The electrical engineering handbook CH089

The electrical engineering handbook

Sherr, S., Durbeck, R.C., Suryn, W., Veillette, M “Input and Output”

The Electrical Engineering Handbook

Ed Richard C Dorf

Boca Raton: CRC Press LLC, 2000

Input and Output

89.1 Input DevicesKeyboards • Light Pen • Data Tablet (Graphics, Digitizer) • Mouse • Trackball • Joystick • Touch Input • Scanners • Voice • Summary • Advantages and Disadvantages89.2 Computer Output Printer TechnologiesClassification of Printer Technologies • Page Printer Technologies • Serial Nonimpact Printer Technologies • Impact Printer Technologies89.3 Smart Cards

Hardware Architecture • Contact ICC, Contactless ICC • Operating Systems • Standards • Applications • Readers • Card-to-System Solutions • Trends

Solomon Sherr

Input devices are those portions of computer, data processing, and information systems that perform theessential function of providing some means for entering commands and data into the system Therefore, inputdevices are found in all such systems, but are treated here as a separate equipment group, independent of thetotal system configuration However, the place of input devices in a representative computer system may beclarified by reference to Fig 89.1(a), which shows the interface of the main input device categories in relation

to the portions of the generalized system that accept the inputs The categories and the devices listed in Table 89.1are the subject of this section

Keyboards

Keyboards are essentially electromechanical devices, and are still ubiquitous, in spite of the inroads of otherinput devices The primary type of keyboard in use as an input device is the alphanumeric (A/N) form, wellknown in its typewriter application, but with various additions and expansions consisting of numeric andspecial function keys This type of keyboard is shown in Fig 89.1(b) with a standard QWERTY format, sonamed because of the layout of the top left alpha keys, for the A/N portion, a separate numeric set to the right,and a group of function keys at the top Other layouts for the A/N portion have been proposed and at leastone (Dvorak) accepted by the American National Standards Institute (ANSI), but it has not received much use

in spite of its advantages in increased efficiency At present, the overwhelming majority of system keyboardsstill use the QWERTY layout, and it is the only one considered here

As illustrated in Fig 89.1, a keyboard consists of a number of keyswitches whose exact structure is of primeimportance in keyboard design The relevant characteristics of keyswitch operation are life, actuation force,travel distance, and feedback Accepted values are shown in Table 89.2 for different keyswitch designs Theelastomer type is preferred to a limited extent over the other two when the electronic audio feedback is included.This indicates that some type of audio feedback is desirable One form of keyswitch design using an elastomer

1 The material contained in this section is a shortened version of that which appears in Electronic Displays, 2nd ed., by Sol Sherr, Chapter 6, Section 6.1, 1993, published by John Wiley & Sons, Inc., and is reprinted here by permission.

A RT OF C OMPILING S TATISTICS

Herman Hollerith

Patented January 8, 1889

#395,781

n excerpt from Herman Hollerith’s patent application:

Having thus described my invention, what I claim as new is (1) The improvement in the art of compiling statistics, which consists in first preparing a series of separate record-cards, each card representing an indi- vidual or subject; second, applying to each card at predetermined intervals circuit-controlling index points arranged according to a fixed plan of distribution, to represent each item or characteristic of the individual

or subject, and third, applying said separate record-cards successively to circuit-controlling devices acted upon

by the index-points to designate each statistical item represented by one or more of said index-points, substantially as described.

This patent, along with two others, describes a system for tabulating statistical items represented byholes punched in cards The 1890 U.S census was completed $5 million under budget and two yearsahead of schedule because of Hollerith’s system The punch card system with encoded holes (the codefor representing alphanumeric characters with holes was named after Hollerith) was widely used forsorting, counting, and tabulating even into the 1980s Hollerith’s original Tabulating Machine Companywas the forerunner to the computer giant, IBM (Copyright © 1995, DewRay Products, Inc Used withpermission.)

A

TABLE 89.1 List of Input Devices Category Designation Operation Mode Keyboard Alphanumeric Electromechanical Keyboard Function Electromechanical Pointing Light pen Screen pointing Pointing Touchscreen Screen pointing Pointing Pen tablet Tablet pointing Coordinates Digitizer X-Y conversion Coordinates Data tablet X-Y location Cursor Mouse Movement Cursor Trackball Movement Cursor Joystick Movement Image Scanner Conversion Verbal Voice Conversion

FIGURE 89.1 (a) Generalized display-system block diagram (Source: After S Sherr, Electronic Displays, New York: John Wiley & Sons, 1979.With permission.) (b) Alphanumeric keyboard (Courtesy of Key tronic.)

TABLE 89.2 Keyboard Parameter Values Parameter Snap Switch Elastomer Foam Pad Key travel 3.8 mm 3.2 mm 3.8 mm Force >60 gm >50 gm >30 gm Life 10 million cycles 10 million cycles 10 million cycles Feedback Audio mechanical Audio electric Tactile

or “molded boot” is shown in Fig 89.2(a), in which the boot consists of two collapsible domes In this design,the internal movement of the keyswitch is completely silent so that some source of sound must be added toachieve the desired audible feedback The snap switch design shown in Fig 89.2(b) has built-in sound andachieves a small reduction in insertion errors over the elastomer design with audio feedback.

The life requirement is estimated on the basis of workstation users operating at approximately half theaccepted rate of 20 million actuations per key used for electronic typewriters The actual layout and content

of the keyboard may vary greatly, ranging from the standard typewriter arrangement, through different binations of alphanumerics and symbols, to the special-function keyboards that contain legends and symbolsspecific to the particular application However, the outputs of each type are the same in that they must containcoded signals that relate the action to be performed by the information system to that defined by the key beingoperated, in terms of the input code of the system Thus, many of the keyboards output the ASCII code, andthe system is usually designed so that it can accept this type of standard code Incidentally, ASCII, the acronymfor American Standard Code for Information Interchange, is the standard means for encoding alphanumericsand a group of selected symbols for transmission to a display system, among others It is the standard codeused in the United States and most other English-speaking countries and corresponds to the ISO seven-bitcode The seven-bit ASCII is usually used, and it should be noted that for serial data transmission an eighthbit is added for parity Various keyboard arrangements are possible, and many variants are found in particularapplications The means for coding the key operation may be through magnetic reed relays, solid-state circuits,

com-or mcom-ore exotic devices such as Hall effect senscom-ors These device characteristics are only incidental to theoperation and beyond the scope of this chapter Similarly, we do not discuss the human-factors aspects ofkeyboard design, not because they are not important, but because, apart from the visual considerations, theother factors have to do with tactile and physical features best left to others

