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Description  |
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TECHNICAL FIELD
The present invention relates in general to information retrieval systems,
such as a videotex system. It more particularly relates to a method and
apparatus for communicating relatively inexpensively between a host
computer and geographically widely-distributed large groups of terminals.
CROSS REFERENCES TO RELATED APPLICATIONS
(1) TERMINAL FOR INFORMATION SYSTEM, by John F. Graham and Daniel L.
Williams, Ser. No. 545,615, filed Oct. 25, 1983;
(2) INFORMATION SYSTEM TERMINAL OR THE LIKE, design patent application by
John F. Graham and Daniel L. Williams, Ser. No. 545,614, filed Oct. 25,
1983;
(3) METHOD AND APPARATUS FOR FAST ACCESS OF REMOTELY LOCATED INFORMATION,
by Peter M. Winter, Thomas R. Ray, and Mary E. Burkhardt, Ser. No.
545,070, filed Oct. 25, 1983;
(4) METHOD AND APPARATUS FOR RETRIEVING INFORMATION, by Anthony Kram, Ser.
No. 805,830, filed Dec. 6, 1985;
(5) METHOD AND APPARATUS FOR RETRIEVING INFORMATION DISTRIBUTED OVER
NONCONSECUTIVE PAGES, by Peter M. Winter and Neil L. Holman, Ser. No.
545,124, filed Oct. 25, 1983;
(6) METHOD AND APPARATUS FOR ASSISTING USER OF INFORMATION RETRIEVAL
SYSTEM, by Anthony Kram, Peter M. Winter, and Neil L. Holman, Ser. No.
545,069, filed Oct. 25, 1983;
(7) METHOD AND APPARATUS FOR INFORMATION RETRIEVAL, by J. William Burk,
Jr., Christopher Fries, and Peter M. Winter, Ser. No. 545,128, filed Oct.
25, 1983.
BACKGROUND ART
Videotex systems interactively connect a host computer to large groups of
geographically widely-distributed terminals. In this manner, remotely
located information can be retrieved by the terminals.
In order to establish communication paths between the terminals and the
host computer, telephone connections have been employed. While such
telephone communications have become popular in videotex systems, the
subscribers to such a system, must not only pay the charges for the
videotex system, but also the telephone charges. The telephone charges can
become very substantial as compared to the videotex service charges.
The telephone charges are prohibitively large, especially where the
terminal is located at a great distance from the host computer. Thus, it
would be highly desirable to have a new and improved method and apparatus
for an information retrieval system, such as a videotex system, to enable
the terminals to communicate relatively inexpensively with a remotely
located host computer over communication paths, such as telephone lines.
Such a technique should reduce the service charges, as compared to
conventional techniques.
Moreover, in such a system where information is retrieved from distant
locations, it is desirable to enable complete computer programs to be
exercised locally by the user interacting with a terminal in an economical
and efficient manner. In this regard, if a program requires a great deal
of real time to execute, the cost of maintaining the communication path
during the execution thereof, may be substantial, and in some instances,
prohibitively high. Thus, a more cost effective and efficient system for
enabling the execution of longer programs, would be highly desirable.
DISCLOSURE OF THE INVENTION
Therefore, the principal object of the present invention is to provide a
new and improved method and apparatus for retrieving remotely located
information in a relatively inexpensive manner and in a highly efficient
and effective manner.
Another object of the present invention is to provide such a new and
improved method and apparatus, which enables a host computer to
communicate with individual ones of a large group of geographically
widely-distributed terminals, over telephone lines, in a cost effective
manner.
Still another object of the present invention is to provide such a new and
improved method and apparatus, which enables an entire program to be
downloaded from a remotely located host computer to a terminal, in a
highly efficient and effective manner.
Briefly, the above and further objects of the present invention are
realized by providing a new and improved information retrieval system,
which includes a method and apparatus for facilitating the information
retrieval in a relatively inexpensive and efficient manner.
The information retrieval method and apparatus includes a group of
geographically widely-distributed terminals, which access a remotely
located host computer. Each terminal includes a memory for storing a
plurality of items of call origination information. A call origination
circuit transmits individual ones of the items of call origination
information via a communication path to the switching system for causing
it to extend the path to the host computer. Logic circuits transfer a
first one of the items of call origination information indicative of the
geographically shortest communication path, to the call origination
circuit in an attempt to extend a communication path, of the
geographically shortest length, to the host computer. If the attempt
proves unsuccessful, the logic circuit automatically sends a second one of
the items of call origination information, indicative of the next shortest
communication path length.
