DISPLAYS FOR INSTRUMENTS: A look
at the characteristics of displays used in electronic instrumentation.
The range of electronic displays available for instrumentation applications has broadened enormously over the last decade: designers are now faced with a bewildering choice of different technologies, each with their own strengths and weaknesses.
by BRIAN ROSE
At present, the range of technologies available for use as instrumentation displays breaks down into six different types: light emitting diodes (l.e.d), liquid crystal displays (LCD), vacuum fluorescent displays (VFD), plasma gas discharge displays (PGD), electroluminescent displays (e.l.d) and cathode ray tubes ( CRT).
Some display technologies are well-established, whereas others have emerged in the last few years as viable displays. Selecting the correct display for a particular application is crucial. particularly as a number of devices may, on initial study, appear to be adequate for a particular requirement.
However, unless the characteristics and limitations of a display are understood, it is all too easy to design -in one part, only to find in the course of further development that some aspect of the display imposes unacceptable constraints on the design of the complete instrument or system for which it is intended. The following review summarizes the characteristics, limitations and advantages of the different display technologies and also describes recent developments that have been made, both in the basic technology and in application.
LIGHT EMITTING DIODES
The l.e.d. is one of the most venerable and well established display technologies available, l.e.d are semiconductor devices that emit light in response to an applied voltage.
Typically, the material used for the junction from which light is emitted is gallium phosphide (CaP) doped with elements such as nitrogen and arsenic. By doping a CaP junction with appropriate quantities of a particular dopant, an energy gap can be created between the electronic valence and conduction bands of the gallium phosphide.
By the application of a voltage, electrons are promoted from the valence to the conductor band, this promotion being continuous once the voltage is applied, as is the return of electrons from the conductor to the valence band. In dropping from the conductor to the valence band a quantum of light is emitted, the frequency of this quantum being proportional to the energy gap. Thus, by tuning the gap by the use the particular dopants, colors from red, through orange, yellow and green to blue can be produced. For the emission of visible light the energy gap has to be larger than 1.8 electron volts, since the spectral range extends from 1.8 to 3.10 eV, corresponding to an optical wavelength range of 700 to 400nm.
Red and green have always been straight - forward colors to produce, although until recently it has been difficult to produce yellow l.e.d with a tight color spread and acceptable brightness variation within the batch. For some time, color and brightness have been selected by batch, but increased control of production has largely eliminated this.
The blue l.e.d. has only been available for a few years and the light output and price still leave a lot to be desired. Consequently, application, for blue l.e.d have been limited, being mostly restricted to spectroscopy applications.
The basic l.e.d. chips can be packaged in many ways: as individual lamps, as seven-segment and alphanumeric displays, as bar-graphs, backlights and dot-matrix displays.
As a single indicator the l.e.d. has no competition and when con figured as a single seven-segment display, it provides a very cost-effective solution.
However, l.e.d are somewhat power hungry. A single l.e.d. lamp running at 5V and 20mA does not improve much of a power requirement on any system, but when multiple, seven-segment displays are used, the power requirement tends to add up and can be prohibitive.
Being solid-state, l.e.d are intrinsically robust and can operate over wide tempera ture ranges ( -25 to + 80°C).
Reliability is excellent with a 500,000 hour m.t.b.f. a conservative figure these days.
In summary, l.e.d provide a simple, cheap and reliable display for single indicator, single or dual -digit applications, but where a greater amount of information needs to be displayed cost and power consumption can be prohibitive.
LIQUID CRYSTAL DISPLAYS
------------ 600 by 400 element electroluminescent display from Sharp.
After the abortive initial launch of I.c. displays using the dynamic scattering type fluid (unreliable with poor environmental integrity and poor contrast) LCD have now come of age with the use of twisted nematic fluids.
The LCD is an interesting technology as it emits no light. Information is conveyed by modification of incident light, in most cases.
LCD consist of two glass panels sealed together, containing liquid -crystal material.
On the inside surfaces of the two glass panels are electrode patterns etched in conductive, transparent indium tin oxide. The inner surfaces of the glass are prepared in such a way as to anchor Liquid-crystal molecules in a fixed position top and bottom and to impose a 90 degree twist in the molecules between the two plates, liquid-crystal material have no effect on normal light but do have the ability to rotate the plane of polarized light. In LCD polarized light is produced by the use of front and rear polarizing filters attached to the outer surfaces of the glass panels. These filters are usually applied with their polarizing axes orthogonal to each other. Thus, with the liquid -crystal molecules in their normal state (twisted and able to rotate the plane of polarized light) any light passing through the LCD and polarizers and reflected back is substantially unaltered. However, when a voltage is applied across the electrodes, the liquid -crystal molecules straighten out, thus losing the ability to rotate the plane of polarization light: hence, any area over which this occurs appears opaque to the viewer.