Light Pen

The light pen initially was a very popular means for accomplishing manual input to the random deflectioninformation display systems, but fell out of favor when raster systems became more popular due to its being

FIGURE 89.2 (a) Elastomer-type keyswitch (b) Snap switch (Source: After H Brunner et al., “Effects of key action design

on keyboard preference and throughput performance,” Micro Switch With permission.)

somewhat difficult to use with raster systems This device goes by a misleading name, as it does not emit lightand is not a pen other than being somewhat similar to one in its physical appearance, as shown in Fig 89.3(a).However, when we consider its functional characteristics, the validity of the term becomes apparent, as it isused to cause the electron beam to “write” patterns on the cathode ray tube (CRT) that are defined by themotion of the light pen on the CRT faceplate.

The light pen operates by sensing the existence or nonexistence of a pulse of light at the point on the screen

of the CRT or surface of any other light-emitting device where the point of the pen is placed This is accomplished

by means of the circuit shown in Fig 89.3(b), where the light pulse is collected and transmitted through thefiber optics to a light-sensitive device that converts the light pulse into an electrical pulse which is shaped bysome form of electronics (of which a Schmitt trigger is one example) We need not concern ourselves with theexact form of the electronics except to note that this pulse is then sent to the computer, as shown in Fig 89.4,and provides a complete, closed-loop system As the electronic pulse occurs at the time when the light pulsepasses under the light pen, the computer is informed of the location at which the designated operation is to

be performed and may proceed accordingly Thus, the light pen is a pointing device that designates a point onthe display screen and can be used as an input device Various light pen programs have been written to expandthe capabilities of the original one, and it should be noted that the light pen is coming back into favor asimprovements in accuracy, ease of operation, and reliability occur

There are two characteristics of light pen operation that affect the capabilities of this input device The first

is the sensitivity, given by

FIGURE 89.3 (a) Light pen (Courtesy of FTG Data Systems.) (b) Light pen schematic (Source: After S Sherr, Electronic Displays, New York: John Wiley & Sons, 1979, p 388 With permission.)

S = ELmpApAmmsmftL (89.1)

where E L = illuminance at photodetector, mp = photodetector sensitivity, A p = preamplifier gain, A m = mainamplifier gain, ms = Schmitt trigger sensitivity, mf = flip-flop sensitivity, and t L = optical loss

Equation (89.1) may be used to calculate the light output required from the display surface, which may be

a CRT or other light-emitting device, but with the limitation that most of the flat panel units are matrix drivenand must track the drive sequence in order to know the location of the light pen from the drive pulse timing.When phosphors are involved as for the CRT, vacuum fluorescent displays (VFDs), thin-film electroluminescent(TFEL) units, and color liquid crystal displays (LCDs), the phosphor delays must be entered into the timing,and the total delay is given by

where E o= voltage at triggering element, E i= voltage equivalent of phosphor light output, t = time, and t =sum of all delays

These delays set limits to the positional accuracy, as the

computer tracking the signal will be in error by this amount

Other inaccuracies are due to the dimensions of the optical

pickup surface, all of which somewhat negate the simplicity

of operation The result is the parameter values shown in

Table 89.3

Data Tablet (Graphics, Digitizer)

A very convenient means for data entry, retaining some of the ease of operation of the light pen but with muchbetter accuracy, are the various forms of data tablets available These tablets differ from the light pen in anothersignificant way in that they do not require a moving spot of light to detect the location of the beam or direct

it to a new location This need for a moving light spot made the light pen difficult to use with the data tabletsinitially designed to overcome this limitation while still using a device with a pen-like input The first successfulexample was the Rand tablet, a digital device that used an X–Y assembly from which a wand placed above somepoint on the X–Y wire matrix could pick up pulse generator output that fed X and Y electrical pulses into thematrix By determining the number of pulses in a time period, the location of the wand is established Anothersimilar device used magnetostrictive rather than electrical signals to accomplish the same result, and this location

is converted into display coordinates used to position a cursor on the CRT screen The cursor may then be

FIGURE 89.4 Block diagram of light pen computer system (Source: S Sherr, Electronic Displays, New York: John Wiley & Sons, 1979, p 389 With permission.)

TABLE 89.3 Light Pen Data Field of View Response Time Sensitivity 0.02–0.08 in 120–150 ns 0.02–0.04 ft.L

used as a visual feedback element so that the operator can correct the position of the wand until the cursor isproperly placed At this time the information from the tablet may also be transferred to either the host computer

or the resident desktop or portable computer, as desired Since the cursor is not used to signal its position to

a pickup device, as is the case with the light pen, it may be used with any type of display system, including thenon-light-emitting flat panel displays Another advantage of the tablet is that it may be used to position cursors

in the blank areas of the display, where no light pulses are available unless they are specially generated by thelight pen

There have been numerous improvements and new developments using a variety of technologies that includemagnetostrictive, electromagnetic, electrostatic or capacitive, scanned X–Y grid, resistive, and sonic Of these,electromagnetic tablets dominate the digitizer market, and sonic is of interest because it does not require atablet, but most of the other technologies are essentially restricted to touch input devices covered later As notedpreviously, electromagnetic is the most popular technology for high-performance digitizer tablets Operation

is based on transformer principles, whereby a conductor carrying ac creates a magnetic field around it thatinduces a current in a second conductor The digitizer tablet uses the amplitude and phase of the inducedcurrent to determine digitizing data The tablet contains an X–Y pattern of conductors beneath its surface, in

a manner similar to the Rand Tablet, but instead of counting pulses in a time period a circular conductor isused as the pick-up element for the induced current This coil is placed on the tablet surface, and its position

is determined by measuring the phase and amplitude of the current in the coil Its center is interpolated bysweeping through the X–Y grid lines and demodulating the signal in the coil to determine the phase reversalpoint, or by calculating this point using digitized data fed into a microprocessor The X–Y coordinates may beresolved to better than 0.025 mm using either of these two techniques Figure 89.5(a) is a photograph of arepresentative digitizer tablet