Other features of the invention relate to the concentrating of groups of
communication paths, at geographically distributed locations, to a smaller
number of dedicated lines, for the purpose of saving added telephone
charges.
Still other features relate to the provision of a method and apparatus for
establishing an initial communication path to a terminal, when it is first
cut into service, to enable the host computer to initialize the new
terminal.
Further advantages of the present invention relate to the downloading of
telesoftware programs from the host computer to a terminal for more
effectively and efficiently executing the program. In this manner, if
desired, the communication path can be disconnected, while the program is
being executed in the terminal.
Alternatively, the host computer can interact with the terminal via the
communication path Additionally, the communication path can be controlled
in response to information contained in the telesoftware program.
BRIEF DESCRIPTION OF DRAWINGS
The above-mentioned and other objects and features of this invention and
the manner of attaining them will become apparent, and the invention
itself will be best understood by reference to the following description
of an embodiment of the invention taken in conjunction with the
accompanying drawings, wherein:
FIG. 1 is a functional block diagram of the information retrieval system of
the present invention;
FIG. 2 is a functional block diagram of one of the terminals of the system
of FIG. 1;
FIG. 3 is a face view of the keyboard unit of the terminal of FIG. 2;
FIGS. 4-7 are illustrative views of pages of displayed information, useful
in understanding the present invention;
FIGS. 8-10 are memory layout diagrams useful in understanding the present
invention; and
FIGS. 11 and 12 are flow chart diagrams of the terminal executive program.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings, and more particularly to FIGS. 1, 2 and 3
thereof, there is shown an information retrieval system 10, which is
constructed according to the present invention. While the system 10 is
shown and described as being a videotex system, it will become apparent to
those skilled in the art that the inventive system 10 may also be other
types and kinds of information retrieval systems. The hereinafter detailed
description of the present invention is organized according to the
following outline:
I. SYSTEM
II. TERMINAL
(a) Detailed Hardware Description
III. KEYBOARD
IV. GENERAL OPERATION
V. SPECIFIC OPERATION
(a) Initial call operation
(b) Automatic Re-dialing Operation
(c) Telesoftware--Multi-tasking Processing
(d) Telesoftware--Multiple graphics control
VI. TERMINAL FIRMWARE
VII. EXECUTIVE PROGRAM
VIII HOST COMPUTER SOFTWARE - SESSION/LINK
Referring now to FIG. 1, the system 10 generally comprises a videotex
office 12 having a host computer 14 for communicating with a plurality of
groups 15--17 of geographically widely-distributed terminals such as the
terminal 20.
The videotex office 12 extends from a calling one of the terminals, such as
the terminal 20, a gateway communication path, such as over the telephone
lines 22 via a telephone central office (not shown), to a selected one of
a plurality of service computers generally indicated at 23, such as the
service computer 24. In this manner, the terminal 20 can communicate
directly with the remotely located service computer 24, which may be a
bank or store computer where the user of the terminal 20 has an account.
Thus, the user is enabled to access the computer 24 to transact desired
business.
In order to communicate economically with the host computer 14, a plurality
of similar multiplexing offices, such as the office 26, 28 and 31,
concentrate a large number of communication paths, to a much smaller
number of communication paths 35, 37 and 39, extending between the outlets
of the respective multiplexing offices 26, 28 and 31, and the respective
inlets of the videotex office 12. The paths 35, 37 and 39 are leased
dedicated telephone lines, which carry a relatively large number of or any
other multiplexing arrangement multiplexing data calls from the terminals.
Thus, the telephone charges for the operation of the system 10 is greatly
reduced. The paths 35, 37 and 39 may also be value added circuits in a
packet switched network or microwave or infrared link.
The cost savings also results from the fact that the multiplexing offices
are distributed uniformly and appropriately over the service area of the
videotex office 12. Moreover, the terminals are arranged in the groups
15-17, which are coupled to conventional telephone central offices 41, 43
and 45 for extending connections to the multiplexing offices 26, 28 and
31.
In this manner, the multiplexing office can be either closely located
relative to the corresponding central offices, or can be physically
located therewithin. As a result, a call from one of the terminals is
charged by the telephone company as a local telephone call, because of the
relatively short, direct distances involved.