An enormous range of standard LCD is available from companies such as Hitachi, Sharp, Epson, Varitronix and Toshiba. Displays from a single, half -inch tall, seven-segment digit through 4, 6, and 16 digit units up to large single digits, many inches tall are available.
Many options are available with LCD: there is a choice of viewing mode, connection type, viewing angle and operating temperature range.
Viewing modes can be chosen as restrictive, transmissive and transflective. Reflective displays can only be seen in ambient light, not in darkness. Transmissive displays are designed to be permanently lit from behind (good in the dark, but poor in strong ambient light unless strong backlighting is used.) Transflective displays combine the features of transmissive and reflective units.
They can be seen in good ambient light but since they will permit light to be shone through from behind, a backlight can be harp switched on in low light levels.
A number of connector types can be used with LCD including push-on pins, bonded pins, elastomeric connectors and heat-bonded connector films. In some cases, due to the number of contacts that have to be brought out in a limited space a high -density elastomeric connector may to be used.
Bonded -pin displays are easy to use and can be inserted in PCB like an i.c. Due to production constraints, a preferred viewing direction must be selected at the time of manufacture. This is normally 6 o'clock, i.e. viewed from below, but 12 o'clock viewing is available for top viewing as are special angles (such as 3 o'clock for a car-radio application). Standard temperature range for LCD is now 0 to +50°C but some types are available spanning -40 to +85°C. Simple LCDs such as 4-digit, half-inch tall digit types are used in great numbers in measuring instruments such as multimeters.
The scope of the LCD, has been greatly increased by the availability of custom -designed facilities. Almost all LCD companies now offer a custom service where an LCD, can be designed and manufactured to a customer's specifications.
This particular area of the market has been tainted in recent years by the activities of certain 'cowboy' companies.
Reputable companies include Sharp, Hitachi. Epson, Varitronix and Lucid.
With a custom LCD design, bar-graphs, legends and even company logos can be incorporated into one display.
When a larger amount of information needs to be displayed such as several lines of alphanumeric characters then a complete module rather than just an LCD, is generally preferred. Modules comprise LCD, connectors, elastomers and driver ICs.
---------- This vacuum fluorescent display dashboard was designed for the Aston Martin Lagonda, by Kemitron. Road and engine speed r.p.m. are shown on bar-graph gauges, all the usual standard warning symbols are incorporated, and a separate dot-matrix display is linked to a speech synthesizer to give system messages.
An area of considerable growth in the past few years has been graphic LCD, modules. Unfortunately, it is in these products that the shortcomings of LCDs become apparent. With large displays (650 by 200 dots for example), each dot needs to be separately addressable.
However, it is impossible to dedicate one contact for each dot: it is necessary to use a multiplex drive system. Many dots share a common electrode and hence unique addressability can only be achieved by a complicated sub -division of drive -voltage levels and reduced address time per drive cycle. The result of a high -duty cycle multiplex drive (100:1 or more) is to give reduced contrast and viewing angle compared with simple LCDs: early graphics LCDs of 640 by 200 dot size were virtually illegible. Continuing development over the past few years has produced a useful improvement, but the release this year of new units using the new super-twisted birefringent LCD, material has finally produced usable large graphics displays: leaders in this field are Sharp, Hitachi and Epson. Sharp have super -twist units available now with Hitachi and Epson due to follow very soon.
LCDs are small and light, if somewhat fragile, due to their thin glass construction.
The great advantage is their very low power consumption, in the order of a few milliwatts even for the largest display. However, since they are passive displays and emit no light they are unsatisfactory in low light conditions. Backlighting can of course be used, but this detracts from the low power consumption advantage of the LCD. Another major advantage of LCDs is their low cost.
For a display of four digits and above, there is no cheaper option and this coupled with the availability of a wide range of c-mos driver helps to explain their popularity.
VACUUM FLUORESCENT
The vacuum fluorescent display has been around for many years and although it has been very popular with Japanese equipment manufactures it remains relatively unpopular in the UK.
-------------- Cathode ray tubes. as well as being instrument display devices in their own right on oscilloscopes etc, can also simulate other displays. This high-resolution CRT. from AEG combines the function of several mechanical meters into one display for use in aircraft all superimposed on a radar map to give all the essential information in a single display.
----- Another example of the versatility of CRT is provided in the Labview software for the Apple Macintosh computer, available from Amplicon in Brighton. It can display the "front panel" of an instrument connected to the computer through a GPIB interface.
