
varying the voltage of these four electromagnets, the electron beam can be
deflected or bent to reach any location on the phosphor coated screen. A final
stage of acceleration is achieved with the high voltage anode. The familiar
suction cup wire that attaches to the side of the CRT is connected to this
anode. This anode is often a metalized surface on the inside of the picture
tube. Many thousands of volts are applied to the anode to pull the electrons
towards the phosphor coated screen. Phosphors can be formulated to emit many
colors though white and green are the most popular for monochrome screens.
A simpler form of a CRT is formed by replacing the external electromagnetic
yoke with four internal electrostatic deflection plates. These plates are
placed in pairs on the horizontal and vertical axes. The electron beam is
deflected by the application of a positive voltage on the plates as the
beam passes through. The beam will bend towards the most positive plate.
Though this type of CRT is not as precise as magnetic deflection it is
much less expensive and quite effective for simple calculator and
oscilloscope displays. Additional circuitry in the calculator can create
numbers, letters, and other symbols by using the control grid to turn the
electron beam on and off, while simultaneously applying positive potentials
to the deflection plates to bend the beam to the desired locations on the
screen. Many early desktop calculators like the Friden EC-130 and the
Hewlett Packard 9100A used Electrostatic deflection CRTs.
In a Nixie Tube display each numeral is a complete, lighted cathode in the
shape of the numeral. The cathodes are stacked so that different numerals
appear at different depths, unlike a planar display in which all numerals are
on the same plane relative to the viewer. The anode is a transparent metal
mesh wrapped around the front of the display. The tube is filled with the
inert gas neon with a small amount of mercury. When an electric potential of
180 to 200 volts DC is applied between the anode and any cathode, the gas near
the cathode breaks down and glows. The digits glow with a orange-red color.
The name Nixie came about accidentally. A [Burroughs] draftsman making
drawings of the device labeled it NIX I, for Numeric Indicator eXperimental No.
1. His colleagues began referring to it as "Nixie," and the name stuck
(Scientific American, June '73, pp. 66).
Interestingly enough the Nixie design is considered "failsafe". If a filament
(cathode) fails, the numeral is not illuminated. Whereas, in a seven-segment
display if one segment fails, a number other than the intended number may be
displayed. The Casio 121-A and the Sharp QT8-B used Nixie tubes.
An Incandescent Filament display is usually housed in a vacuum tube like the
either the Nixie tube or the early Vacuum Fluorescent tubes. This display is
typically a seven segment style of display where each display segment is formed
with a conductive anode tungsten filament. A small voltage placed across a
filament will cause it to heat to incandescence. They emit a yellowish-white
light that can be filtered to any desired color. The filament voltage (3-5vdc)
can also be varied to change the brightness level of the display. The biggest
problem with Incandescent displays is they have a slow response time and they
consume a large amount of current. A popular version of this type of display
was the RCA Numitron. Some early electronic kits used the Incandescent
Filament display.
A Planar Gas Discharge or Plasma Display Panels (PDP) display utilizes the same
principle the Nixie tube does. It's construction consists of sandwiching a
hollow center layer filled with neon and a small amount of mercury between a
glass front and a ceramic back. A thick conductive paint forms the Cathodes on
the inside of the ceramic back. The Cathodes form the segments of each digit.
Each digit is covered by a separate Anode that is deposited on the inside of
the glass front. The Anodes are formed from a thin transparent layer of tin
oxide. When a sufficient voltage is applied between a cathode segment and it's
anode, the gas around the cathode segment breaks down and begins to glow. Like
the Nixie tube, the digits glow with a orange-red color. Voltage requirements
for these displays are typically 180-200 volts DC).
Burroughs manufactured a common brand of PDPs called Panaplex II. PDPs were
used in many early calculators including CompuCorp's Scientist series.
The Vacuum Fluorescent display (VFD) consists of a vacuum tube in which there
are three basic types of electrodes, the filament (cathode), the anode
(segment), and the grid. The VFD is essentially a small Cathode Ray Tube. The
filament (or filaments) is a very fine wire that is heated to a temperature
just below incandescence. At that temperature it remains virtually invisible
but it emits electrons. A transparent metal mesh grid covers each digit and
controls the electrons emitted from the filament toward that digit. Seven
phosphor coated anodes, arranged in the seven-segment configuration (that form
a square eight), glow when struck by the electrons. When a positive voltage of
12 to 25 volts is applied to the grid and the anodes, the electrons emitted by
the cathode filament are accelerated and attracted to the positive anode
segments which in-turn glow. If the grid has a negative potential then it will
block the electrons from passing regardless of the potential of the anodes
under the grid.
VFDs were developed in Japan in 1967. Early versions of VFDs were individual
digits housed in vacuum tubes like the Nixie tube and Incandescent Filament
displays. VFD Phosphors can be formulated to emit red, yellow, and green as
well as the more common blue-green color. Later versions would house all of
the digits (and other graphics and indicators) in one large glass assembly.