Thus, because of the short distances involved, the call from a terminal
through the central office to the multiplexing office, is charged at a
minimum rate. Also, the connection between the multiplexing office and the
videotex office is charged at a low rate, because of the economical use of
multiplexed leased telephone lines. Therefore, the overall charges are
maintained at low rates, regardless of the location of the terminals
relative to the host computer 14.
Considering now the multiplexing offices, each one of them is similar to
one another, and thus only the office 26 need be described in greater
detail. The office 26 includes a group of modems, such as the modems 47
and 49, which have their outputs connected to the inlets of a multiplexer
52. The outlet of the multiplexer 52 is connected to the inlet of a modem
54, which has its outlet connected to the single communication path 35.
A modem 56 in the videotex office 12, is one of a group of input modems,
such as the modems 57 and 59 connected to the respective lines 37 and 39,
and the modem 56 receives the data from the communication path 35. A group
of multiplexers, such as the multiplexers 56A, 57A and 59A, de-multiplex
the outputs of the respective input modems 56, 57 and 59 for supplying the
information to the host computer 14.
The inlets of the modems, such as the modems 47 and 49, are connected to
telephone communication lines, such as the lines 58 and 60 extending
between the central office 41 and the respective modems 47 and 49. The
telephone lines 58 and 60 are part of a hunt group 59 of lines extending
between the central office 41 and the multiplexing office 26. In this
regard, the central office includes conventional equipment (not shown),
which automatically hunts for the first idle line, when a call is made to
a single telephone number for the hunt group. The multiplexing offices 28
and 31 include similar other hunt groups 61 and 63 of telephone lines
extending from their respective central offices 28 and 31.
Considering now a communication path extending from a calling terminal and
a service computer. The path to be described is shown in heavy lines in
FIG. 1. Assume that the user of the terminal 20 desires to perform a
service transaction, utilizing the service computer 24, which may be, for
example, the computer located at the user's bank. Assume further that all
of the lines for both of the closer multiplex offices 28 and 31 are busy,
and thus the office 26 must be used. It should be noted that the actual
establishment of these connections is hereinafter described in greater
detail.
The communication path indicated generally at P extends from the terminal
20 over a telephone line 62 through the central office 45, over a
telephone line 64 to the central office 41. From there, the path P extends
through the central office 41, over the telephone line 60 (or an internal
line, if the multiplexing office 26 is co-located in the central office
41) to the input to the modem 49, which has its output multiplexed by the
multiplexer 52, with the outputs from the other multiplexers in the office
26.
The multiplexed output of the multiplexer is connected to the input to the
ouput modem 54, which, in turn, transmits the multiplexed data signals
over the line 35 to the modem 56 in the videotex office 12.
From there, in response to user information keyed into the terminal, the
communication path P extends to the input of a multiplexer 56A, and from
there to the host computer 14. The computer 14, in turn, communicates over
an output modem 66 via the telephone line 22 to a modem 68 to the service
computer 24, thereby completing the communication path P.
It should be understood that the portion of the communication path P
extending between the terminal 20 and the videotex office 12, is also
referred to as a communication line, since the terminal 20 communicates
with the host computer 14 over this portion of the path P. The portion of
the path P extending between the videotex office 12 and the service
computer and including the telephone line 22, is also referred to as a
gateway path, or simply a gateway. As desired, the terminal can
communicate with the host computer 14, as well as the service computer.
While the presently preferred form of the present invention employs
telephone connections to serve as the communications path, it will become
apparent to those skilled in the art that other forms of communication
paths may also be employed. For example, microwave links may also serve as
communication paths for the system 10.
In the preferred form of the present invention, the host computer is a
Honeywell DPS 8, which employs a CP-6 operating system.
Referring now to FIG. 2, the terminal 20 includes a decoder 69 and a
keyboard unit 71, and communicates with the host computer 14 over the
communication path by means of a modem/dialer 70, which in turn
communicates by a pair of asynchronous communication interface adaptors 72
and 74 to and from a bus 76. The bus is a 28 bit parallel, 8 bits for data
and 20 bits for the address information. The interface adaptors convert
between parallel bus information and serial information for the telephone
line communication path P between the host computer 14 and the terminal
20.
A bus extender connector 77 enables peripheral devices, such as a printer
(not shown), a disk drive (not shown) or the like, to be coupled directly
to the terminal, without the need for special interface circuits. Thus,
the terminal can be expanded for greater utilization thereof, if desired.