VFDs operate in a similar manner to valves. Electrons from a cathode are accelerated by a potential difference through a grid to impinge (fluoresce) upon a phosphor layer. VFDs are identifiable by their attractive bright blue-green color which is filter able to other colors. Their familiarity in the UK comes from their use in almost every video-cassette recorder: however. they are rarely seen in instrumentation. VFDs are bright with good viewing angles. but the vacuum envelope is intrinsically fragile, particularly the "neck" where they are sealed after evacuation. Although attractive. VFDs are not a particularly cheap technology, especially when driver costs are borne in mind. There are only three major manufactures--Itron, NEC and Futaba - all Japanese.
PLASMA GAS DISCHARGE DISPLAYS
Plasma displays take the form of a gas -filled space between two electrodes. When a high voltage is applied across the space, the gas molecules ionize and emit light at the electrode surfaces. Neon is the gas most normally used: this requires a discharge of 180V+ and gives a bright orange display.
Small displays are not practical with PGDs and available units range from 4 digit units to 640 by 400 -dot flat panels. Very high brightness (up to 300 foot-lamberts) can be obtained from PGDs and this partially ex plains their popularity as bar graphs in process control instrumentation.
PGDs are most popular in the United States where many years ago Burroughs were the world leader in this technology.
Unfortunately, the demise, resurgence and demise again of Burroughs and other spin off PGD, manufacturers led for many years to erratic supply. Japanese companies such as NEC now also manufacture PGDs and supply has stabilized somewhat. PGDs have been unlucky in attracting a bad press over the last few years. Their flicker has attracted criticism and at one time there was talk of banning PGDs for computer terminal applications.
There has also been a (surely) apocryphal story about the British defense instrumentation manufacturer whose marine PGD, displays failed to operate in the darkness of the Arctic circle. Apparently, some ionization of the neon gas needs to be present before an initial discharge can be struck, this ionization being normally provided by light photons which, if absent, prevent strike -up.
One method of avoiding this problem is to incorporate minute quantities of radio active tritium to ionize the neon.
Although popular in the US, PGDs have never really taken off in the UK although many large PGD, graphics panels now look attractive. Many PGD, manufacturers still have not solved the display/driver integration problem and it is not unusual to see PCBs stacked four deep on the back of a PGD.
ELECTROLUMINESCENT DISPLAYS
Like LCDs, e.l. displays had a bad start. There are four types, thin film, thick film, a.c. drive and d.c. drive. Thick film a.c. drive panels have been renowned for their short half-life and this has unfortunately rubbed off on the other variety. The dominant technology is thin-film a.c. drive e.l. panels. These have been pioneered by Sharp in Japan.
E.I. panels consist of two sheets of glass with electrodes etched in indium tin oxide on the internal surfaces. One set of electrodes is orthogonal to the other - when assembled an x-y grid is produced. Sandwiched between the two plates of glass is a thin layer of zinc manganese sulfite. When a potential difference is applied across the phosphor it emits a pleasing yellow -glow.
Hence, the panel is completely solid-state with no fragile envelope containing a gas or a vacuum. Integration of display and drivers has been taken to an advanced level with surface -mounted drivers sitting on flexible printed -circuit boards which are attached to the four sides of the display. These boards fold tidely behind the display, resulting in a compact package.
Current Sharp units offer viewing angles of more than 120 degrees in all axes, brightness greater than 20 foot lamberts (68cd/ m^2), and high contrast, all in a package less than 35mm thick.
E.I. displays have only been in production for the last five years, but even so, they show signs of becoming the flat -panel display of the future. Unfortunately, e.l. displays are not viable as small units. The smallest display in production is 320 by 240 dots, but the largest is 640 by 400 dots, and is most suitable where a large amount of information needs to be displayed. An interface very similar to that of a CRT, has enhanced acceptance and has given e.l. displays an advantage over VFDs and PGDs.
Outstanding features of e.l. displays are robustness, compactness, brightness, wide angle, stable image and good system integration.
CATHODE RAY TUBES
The cathode ray tube is the oldest of the electronic displays currently in use. In terms of value, more CRTs have been sold than all other display types combined. The crude units used for television before the second world war have developed into today's high resolution monochrome and color displays. The basic technology of CRTs is well known - electrons from a heated cathode are accelerated in a vacuum towards a grid and guided by external magnetic or static forces to impinge on a phosphor coated screen.
Except for esoteric units, no major changes have been made in the basic technology used in the vast majority of CRTs supplied today. Instead, detailed improvements in the performance and variety of phosphors, the resolution of screen etching and better yoke design have taken place. The huge volumes of CRTs in productions mean that costs are very competitive.