Another digitizer technology is the one that uses the measurement of the time required for sound waves totravel from a source to movable microphone pickups.This sonic technology has the advantage that no specialdigitizing board is required, and either a stylus or a cursor can be used as the digitizer Two sonic sources arecontained in an L frame so that both X and Y coordinates can be determined by calculating the time it takesfor the sound wave to reach the microphones contained in the pickup device This calculation is made on thebasis of sound traveling at 345 m/s at 20°C, and the accuracy is dependent on stable ambient conditions Thistends to limit the resolution to about 300 lpi, and the accuracy to ±0.1% The device may also be implementedwith a single sonic source as the digitizing means and a pair of microphones located outside the digitizing area

In this case the location of the transducer is calculated by triangulation and converted into Cartesian coordinates.Digitizers are used primarily for inputting accurate coordinate data from maps and engineering drawings.Their high accuracy requirements have led to relatively high prices Alternative means for inputting data arethe data and graphics tablets that meet most input requirements at a lower cost and accuracy The maintechnology is still electromagnetic, and the units are essentially the same as the digitizers, but with loweraccuracies However, several of the other technologies have also been used to achieve lower costs Most successfulamong them are the capacitive and resistive versions, which may also be used as digitizers The capacitive units,also termed electrostatic, use capacitive coupling where the coupling between the tablet and the cursor or stylus

is determined by the capacitance made up of the tablet surface as one plate and the pickup element as the other

In this case, the capacitance is given by

where C = capacitance, e = permittivity of dielectric, A = relative area of two plates, d = distance betweenplates, and f = proportionality factor

A scanned grid approach is used to determine the location of the cursor As in the electromagnetic tablet,

an X–Y grid of conductors is embedded in the tablet, with semiconductor switches on each line providingcontact on a scanned basis The charge flowing from each capacitance is summed through a summing amplifier

as shown in Fig 89.5(b) The resultant voltage peaks twice, once for the X and once for the Y lines, as they arescanned The peak positions are digitized by means of a counter that starts at the beginning of the scan, andruns at some multiple of the scan rate The digital values represent the coordinates of the cursor location

The mouse has gone a long way from its original invention by Engelbart in 1965, through its redesign at Xeroxand introduction by Apple as a main input device, and its general acceptance by computer users as an importantaddition to the group of input devices It should be noted, in passing, that the mouse is essentially an upside-down trackball, although the latter is now being referred to as an upside-down mouse However, the trackballcame first and is described further in the next section

FIGURE 89.5 (a) Digitizer tablet (Courtesy of Numonics.) (b) Capacitive technology (Source: After T E Davies et al.,

“Digitizers and input tablets,” in Input Devices, S Sherr, Ed., New York: Academic Press, 1988, p 186 With permission.)

Mice contain motion-sensing elements and are operated by moving mechanical or optical elements One

form uses wheels and shafts to drive the sensing elements, as shown schematically in Fig 89.6 The angular

velocity (w) of the wheel and shaft is given by

where V r= velocity of wheel and R = wheel radius

The rotation angle (q) is given by

where X = distance moved

This type of mouse has two sets of wheels and shafts, one for horizontal and the other for vertical motion

A more popular type of mechanical mouse is the one that uses a ball for the motion sensing device, as shown

in Fig 89.7 Again, the velocity of the ball circumference equals the velocity of the mouse, and the angular

velocity is given by

where R1 = shaft radius

The smaller the shaft the more rapid its rotation for a given

mouse velocity Another form of the ball-and-shaft mouse is

the one that uses an optical interrupter, as shown in Fig 89.8

In this form, the light from the light-emitting diodes (LEDs)

is interrupted by the coded disks that are rotated by the shafts,

and is then picked up by the phototransistors and converted

into the digital signal that represents the disk rotation An

optical interrupter is also used for the optomechanical mouse,

and here the interrupter contains a set of slots; as the

inter-rupter rotates quadrature signals are created that correspond

to the shaft rotation

In addition to the shaft and optomechanical mice, an early form of mouse used multiturn potentiometers

connected to the wheels, and the output voltage that represented the motion varied in direct proportion to the

mouse motion The voltage was then converted by means of an analog-to-digital converter into digital form

for input to the computer

Finally, there are the true optical mice that use a special surface that is printed with a set of geometric shapes,

usually a grid of lines or dots, that are illuminated and focused on a light detector The most common form

uses a grid made up of orthogonal lines, with the vertical and horizontal lines printed in different colors These

colors absorb light at different frequencies so that the optical detectors can differentiate between horizontal

FIGURE 89.6 Wheel showing velocities and slip angle (Source: After C Goy, “Mice,” in Input Devices, S Sherr, Ed., New

York: Academic Press, 1988, p 225 With permission.)

FIGURE 89.7 Ball and shaft (Source: C Goy,

“Mice,” in Input Devices, S Sherr, Ed., New York:

Academic Press, 1988 With permission.)

and vertical movement of the mouse If such a structure is used as the mouse, then the photodetector will pick

up a series of light-dark impulses consisting of the reflections from the mirror surface and the grid lines andconvert them into square waves A second LED and photodetector that is mounted orthogonally to the first isused to detect motion in the orthogonal direction, and the combination of the two inks avoids confusionbetween the two directions of motion The system then counts the number of impulses created by the mousemotion and converts the result into motion information for the cursor This type of mouse has the advantagethat no mechanical elements are required

The typical trackball has an X and Y optical encoder that generates a pulse for each 0.76 mm of incremental

motion of the ball This means that the pulse train may range from 10 to 2500 pulses per second (pps), depending

on how fast the ball is rotated This is much more rapid than required for satisfactory updates, which need not

be greater than about 100 times per second This can easily be accomodated by the RS-232 protocol using aneight-bit word Thus, the trackball is an excellent alternative for the mouse, and is rapidly returning to apreferred position as an input device

Joystick

The joystick has not achieved much acceptance as an input device for electronic display systems, except forvideo games, although it has been the preferred control for many types of aircraft However, it can be used tosome extent in display systems other than those used in video games, and therefore warrants inclusion in thissection There are two basic types of joysticks, termed “displacement” and “force-operated” A typical displace-ment joystick is shown in Fig 89.10, and may have two or three degrees of freedom The activating means mayvary from as few as four switches mounted 90 degrees apart, to full potentiometers for analog output, andoptical encoders for digital output A third axis may be added by allowing the handle to rotate and drive a thirdpotentiometer Spring forces of 5 to 10 lbs are usual for the other two axes, and displacements go from 6 to

30 degrees

FIGURE 89.8 Optical interrupter (Source: C Goy, “Mice,” in Input Devices, S Sherr, Ed., New York: Academic Press, 1988,

p 229 With permission.)