A microprocessor unit 81 communicates with the bus to control the
functioning of the decoder of the terminal. An interrupt controller 82
responds to the microprocessor 81. Examples of interrupts include the
initiation of keyboard information, the initial receipt of communication
line information, and others.
The decoder 69 communicates with the host computer 14 to display
information on a television screen (not shown) of a television receiver
83. The decoder receives manual input information from a keyboard 85 of
the keyboard unit 71 over an infrared link, which comprises an infrared
transmitter 87 of the unit 71 and an infrared receiver 89 of the decoder
69. The transmitter 87 includes a conventional semiconductor laser diode
(not shown), which transmits a semidirectional infrared signal to a
conventional infrared-sensitive diode receptor (not shown) in the decoder
receiver 89, whenever the cone of transmission is pointed in the general
direction of the decoder 69. In this manner, the keyboard unit 71 can be
used remotely from the decoder 69 by means of a wireless infrared
radiation link.
An asynchronous communication interface adaptor 92 couples the information
from the I.R. 89 receiver to the bus 76.
In order to control the formation of information for the television screen,
a TV control logic 94 causes the sending of pixel information to the
television receiver 83, via a jack 83A. The video set can either be a
conventional television receiver, or a component video set (not shown). In
the latter case, the signals are sent directly thereto via an amplifier
and filter circuit 95 to a jack 95A. In the case of the conventional
television receiver 83, the signals from the TV control logic 94 are sent
to an R.F. modulator 96 via an amplifier and filter circuit 97, which
modulator in turn is coupled to the conventional television receiver for
operation on either channel 3 or channel 4 thereof.
A video or screen random access memory 98 is connected to the bus 76 and is
accessed in either bytes (8 bits ) or nybbles (4 bits). A data buffer 99
writes data received from the microprocessor 81 via the bus 76 in either
bytes or nybbles. An address buffer 100 receives address data from the
microprocessor 81 via the bus 76, and, in turn, supplies it to video
memory 98 for addressing it. The video or screen random access memory 98
contains information representing the picture elements (pixels) stored
therein in a conventional dot matrix arrangement. This information also
determines the positioning for the screen. In this regard, the control
logic 94 responds to the information in the video RAM memory 98 to
formulate the pixel information for the television receiver 83.
The pixels are areas of light or dark on the video screen. Each pixel is
represented by 4 bits of information stored in the video random access
memory 98.
A color map memory 101 stores color information, which combines with the
pixel information stored in the video memory 98 to determine the color of
the pixels.
The video memory 98 is connected to the bus 76 so that the microprocessor
unit 81 can control the content and operation of the video memory.
A CRT controller 103 receives data from the microprocessor 81 via the bus
76, for the purpose of controlling the graphic display for the receiver
83.
In this regard, during normal operation, the microprocessor 81 receives a
stream of data from the host computer 14 via the path P, the modem/dialer
unit 70, the ACIA unit 74, and the bus 76; and continuously, on the fly,
generates graphic display information for the CRT controller 103 under
local firmware control, as hereinafter described in greater detail.
In order to provide the proper information for the video control logic 94,
the controller 103 causes address data to be sent to the video memory 98
via the address buffer 100 and the bus 76, to cause the graphic
information, arranged in the three color bit-plane configuration, to be
read from the video memory to a multiplexer 105, via the bus 76.
The multiplexer 105 receives 32 data bits of information at a time, and, in
turn, gates four bits (a nybble) to the color map memory 101. The four
bits represent a single color dot.
There are three outputs, generally designated 106 and individually
designated B, G and R, from the color map memory 101 and presented to a
data buffer 107. The outputs each comprise four bits of information. The
three outputs represent the respective three colors of blue, green and
red.
A color composite video output circuit 108 receives three color outputs,
generally designated 109, and also specifically designated B, G and R,
representing the same colors received from the memory 101.
The video output circuit 108 receives the color outputs 109, and a signal
BLANK from the color map memory 101, to, in turn, supply the necessary
information to the video control logic 94. The CRT controller 103 also
supplies the horizontal and vertical synchronization signals directly to
the video output circuit 108, for mixing with the video picture
information and supplied in synchronism therewith.
In order to utilize the bit plane memory information built up and stored in
the memory 98, under microprocessor control, the video memory 98 is
addressed by sending address information thereto via the address buffer
100 and the bus 76. As a result, groups of 32 bits of color data are
transferred, repeatedly and sequentially via the bus 76 to the multiplexer
105, which in turn, presents four bits (one nybble) of the information to
the color map memory 101 for storage therein temporarily, before moving to
the video output unit 108.