The design of a CRT, display package involves trade-offs. Increased resolution is achievable only at the expense of brightness, for example. Tube depth can be reduced by using tubes with large deflection angles, but these angles may jeopardize display quality, due to loss of linearity and poor edge resolution.
Higher scanning rates (up to 64kHz for color) are now possible, which gives higher resolution and this, together with the achievement of dot pitches down to 0.26mm, has produced CRTs more suited to the demands of computer terminals and c.a.d./c.a.m. units. It is, however, in the field of monitors (the integration of CRT. into a display system) that most advances have been made.
In general, a monitor is engineered to Display data at a single fixed frequency.
However, new advances have produced a single monitor that can accommodate a variety of scanning (data) rates, which means that the circuit can operate from multiple sources showing varying resolution. The NEC Multi-sync is an example of this new type of monitor accommodating frequencies of 15kHz to 35kHz.
QUICK GUIDE
No one display technology will suit every application. The criteria that should influence selection of a display include size, power consumption, legibility, robustness and interfacing.
It is possible to get a more meaningful view of the way in which these factors influence selection by grouping types of displays together by the amount of information they can display.
---------- Touch -sensitive pads are incorporated into the face of this British electroluminescent display to provide an interactive system for control and display. The display offers 640 by 256 pixels and has the advantage of a wide viewing angle, suitable for both text and graphics. The 8 by 16 capacitive switches can be programmed to match the corresponding areas of the display by high-level language so that. e.g.. a read-out from a required function can be obtained by touching that spot on the screen. By Phosphor Products of Poole.
------- Intended for use in portable computers but also suitable for laboratory instrument applications, this electroluminescent display comes from Finland (Lohja corporation). It has all the electronics necessary to be compatible with the output of an MS DOS computer.
------ This fluorescent indicator from NEC includes all the support
and control electronics to allow the unit to be connected directly to
the data and control signals.
Low information -content displays
Here LEDs reign supreme. They are small, robust, with good view-ability and are comparatively cheap, so long as the number of LEDs used is restricted. Power consumption is acceptable even in portable equipment. Towards the higher end of this category (say four digits and above) LCDs start to become more attractive so long as they are to operate in normal lighting conditions. Their low power consumption is a great advantage and as long as LCDs are carefully mounted, their robustness is quite good.
Medium information-content displays
If we define this category to commence with, say, a single line of sixteen alphanumeric characters, then the choice is between LCD, modules and VFDs. LCD, modules usually have a price advantage and are to be preferred unless the pleasing blue/green characters of a VFD, are considered aesthetically more important than the unimpressive black character of the LCD. Even with a backlight, an LCD, module still consumes less power than a VFD, especially if the backlight need only be used intermittently in low-light conditions.
Although plasma displays can also be attractive, their integration with driver and their expense tend to militate against them in this category.
High information-content displays--If this category is considered to begin at 4 lines of 40 characters, then the choice here is be wildering. All the displays mentioned qualify.
For smaller units, LCD. modules are probably the best choice, up to about eight lines of characters. VFD. and PGD, units are also available at this size and integration of display and drivers rivals that of the LCD module, albeit at higher cost and greater power consumption. Here if the power consumption and cost penalties are acceptable, the decision to be made is one of aesthetics.
Both VFD and PGD, are better-looking displays than LCDs.
At the top end of this category where an entire screen of information needs to be shown, then the choice is between LCD., PGD., e.l. and CRT types.
The CRT. is still the cheapest way to display a large amount of information, so long as the weight and size is acceptable. The neck of a CRT. always makes the depth of a unit problematical. Several companies, including Sinclair, Sony and Philips have produced CRTs with reduced depth, but costs are high and they show no sign as yet of taking a significant share of the total market.
If a CRT. is unacceptable then the choice is between LCD. modules, PGD, and e.l. LCD modules offer the lowest power consumption, but even the new super -twist units offer an unexceptional display compared with PGD. and electroluminescent displays.
For a flat -panel display with good view-ability, the choice is between PGD. and e.l. PGDs have a slight edge on brightness, but are usually larger and heavier than an equivalent e I. display (this may be important for portable equipment). Interfacing to an e.l. display is usually easier - in many cases CRT. controllers may be used.
Unlike the majority of electronic components, the way a display looks has a great influence upon its selection. It is not un known for the selection of a display to be conditioned by its match to a company's color scheme. Designers are spoilt for choice as far as displays are concerned -- even though some hard work may be necessary to select the correct display for a particular application.
Brian Rose is the product manager of lmpectron Ltd of Horsham.
------------
Also see:
==========
(adapted from: Wireless World , 1987)