The force joystick operates by responding to pressure on the handle to generate the X–Y coordinates It may

be either a two- or three-dimensional version, with the same types of handles as for the displacement joysticks.However, it is difficult to use a rotating handle for the third dimension because some force is usually transmitted

to the other dimensions causing crosstalk Therefore, a separate lever is preferred The force is detected bymeans of piezoelectric sensors that are bonded to the handle rod, and a voltage source is applied across thenetwork, as shown in Fig 89.11 The output is taken from the strain gauge and the analog voltage will beproportional to the amount of force The same type of protocol and output circuitry may be used as for thedisplacement unit, and both can generate either position or rate data An exponential curve with a dead zonethreshhold is preferred for pulse rates in order to avoid starting pulse rate uncertainties, with the first pulsestarting as soon as the threshhold is exceeded

Touch Input

Touch input devices come in two basic forms, either placed directly on the display surface, or as a separatepanel attached to the computer system In its second form it is essentially a data tablet differing mainly in that

it acts as another display unit with some form of a touch-sensitive surface In this implementation it is the

FIGURE 89.9 Trackball (Courtesy of CH Products,Vista,

Calif.)

FIGURE 89.10 Three-axis displacement joystick (Courtesy of CH Products, Vista, Calif.)

FIGURE 89.11 Schematic connections in a force joystick (Source: After D Doran, “Trackballs and joysticks,” in Input

Devices, S Sherr, Ed., New York: Academic Press, 1988, p 260 With permission.)

same as the Touchscreen input device, and this discussion concentrates on the technologies used for

Touch-screens There are five different technologies used for touch input devices, which are capacitive or resistiveoverlays, piezoelectric, light beam interruption, and surface acoustic wave The system may be divided into thesensor unit, which senses the location of the pointing element, and the controller that interfaces with the sensorand communicates the location information to the system computer Since the controller is an electronic devicethat does not use technology different from the computer it is not covered here The main differences amongthe different touch input devices are due to the choice of sensor technology, and the discussion concentrates

on these technologies

Capacitive Capacitive overlay technology is illustrated in

Fig 89.12 where a transparent metallic coating is placed over the

display screen and the finger or stylus capacitance is sensed to

determine the touch location The overlay may consist of a group

of separate sections etched into the surface with each separate

section connected to the controller, or a continuous surface

con-nected at the four corners The first form is termed discrete

capac-itive, and touch location is determined by having each section

sequentially connected to an oscillator circuit where the frequency

of oscillations is affected by the pointing device The oscillation

frequency is measured and compared to a stored reference

fre-quency If the frequency difference is large enough then it is

rec-ognized as a touch at that location It is a simple system, but suffers

from low resolution and slow response so that it is only practical

for menu selection

The analog capacitive system uses the same metallic overlay, but

the metallic surface is continuous rather than etched The

connec-tions at the four ends are each connected to a separate oscillator,

and the frequency of each is measured and stored Then when the

overlay is touched the change in capacitance will have a different

effect on the frequency of each oscillator These are measured and

the differences are used to determine the coordinates of the touch

by means of an algorithm This technique is capable of much higher

resolution (250 ´ 250) than the digital approach and is preferred for graphics or other high-density displays

in Fig 89.13 The layers both contain transparent metallic surfaces and are separated by spacers so that an airgap exists between the layers in the absence of any pressure on the touch panel The metallic layers face eachother and when the outer panel is pressed the metallic layers make contact and form a conductive path at thepoint of contact When a voltage is applied between the top of the outer layer and the bottom of the inner

layer, the two layers act as a voltage divider, and the voltage at the point of contact may be measured in the X and Y directions by applying the voltage in first one and then the other direction The measured voltages are

then transmitted to the controller where they are converted into coordinates which are then sent to the computer.The panel may be discrete, in which the conductive coating on the top layer is etched in one direction andthat on the bottom layer in the other direction, or analog, where the conductive coatings in both layers are

continuous In the discrete case, the panel then acts as an X–Y matrix, and the resolution is determined by the

number of etched lines The analog configuration requires the addition of linearization networks on each edge

of the panel so that a large-area resistor is created with a voltage drop in one direction Other linearizationtechniques are also possible, but only the four-element system is described here as shown in Fig 89.14 In thisarrangement, one of the layers acts as the large-area resistor and the other as a voltage probe where either can

function in either role For the Y coordinate value the top layer is the voltage probe, and the voltage is applied

by the controller to the bottom layer Similarly, the X coordinate is found by connecting the voltage to the top

layer and making the bottom layer into the voltage probe In either type of system, the resolution can be veryhigh, but the transmissivity is reduced to under 80% due to the multiple layers

FIGURE 89.12 Capacitive overlay technology.

(Source: After A B Carrell and J Carstedt,

“Touch input technology,” SID Sem Lecture

Notes, p 15.30, 1987 With permission Courtesy

Society for Information Display.)

Piezoelectric The piezoelectric technology uses pressure-sensitive transducers as the means for determining

the location of the touch, as shown in Fig 89.15 The sensor is a glass plate with transducers connected to thefour corners Pressure on the plate causes readings to occur at each of the transducers, which depend on thelocation of the pressure Thus, the controller can measure the readings and obtain the coordinates by means

of a proper algorithm This technique allows a high-transmissivity plate to be used that can be curved to followthe CRT face plate curvature, but it allows only a limited number of touch points to be used

and detectors facing each other in the X and Y directions When the beams from the X and Y light sources are

interrupted, this is sensed by the facing light detectors and the signals are sent to the controller The light beams

FIGURE 89.13 Resistive overlay technology (Source: After A B Carrell and J Carstedt, “Touch input technology,” SID Sem.

Lecture Notes, p 15.31, 1987 With permission Courtesy Society for Information Display.)

FIGURE 89.14 Four-wire analog resistive (Source: A B Carrell and J Carstedt, “Touch input technology,” SID Sem Lecture

Notes, p 15.32, 1987 With permission Courtesy Society for Information Display.)