The red, green and blue color dot pixel information is transferred from the
color map memory 101 to the video output unit 108 via the buffer 107 in a
sequential mode of operation. The unit 108 causes a graphic screen
resolution for the television receiver 83, of 256 horizontal pixels by 200
vertical pixels.
The sequence commences by the controller 103 sending a horizontal
synchronization signal HSYNC to the output unit 108. Thereafter, 256 pixel
information elements are transferred sequentially from the color map
memory 101 via the data buffer 107 to the output unit 108. A BLANK is also
supplied to the unit 108.
After the last element of pixel information is received, another horizontal
sychronization signal is sent to the video output 108, followed by another
such series of color dot pixel information elements being transferred with
the associated BLANK signal. After this operation is repeated 200 times, a
vertical sunchronization signal VSYNC is sent to the output unit 108. This
entire operation can then be repeated.
This sequence of information is, in turn, supplied from the output unit 108
to the control logic 94, and from there, to the RF modulator 96, via the
amplifier and filter unit 97, for transferring to the television receiver
83 to provide the desired graphical display on the screen thereof.
A data buffer 112 is adapted to supply color data information from the
microprocessor 81 to the color map memory 101 for altering the information
stored therein. In this manner, the graphical presentation can be quickly
altered, and thus, certain effects, such as animation, can be conveniently
realized.
An electronically erasable read only memory (EEROM) 114 stores terminal
identification information, telephone numbers for automatic calling
purposes, and host managed data. The memory 114 is used to store telephone
information for enabling the terminal 20 to set up an initial telephone
call to a particular multiplex office port for establishing a connection
to the host computer 14, whereby the terminal can send terminal
identification information to alert the host computer 14 to the fact that
this particular terminal is now functional. The host computer then
initialized the newly functional terminal by downloading other telephone
information for storage in the memory 114. In the event a particular port
of a multiplex office is busy, other connections may be established.
In this regard, a plurality of such telephone numbers are stored in the
non-volatile memory 114 of the terminal 20, because if one of the
telephone lines is busy, the next geographically closest telephone line is
then dialed automatically under firmware control, in the terminal 20.
A PLPS read only memory 116 stores videotex presentation level protocol
syntax firmware for the terminal 20, and communicates with the other
subsystems of the terminal 20 via the bus 76. A PLPS "scratch pad" random
acces memory 118 communicates with the memory 116 via the bus 76. The
video memory 98 and the memory 118 are collectively referred to as "main
memory".
A telesoftware random access memory 121 stores downloaded software programs
from the host computer 14. The telesoftware program is sent from the host
computer via the communication path P (FIG. 1), the modem/tone dialer unit
70, the ACIA unit 74, the bus 76, and via the microprocessor 81 to the
memory 121. The telesoftware is used to control the operation of the
terminal 20, either independently of the host computer 14, or in
conjunction therewith. The memory 121 is addressed by address information
received from the microprocessor 81 via the address buffer 100.
A read only memory 123 stores a multi-tasking executive firmware program
for the terminal 20 for controlling its functioning, as hereinafter
described in greater detail. Also stored therein are the keyboard handler
firmware, the input/output handler firmware, and the power on page
firmware.
A session and link read only memory 125 stores the session and link
firmware for the terminal 20, as hereinafter described in greater detail.
The session firmware program controls the sending of data to the host
computer 14. It also oversees the reception of data from the host
computer. In this regard, it directs the flow of incoming data within the
terminal. The link program starts and stops the flow of data to and from
the host computer, in the event that either the terminal or the host
computer should be unable to accept the information at the current rate.
The link firmware program controls the communication protocol. It
determines data flow control, in that it controls the speed of the flow of
data.
The data is first subjected to the link program layer, and then to the
session program layer.
A terminal clock 127 provides the necessary timing signals for the various
terminal subsystems via the bus 76.
During the initial call dialog between the terminal 20 and the host
computer 14, the host computer requests and then receives the terminal
identification information and looks up the information in the table
stored therein to determine whether or not the terminal is a maximum
convenience terminal. If so, the host computer looks up in the table in
the host computer to learn the user's identification and the user's
password. The host computer then causes a master index screen to be
displayed on the television receiver 83 via the terminal 20. The master
index requests the user to determine which type of service is desired. In
this regard, it requests the user to press one of the service keys.