Currently VCRs account for 30% of the VFD market and Audio/Video products
account for another 30%. Many early series of calculators like the Commodore
412F, Brother 310, and the MITS 816 used the individual digit VFD tubes. Later
manufacturers such as TI and Rockwell used the integrated multidigit VFDs in
both handhelds and desktops.
Thin-film Electroluminescent Displays (ELDs) use a thin film of phosphor (zinc
sulfide (ZnS); ZnSe; ZnSMn or other fluorescent materials) sandwiched between a
dielectric layer that is sandwiched between two glass plates. Transparent
electrodes (tin-oxide) are deposited on the insides of the glass plates. When
a sufficient AC voltage (>100 volts) is applied to any of these electrodes
the phosphors will be excited and will emit light. ELD phosphors can be mixed
with pigments to emit many colors of light including green, blue-green,
lemon-yellow, orange, red as well as white light.
This type of solid state display can endure extreme conditions with exceptional
tolerance to shock, vibration, temperature, and humidity, while response times
remain less than one millisecond. I have not seen ELDs used in calculators but
they are used in some laptops, office machines and in the cockpit of the
Spaceshuttle. They are also used to backlight LCD panels.
A Light Emitting Diode (LED) is an special type of diode that emits light when
electricity applied to it's anode and cathode. A typical LED requires about 3
volts DC at 10 milliamps to begin emitting light. LEDs usually produce red
light but yellow, green and blue versions are also now available. The LED was
first marketed by Texas Instruments around 1962. LED displays (7 or more
individual LEDs) were introduced around 1967 but were very expensive.
Calculators used LEDs that were arranged to form either a seven-segment display
or a dot-matrix display.
Early seven segment displays formed each segment with many LEDs, later
seven-segment displays would use one LED per segment with a light pipe to
spread it's light across the segment. Also early LED displays were made small
in order to keep power consumption down. A clear plastic bubble lens was
fabricated into the package to magnify the display for easier viewing.
The dot-matrix style of display would form characters shaped similarly to that
of a dot-matrix printer. A dot matrix of 4x7 or 5x7 is typically used. Notice
how the 4x7 matrix makes up for the missing 5th column by slightly slanting the
columns. LEDs require much more power than LCDs and are more expensive to
manufacture. This is the simple reason for their demise from being used in
calculators.
The Liquid Crystal Display (LCD) was first developed at RCA around 1971. LCDs
are optically passive displays (they do not produce light). As a result, LCDs
require all most no power to operate. Many LCD calculators can operate from
the power of a solar cell, others can operate for years from small button cell
batteries. LCDs work from the ability of liquid crystals (LC) to rotate
polarized light relative to a pair of crossed polarizers laminated to the
outside of the display. There are two main types of LCD displays used for
calculators today: Twisted nematic (TN) and supertwisted nematic (STN). TN
displays twist polarized light to 90 degrees and have a limited viewing angle.
STN displays were developed to twist polarized light between 180 to 260 degrees
resulting in better contrast and a wider viewing angle.
A LCD consists of two plates of glass, sealed around the perimeter, with a
layer of liquid crystal fluid between them. Transparent, conductive electrodes
are deposited on the inner surfaces of the glass plates. The electrodes define
the segments, pixels, or special symbols of the display. Next a thin polymer
layer is applied on top of the electrodes. The polymer is etched with channels
in order to align the twist orientation of the LC's helix shaped molecules.
Finally, polarizing films are laminated to the outer surfaces of the glass
plates at 90 degree angles. Normally, two polarizing films at 90 degrees
should be dark, preventing any transmission of light but due to the ability of
LC to rotate polarized light the display appears clear. When AC voltage is
passed through the LC, the crystals within this field align so that the
polarized light is not twisted. This allows the light to be blocked by the
crossed polarizers thus making the activated segment or symbol to appear
dark.
Many other types of LCD displays are being developed for the laptop and
CRT replacement market including full color versions. These include double and
triple twisted nematic (DSTN and TSTN) displays and the Active-matrix Thin-film
Twisted Nematic and Metal-Insulated-Metal Twisted Nematic (TFT-TN and MIM-TN)
displays. Unfortunately these advanced display are too expensive for most of
the calculator market. TN LCDs almost completely dominate todays calculator
market due to their extremely low power requirements, thin size, and low
cost.
References:
Alan Sobel, "Electronic Numbers", Scientific American, pp. 64-73, June 1973.
"Note on the Liquid Crystal Display Industry,http://www.duc.auburn.edu/~boultwr /lcd/lcdpg1.html.
"Display Technologies in Japan", http://www.itri.loyola.edu/dsply_jp/toc.htm
"Sharp -- World of LCDs", http://www.sharp.co.jp/sc/library/lcd_e/indexe.htm
The Calculator Reference by Rick Furr (rfurr@vcalc.net)
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