FIGURE 89.15 Piezoelectric technology (Source: A B Carrell and J Carstedt, “Touch input technology,” SID Sem Lecture

Notes, p 15.34, 1987 With permission Courtesy Society for Information Display.)

are turned on sequentially by pulsing the LEDs and thus create a full matrix of light beams without requiringeach of them to be on continuously This system does not reduce the screen transmissivity as there is noobstruction of the screen output, but it is limited in resolution to the number of LED detector pairs that can

be placed on the periphery of the screen

Another approach to light interruption is to use a rotating beam of light, which has the advantage that onlyone light source and detector pair is required This technology is depicted in Fig 89.16 and consists of a LEDand a light detector placed inside a rotating drum which has a slit that allows light to be transmitted outsidethe drum The light is swept across the surface and strikes the retroreflectors that sends it back directly to thedetector The beam scan is sampled 256 times on each scan, and Fig 89.16 shows how two angles of interruptionare created, angle B by direct interruption, and angle C by mirror reflection interruption The result is that thelocation of the interruption can be calculated by comparing the two angles Again, there is no obstruction ofthe screen but a moving element must be added, and parallax errors may occur

input device comes in several forms, each of which can recognize hand printing with the special operatingsystem and software recognizing this type of input The pen-based input device comes in several forms, ofwhich the one termed TouchPen™ can function both as a digitizer with a touch tablet, and as the touch inputdevice with a touch input pen-based computer system A second one is that developed by Wacom, Inc., primarilyfor the GO Systems computer, but used by other pen-based systems as well Finally, a third unit is that made

by Scriptel Corp and used by Wang Laboratories in its system

TouchPen™ was developed by Microtouch Systems, Inc., initially for use in GridPad made by the Grid SystemsCorp It is essentially a high-resolution digitizer consisting of an all-glass tablet that can be used with a number

of stylus input operating systems to digitize handwriting It is basically a touch input device using resistivetechniques to digitize the handwriting appearing on the display surface of pen-based computer systems Theglass tablet is placed on the display surface and the system pen is used to transmit the digitized data to thecomputer As noted previously, the tablet may also be used as a standard touch input device

The second form of pen-based input device is one that uses electromagnetic technology and consists of agrid of wires that transmit radio waves that are picked up by a tuned circuit in the stylus This circuit resonates

at its own frequency and transmits that signal back to the wires at the grid location it is touching The pen alsotransmits its signal to the computer, which turns off the grid transmission, and locates the position of the pen

by determining which of the grid wires pick up the pen signal The pen does not need to actually touch thedisplay surface and does not require any power, which is an advantage somewhat counteracted by the higher cost.Finally, the Scriptel unit is similar to that made by Microtouch, but differs in that it uses electrostatictechnology and is also similar to the capacitive touch panel

FIGURE 89.16 Rotating infrared beam technology (Source: A B Carrell and J Carstedt, “Touch input technology,” SID

Sem Lecture Notes, p 15.34, 1987 With permission Courtesy Society for Information Display.)

Surface Acoustic Wave (SAW) This technology is

more recent than the others and has not received wide

acceptance as yet It is based on the transmission through

the glass of SAWs generated by transducers mounted on

the glass overlay These waves are detected by receivers

also mounted on the glass, and the time of arrival of the

waves at the receivers is known because the wave velocity

is known The placing of a finger on the glass weakens

the signal and the location of the finger can be

deter-mined by the difference in its effect on the SAW

There are two types of SAW systems in use, namely

those using reflective techniques and those using

atten-uation as the source of position information The

reflec-tive systems are similar to sonar where the time from the

source to the pointing finger and then from the finger

to the receiver is measured to arrive at finger location

The attenuation technology is illustrated in Fig 89.17

and consists of two transducers, two receivers, and four

reflector strips, all mounted on a glass substrate One

transducer-receiver pair is used for X and the other for

Y location Figure 89.17 shows the X axis pair, and the

transducer transmits a burst of acoustic energy in a

hor-izontal wave The wave is partially reflected by the top

reflector strips and travels down to the bottom strip where the reflectors are at an angle such that it is reflected

to the lower left corner receiver The wave now has a long rectangular shape, and each point in time corresponds

to a specific vertical path across the substrate The Y axis is scanned in the same fashion after the X wave dies

out Then, when the finger touches the substrate, its water content absorbs some of the energy in the wave,and the wave is attenuated The dip in the wave amplitude corresponds to the amount of absorbed energy, andthe time of the lowest point can be determined, allowing the location of the finger to be calculated Finally, in

addition to the X and Y coordinates, a Z coordinate can be determined, depending on how hard the user presses.

This depends on surface contact, which affects the amount of attenuation The advantages of this system are

high resolution, speed of transmission, and the availability of a Z axis component Its main disadvantages are

the variation in moisture content in fingers and sensitivity to local moisture on the substrate However, it isbeing used in developmental units and should be considered as another input device technology

Scanners

Scanners are a means for inputting text and/or images directly into the computer system, thus avoiding theneed for retyping and redrawing information contained in other sources It is a relatively convenient way toavoid repetition if the data to be entered already exist in readable form This is done by special image-recognitionsoftware that accompanies the scanning hardware, and can transfer an entire image containing both text andillustrations, but without the capability to modify the image However, the addition of optical characterrecognition (OCR) software allows the entered text to be modified as if it were entered by typewriter This cangreatly simplify entering and editing text from some preexistent source and has resulted in a proliferation ofdevices that can perform this function

These devices come in two main forms, hand-held and page scanners, with or without OCR software in

addition to the standard image-recognition software A typical hand-held scanner is shown in Fig 89.18 and

it consists of a light source, a light-sensitive device such as a charge-coupled device (CCD) array, and theelectronics to actuate the elements of the array sequentially under software control The scanner window isplaced over the page, and is moved down or across the page so that the window covers as much of the page asfalls within the capability of the software The light source is reflected from the page to the CCD and the charge

in the CCD is modified by the reflectivity of the printed material

FIGURE 89.17 Attenuation SAW technology (Source:

A B Carrell and J Carstedt, “Touch input technology,”

SID Sem Lecture Notes, p 15.35, 1987 With permission.

Courtesy Society for Information Display.)