If the user has requested maximum security, instead of looking up the
information in the table, the host computer causes the terminal to display
a request of the user to input the user's identification number and the
user's password. Once this has been accomplished, the host computer checks
the information entered to determine whether or not the user is
authorized. If the user is authorized and the terminal is not locked, the
host computer 14 causes the terminal to display the master index.
In either the maximum convenience or the maximum security modes of
operation, once the initial steps have been taken, the terminal 20 can be
used to gain access to the host computer 14 and the gateways which can be
established therefrom.
II. (a) Detailed Hardware Description
The modem 70A is a conventional two-way simultaneous communication device
for communication with the host computer 14 over standard telephone
circuits. The modem has a receive channel operating at 1200 baud for data
from the host computer to terminal. The carrier frequencies are: Mark=1200
Hz, Space=2200 Hz.
A transmit channel is provided for operating at 150 baud for data from the
terminal 20 to the host computer 14. The carrier frequencies are:
Mark=387, Hz, Space=487 Hz. Automatic dialing is provided for both
Touch-Tone and rotary dial circuits. A call waiting feature is implemented
by timing the loss-of-carrier period. (Refer to the description
hereinafter for operational message consideration.) Echo suppression (if
required) is the responsibility of the network. Failure-to-connect is
detected by an absence of the carrier. This condition can occur from
ringing, busy, or answered-without-carrier events.
Modulated RF and composite outputs are provided at the respective
connectors 83A and 95A for connecting to the standard color, or a standard
black and white TV receiver on channels 3 or 4 (switch selectable). The RF
output signal complies with the NTSC specification. The specification is
promulgated by the Federal Communications Commission, Office of Science
and Technology, "NTSC Rules for Radio Broadcast, Volume 3, Part 73,
Section 73.682," U.S. Government Printing Office, Washington D.C.
The composite video signal complies with the RS 170 specification, which is
the Electric Industries Association, "Electrical Performance Standards -
Monochrome Television Studio Facilities," RS-170, Electronic Industries
Association, Engineering Department D.C., 1957.
The keyboard 85 is a flat membrane type, with an embossed surface around
each keycap to enable finger positioning, and is housed as a cordless
unit, detachable from the main enclosure for the decoder 69, which
enclosure confines the terminal electronics. For additional information
concerning the terminal enclosure, reference may be made to the first two
hereinbefore-mentioned cross-referenced, co-pending U.S. patent
applications.
A maximum of 256 key codes are possible. However, only those listed are
supported.
Audio feedback via a conventional audible annunciator (not shown) located
in the decoder 69, is activated by a conventional beep timer (not shown).
One beep tone (400 Hz) for 70 milliseconds indicates that a keystroke of
the keyboard 85 has resulted in a code being received by the decoder 69
without error. The second beep tone (800 Hz) for 70 milliseconds is used
for error conditions. A rear-mounted volume control (not shown) is also
provided.
The 800 Hz beep tone is activated, if an ASCII BELL Code is received from
the host computer 14.
The terminal 20 is also provided with a keyboard repeat function. When a
keyboard depression occurs for more than a half second, the associated
code is transmitted from the keyboard 85 at a rate of 10 per second until
key release. In the event of a two key rollover, the keyboard 85 responds
to each key depression, provided that no more than two keys depressions
occur simultaneously. If a second key depression occurs while the
character codes for the first key depression are being transmitted,
transmission of the first key code ceases and only one code for the second
key depression is transmitted (with repeat function disabled), until one
of the two key depressions is released. After release of one of them, the
remaining key depressions cause the engagement of the repeat function. A
full character code is always transmitted.
Using the IR transmitter 87 and the IR receiver 89, data is transmitted to
the decoder 69 (at an effective rate of 150 baud) by means of high
frequency infrared energy pulses (e.g. pulsed infrared light beam). The
range of transmission is approximately 30 feet. The operational radiation
cone is approximately 60 degrees.
Data to transmitted to the host computer 14 starts to exit the terminal
within 85 milliseconds following a key depression in the PLPS mode,
provided that transmission has not been disabled by the host computer 14
(FIG. 1). The complete key character exits the terminal within 145
milliseconds.
Power for the keyboard unit 71 is supplied by a non-rechargeable battery
(not shown). The keyboard design minimiz | | |