The window area ranges from 4 to 5 in in width by 0.5 in in height

and may be moved through 14 to 20 in., so that a fairly large area may

be covered in a single manual scan Images wider than the maximum

window may be scanned in two passes, and the OCR software can stitch

the two scans together into a single image, although this procedure

requires considerable care in scanning so that the scans line up properly

Therefore, when images wider than the window of the hand-held

scan-ner are to be scanned, it is advisable to use a flatbed scanscan-ner of the type

shown in Fig 89.19 which can handle a full 8.5 in by 11 in page, or

some of the larger scanners than can accept large drawings and input

them into the computer system Resolutions of 400 dpi and higher, with

up to 250 levels of gray and 24 bits of color resolution are available

Thus, scanners offer a wide variety of choice and performance

capabil-ities, and are powerful input devices when prepared data in visual form

is to be entered into the computer system

Voice

Voice input is an intriguing approach to data input, with particular

attractiveness to managers who want a simple and direct means for

inputting data and commands For many years, this technology tended

to promise more than it could achieve, but recent developments have brought it to the point where it can beconsidered as a viable input means This has been due to new developments in software that make it possible

to minimize the amount of training required and increase the success rate to close to 100%

One basic approach to speech recognition is represented by the block diagram shown in Fig 89.20 This is

a system that is built around a special chip developed by Texas Instruments This system uses templates andspecial algorithms for recognizing the input speech patterns The system is speaker dependent, with thecapability of storing up to 32 word templates and user-defined phrases The output portion may be superfluouswhen the system is used only for inputting data and commands, but can be a useful adjunct to the visualresponse Other techniques such as speaker-independent and phoneme-recognition systems are also available.Vocabularies range from 50 to 5000 active words, and both isolated and connected words can be recognized,although the larger numbers tend to be associated with isolated word systems In general, it seems feasible that

a combination of speech input and pen-based computing may find a viable market

FIGURE 89.18 Hand-held scanner (Courtesy of Logitech, Freemont, Calif.)

FIGURE 89.19 Page scanner tesy of Chinon, Torrance, Calif.)

The multiplicity of input devices that are available makes it difficult to determine which is most suitable forany specific set of requirements However, the limited functional comparison of the input devices covered inthis section shown in Table 89.4 may be of some use, and in any event is a starting point in this evaluation Itshould be noted that what appears best at one time may become unpopular or obsolete at a later time, asoccurred for light pens and trackballs, both of which have come back into favor

In addition to the generalized evaluation shown in Table 89.4, it is also of interest to examine representativeperformance parameters These are shown in Table 89.5 and while representative do not necessarily cover therange of performance parameters offered More data may be obtained from the vendors of these devices

Advantages and Disadvantages

Input devices make up one of the functional groups of the display systems, and their technical characteristicsare covered in some detail at the beginning of this chapter, with performance information provided in Table 89.5containing characteristic parameter values for each type, as available The following material expands somewhat

on that information by placing these devices in the context of a full graphics display system and evaluating thefunctions that the various types of input devices perform in that type of system in terms of their advantagesand disadvantages It is of some interest to compare the advantages and disadvantages of each type at this point,

as listed in Table 89.6 This is an imposing list and may be used to aid in choosing the best input devices forspecific applications It also concludes this section on input devices

FIGURE 89.20 Block diagram of speech recognition and synthesis chip (Source: After M Leonard, “Speech poised to join man-machine interface,” Electronic Design, pp 43–48, September 26, 1991 With permission.)

TABLE 89.4 Input Device Functional Evaluation

Function Input Device Control Data/Text Data/Graphics Total Keyboard E E P 9 Light Pen G G E 10 Tablet E G E 11 Mouse E F E 11 Trackball E G E 11 Joystick F F G 5 Touchscreen G F G 8 Scanner F E G 9 Voice G F P 6 Total 29 23 28 80

E = Excellent = 5; G = Good = 4; F = Fair = 3; P = Poor = 2

Defining Terms

Data tablet/digitizer: A device consisting of a surface, usually flat, and incorporating means for selecting aspecific location on the surface of the device and transmitting the coordinates of this location to acomputer or other data processing unit that can use this information for moving a cursor on the screen

of the display unit

Joystick: An input device somewhat in the form of the navigation control device found in early aircraft andoperating in a somewhat similar manner by generating series of pulses whose frequency or numberdepend on how far, with what force, and in what direction the control stick is moved from the centralposition

Keyboards: Electromechanical devices consisting of sets of keys labeled with alphanumeric, numeric, andfunctional designations that enable the user to describe and define the operation to be performed

Light pen: Neither a pen or a light source but rather an input device in the shape of a pen that operates bysensing the existence or nonexistence of light pulses at specific locations on the surface of a display deviceand uses this information to signal the computer as to the location of the pen

Mouse: An input device based on a much older type known as a trackball and fancifully named because itbears only a casual resemblance to a mouse It consists of a roller ball that is moved on a flat surface and

causes orthogonal potentiometers or other types of X–Y-position signal generators to move and produce

electrical signals defining the desired coordinates of the cursor on the screen so that the cursor can bemoved to that position

TABLE 89.5 Representative Performance Parameters Input Device Parameter Value Light pen Response time 150–500 ns

Spectral response 4200–9500 A Luminous sensitivity 0.03–0.7 nts Field of view 0.02–0.1 in.

Ambient rejection 350 nts Data tablet (digitizers) Resolution (l/in.) 100–2000

Accuracy (in.) 0.0005–0.02 Active area (in.) 12 ´ 12–60 ´ 120 Active height (in.) 0.02–2.5 Digitizing rate (pps) 100–350 Transducers Stylus, puck, cursor

Speed 1–20 in./s Accuracy 25–1000 dpi Trackball Resolution 100–1000 cpi

Speed 1200–9600 BPS Accuracy 100–1000 dpi Ball diameter 1.5–2.5 in.

Scan rate (in./s) 0.5–2.0 Scanning width (in.) 4.1–36 gray shades 32–256 Scan time (s/page) 1–30

Voice Active vocabulary 13–5000 words

Scanners: Means for converting hard copy into electrical signals that can be entered into a computer or dataprocessing system The usual means for accomplishing such conversion is to move a light beam over thesurface containing the data either by hand or automatically and using arrays of light-sensitive devices toconvert the reflected light into electrical pulses.

Touch input: A means for selecting a location on the surface of the display unit using a variety of technologiesthat can respond to the placing of a finger or other pointing device on the surface These are essentiallydata panels placed either on the display surface or between the user and the display surface

Trackball: The earliest version of an input device using a roller ball, differing from the mouse in that the ball

is contained in a unit that can remain in a fixed position while the ball is rotated It is sometimes referred

to as an upside-down mouse, but the reverse is more appropriate as the trackball came first

Voice: Means for enabling a computer or data processing system to recognize spoken commands and inputdata and convert them into electrical signals that can be used to cause the system to carry out thesecommands or accept the data Various types of algorithms and stored templates are used to achieve thisrecognition

Related Topic

89.2 Computer Output Printer Technologies

TABLE 89.6 Input Devices—Advantages and Disadvantages Device Advantages Disadvantages Keyboard Simple operation Requires many keys

Well known Requires training Standard interface No graphics Light pen Eye-hand coordination Arm fatigue

Low cost models Limited resolution

No desk space required May block display Graphic tablet Natural hand movements Eye-hand conflict

Screen not blocked Requires desk space

No parallax Breakable stylus Good for graphics Poor for A/N entry Mouse Small space needed Some space needed

Low cost Slow transmission Screen viewing Low resolution Any surface may be used Grid for optical (Optical) noiseless Mechanical noise Trackball High resolution Poor for A/N input

Fixed desk space Slow transmission Screen viewing Mechanical noise Tactile feedback 3-D difficult Joystick Fixed desk space Low accuracy

Low fatigue Low resolution Low cost No A/N input Touchscreen Eye-hand coordination Arm fatigue

Minimal training May block display Minimum input errors Varied resolution User acceptance Parallax

No special commands Slow data entry Scanner Full A/N page input Hand scanner width

Color scan input High cost for color High resolution Slow input OCR software Compatibility Voice Ease of use Limited words

Minimal training Machine training

No special devices Graphics difficult

H Brunner et al., “Effects of key action design on keyboard preference and throughput performance,” MicroSwitch

A.B Carrell and J Carstedt, “Touch input technology,” SID Sem Lec Notes, pp 15.30–15.35, 1987.

T.E Davies et al., “Digitizers and input tablets,” in Input Devices, S Sherr, Ed., New York: Academic Press, 1988,

p 186

D Doran, “Trackballs and joysticks,” in Input Devices, S Sherr, Ed., New York: Academic Press, 1988, pp.

251–262

C Goy, “Mice,” in Input Devices, S Sherr, Ed., New York: Academic Press, 1988, pp 225–232.

M Leonard, “Speech poised to join man-machine interface,” Elec Des., pp 43–48, Sept 26, 1991.

S Sherr, Electronic Displays, New York: Wiley, 1979, pp 323, 388–389.

Further Information

Electronic Displays, 2nd ed., by Sol Sherr and published by John Wiley & Sons, Inc., contains an extensive and

detailed discussion of other aspects of display systems and technology, as well as a somewhat expanded version

of this section In addition, Input Devices, edited by Sol Sherr, and Output Hardcopy Devices, edited by Robert

C Durbeck and Sol Sherr, both published by Academic Press, include extensive discussions of a wide variety

of devices

The Society for Information Display (SID) sponsors a yearly symposium at which a large amount ofinformation on new developments in information display as well as tutorials and seminars on basic informationdisplay topics are presented and made available in published form In addition, it publishes two journals,

namely, Proceedings of the Society for Information Display and Information Display Other relevant meetings and

publications are those sponsored by the Computer Society and Electron Devices groups of the IEEE, theSIGGRAPH group of the Association for Computing Machines (ACM), and the National Computer GraphicsAssociation (NCGA)

89.2 Computer Output Printer Technologies

Robert C Durbeck

Electronic printers for computer output represent a very important part of the computer industry They rangefrom small, inexpensive printers for personal computers and workstations to very large and fast page printersused for bulk printing output for large-scale computer systems The technologies employed for this wide scope

of printing requirements are diverse: some based on “impact” methods to transfer ink from a sheet or ribbon

to paper, others based on more sophisticated “nonimpact” methods Today, there is no single technology whichcompletely dominates A wide range of user needs has led to the present proliferation of printer technologies.The most prevalent are discussed in the following

Classification of Printer Technologies

Table 89.7 illustrates the two main classifications of printer technologies and also the wide range of extanttechnologies Those technologies listed under line impact and page printing are used for large computer system

printing; by far the most popular are fully formed character and electrophotographic (so-called laser printers) The most common printing technologies used for personal/workstation computer systems are electrophoto- graphic, ink jet, and serial wire matrix Emphasis in the following is directed to these favored technologies, with

brief descriptions of the others included

Page Printer Technologies

By far the most important page printer technology is electrophotography (EP) EP, as well as the much lessemployed ionographic and magnetographic technologies, uses “powder toning” development of an intermediate

“image” created in the process Liquid toners are also used to develop electrostatic images Thermal page printersemploy a full-page-width linear array of thin-film heater elements to melt and transfer ink from a ribbon tothe paper or to mark special thermal-sensitive paper.

Electrophotographic Printing

EP printers use essentially the same technology found in most “plain paper” copiers, the major exception beingthe printhead Instead of using page- or line-imaging optics as in a copier, the printhead utilizes a solid-statelaser (usually GaAlAs) or gas laser (typically HeNe) to scan across and expose a photoconductor drum or belt

to create a “latent image” (see below) A few EP printers use stitched arrays of light-emitting GaAs1–xPx diodes(LEDs) with SelfocÔ glass fiber optics or an array of liquid crystal shutters, the latter to modulate light from

a bright line light source Other possibilities are electroluminescent, magnetooptic or electrooptic arrays, butthese have not been commercialized to any extent

There are basically six major steps employed in the EP printing process (see Fig 89.21): (1) uniform charging

of the photoconductor (PC) electrostatically; (2) exposing the PC to the image light pattern, which results inselective discharge of the area charge created in Step 1, creating an electrostatic image; (3) developing the PC

by bringing electrostatically charged toner particles (black or colored) to the surface of the PC where theyselectively adhere to appropriately charged regions; (4) electrostatically transferring the toned image from the

PC to the final medium (usually paper); (5) thermal fusing of the toner to the paper; and (6) cleaning residualtoner from the surface of the PC to allow reinitiation of the six step cycle

TABLE 89.7 Types of Printer Technologies Impact Nonimpact Line impact Page printing Fully formed character Electrophotographic Dot band matrix Ionographic Shuttle hammer matrix Magnetographic

Electrostatic Thermal Serial impact Serial nonimpact Fully formed character Ink-jet Serial wire matrix Continuous

Piezoelectric/impulse Thermal/bubble-jet Thermal

Direct (thermal paper) Thermal transfer Resistive ribbon

Figure 89.21 The six basic electrophotographic printer process steps: charge, image, develop, transfer, fuse, and clean.

Step 1—Charging the PC. The most common approach used is corona charging One or more thin coronawires (typically tungsten) are supported directly above the PC and are energized to 5–8 kV The resultant highelectric field surrounding the wire causes electrons in the immediate region to be accelerated to energiessufficient to ionize local air molecules Either positive or negative ions are then attracted to the outer surface

of the PC (which, when unexposed to light, acts for a short period of time as an insulator) depending on thesign of the potential difference At the same time, a counter-image charge is formed on the inner side of the

PC The two corona structures most commonly used are the corotron and the scorotron (see Fig 89.22) Thegrid on the scorotron is used to more precisely control the resultant voltage charge level on the PC (approximatesthe grid voltage) Both dc and ac designs are used; the latter usually include a glass sleeve around the coronawire to reduce localized high-emission spots on the wire due to contaminants To save on cost for very low-cost EP printers and to reduce corona by-products (e.g., ozone), a lower-voltage conductive elastomer chargeroll in direct contact with the PC has also been used in place of the corona wire

the PC If the PC is discharged in areas that will be printed white, the overall process is termed charge areadevelopment (CAD); if the discharged area will be printed black (or color), the process is called discharge areadevelopment (DAD) Both CAD and DAD processes are used in EP printers but CAD is the only commonprocess used in copiers

Selective discharge of the PC involves two steps: (1) photogeneration of electron-hole pairs and (2) transport

of the electrons and holes in opposite directions under the influence of a high-dc bias field, locally dissipatingthe surface charges created in Step 1 (see Fig 89.23)

Both organic and inorganic PC materials are used (see Fig 89.24) A variety of generation and transport material systems have been developed for organic PCs; most use separate layers for charge generationand charge transport Examples of efficient organic charge-generation materials sensitive at both GaAlAs (7800Å) and HeNe (6328 Å) wavelengths include squarylium and thiopyridium dyes in an appropriate binder layer(»0.5 mm thick) The charge-transport layer (CTL) consists of a thicker layer (20–30 mm) of a charge-transport

charge-Figure 89.22 Two types of coronas for charging the photoconductor: the scorotron and the corotron.

Figure 89.23 Light is used to selectively discharge the photoconductor Electron-hole pairs are photogenerated in the charge-generation layer, followed by charge transport under a dc bias field and then selective neutralization of the surface charges, thereby creating an electrostatic image.

molecule dispersed in an inert binder, or simply a good charge-transporting polymer (e.g., polyvinylcarbazole).Since most polymer transport materials are essentially hole-transport materials, this requires that the PC surfacecharge produced in Step 1 be negative The CTL must be transparent at the imaging wavelength.

Examples of inorganic PC materials include amorphous chalcogenide alloys such as a-Se, a-As2Se3 and doped Se Most do not have a high sensitivity at GaAlAs wavelength, but a-Si does a-Si also offers other desirableproperties (e.g., durability and lack of fatigue), but is quite expensive to produce

Te-The laser beam is scanned linearly over the PC in a direction orthogonal to the PC motion; the combinedmotions covering a “page” The most common gas laser scanning technique employs a high-speed rotatingpolygon mirror along with beam-expanding optics, an acoustic-optic modulator (not required for solid-statelasers) and an f-q imaging lens To produce a quality image, the multifaceted scanning mirror system must beessentially free of facet defects, up-and-down wobble, variations in polygon rotational velocity and lack ofsynchronization with the pixel clock Some nonplanarity of the facet surfaces can be corrected with anamorphicoptics LED arrays and liquid crystal shutter systems do not have these same technical challenges but, so far,they are more expensive to produce

Step 3—Developing the PC There are basically three development techniques: dual component,

monocom-ponent and liquid development The first two use powder toner Until the advent of EP printers for personaland workstation computers, the most common method of development was dual component, where polymer-coated magnetic carrier beads are mixed with the toner particles and development is done with a “magneticbrush.” This technique is still prevalent in high-end printers With this approach, the 5–20 mm toner particles(consisting mainly of resin plus carbon particles or colorant) are triboelectrically charged by repeated contactsduring mixing with the much larger (60–250 mm) magnetic carrier beads The toner particles then electrostat-ically adhere to the opposite-sign charged carrier beads Charge control agents (e.g., complex organometallicsalts) are often included in the toner composition to control the charge level, rate of charging, and consistency

of charge The mix is mechanically directed to a nonmagnetic rotating shell which has fixed magnets locatedwithin its core, adjacent to the gap between the PC and the shell (see Fig 89.25) The gap is typically 0.5 to 6

mm, depending on the specific system As the shell rotates, the mixture is carried to the gap, and chains ofcarrier beads (coated with toner—the “magnetic brush”!) form along the local magnetic field lines These fieldlines are approximately perpendicular to the shell at the smallest gap In addition, a development voltage(200–500 V) is applied between the PC and shell This provides a high field in the gap whose local value isdetermined by the applied voltage, gap dimension and the electrical properties of the material mix in the gap

Figure 89.24 Spectral absorption characteristics of several photoconductor materials: (a) squarylium dye, (b) SeTe, (c)

As2Se3, and (d) a-Si.

The electric field at the end of the last carrier bead (next to the PC) in a chain may be up to 50 times thenominal unfilled gap field, the value being greatest with uncoated carrier beads It can be shown that for coatedcarrier beads, the mass of toner per unit area developed on the PC is approximately

PC, and the toner is then electrophoretically transferred to the latent image areas on the PC

Color can be accomplished by using multiple development stations, one each for the subtractive colors (cyan,yellow and magenta) plus black Toners are colored by either dyes or pigments The four-colored images may

Figure 89.25 Dual-component magnetic brush developer Toner particles, adhering electrostatically to much larger

mag-netic carrier beads, are transported into the photoconductor-developer gap to tone the electrostatic image (Source: R C Durbeck and S Sherr, Eds., Output Hardcopy Devices, San Diego, Calif.: Academic Press, 1988, p 242 With permission.)

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