U.S. patent number 3,774,158 [Application Number 05/215,817] was granted by the patent office on 1973-11-20 for multiple terminal display system.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Robert John Clark.
United States Patent |
3,774,158 |
Clark |
November 20, 1973 |
MULTIPLE TERMINAL DISPLAY SYSTEM
Abstract
Blocks of digital data representing different pictures are
available at a remote data processing center for supply to the
control channels of a DIVCON system. Each channel includes a
refresh memory for storing a block of digital data and circuits for
translating the stored data to a frame of video signals but there
are substantially fewer channels than display terminals. In
response to a request for service by a terminal, a data channel
exchange hunts for a "not busy" control channel and when it finds
one, connects the channel to the terminal. The terminal may then
communicate with the remote data processing center both for
receiving data via the channel's refresh memory for display on the
terminal and for sending data to the remote data processing center
also via the channel's refresh memory.
Inventors: |
Clark; Robert John (Dorion,
CA) |
Assignee: |
RCA Corporation (New York,
NY)
|
Family
ID: |
22804525 |
Appl.
No.: |
05/215,817 |
Filed: |
January 6, 1972 |
Current U.S.
Class: |
345/2.1; 715/255;
715/273; 340/2.71; 345/180; 710/200; 715/733; 345/181 |
Current CPC
Class: |
G06F
13/22 (20130101); G06F 3/02 (20130101) |
Current International
Class: |
G06F
3/02 (20060101); G06F 13/20 (20060101); G06F
13/22 (20060101); G06f 003/00 () |
Field of
Search: |
;340/172.5,324A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Henon; Paul J.
Assistant Examiner: Vandenburg; John P.
Claims
What is claimed is:
1. A multiple terminal display system comprising, in
combination:
means for producing digital data corresponding to different frames
of video information;
m separate video display terminals, where m is a first plural
integer, each of said terminals including means for selectively
supplying a request for service signal;
n separate control channels coupled to said means for producing
digital data, where n is a second plural integer smaller in value
than said first plural integer, each of said control channels
including request-for-service means responsive to a request for
service signal supplied thereto from any terminal initially without
service for causing an idle one of said control channels to be
seized and retained by that terminal for its exclusive use only so
long as that terminal continues to supply the said seized control
channel with said request for service signal , and
wherein each control channel includes refresh memory means for
storing a block of digital data and character generator means for
translating that data to a frame of video information which, in
response to that control channel being seized by a terminal, is
forwarded for display by its seizing terminal.
2. The system defined in claim 1, wherein said means for
selectively supplying a request for service signal of any of said m
terminals includes associating means for associating that terminal
with a single one of said control channels at a time, wherein said
request-for-service means of a control channel includes
busy-determining means operative when that channel is idle for
returning a not-busy signal to a terminal without service
associated therewith and operative when that channel is already
seized by another terminal for returning a busy signal to a
terminal without service associated therewith, and wherein an
associated terminal includes signal-responsive means responsive to
the return of a not-busy signal to its initial request for service
for seizing that single one of said control channels with which it
is then associated and responsive to the return of a busy signal to
its initial request for service for associating that terminal with
a different single one of said control channels.
3. The system defined in claim 2, wherein a frame of video
information is composed of L display scan lines arranged in a
raster, where L is a plural integer at least equal in value to m,
and
wherein said busy-determining means returns said not-busy signal or
said busy signal during each scan line, wherein said
signal-responsive means of each different terminal has a different
preselected single scan line assigned thereto for responding to a
not-busy signal returned thereto only during that single scan line
assigned thereto, whereby a simultaneous attempt at seizing an idle
control channel by more than one terminal is prevented.
4. The system defined in claim 2, wherein each terminal includes a
display portion normally decoupled from said control channels, and
video-signal transmission means including said associating means
for extending a video connection from a control channel to the
display portion of a terminal only while that control channel is
seized by that terminal.
5. The system defined in claim 4, wherein each terminal includes
means for generating a data signal which is normally decoupled from
said control channels, and data-signal transmission means including
said associating means for forwarding said data signal from a
terminal to a control channel only while that control channel is
seized by that terminal.
6. The system defined in claim 4, wherein each terminal includes
means for generating a light-pen signal which is normally decoupled
from said control channels, and light-pen signal transmission means
including said associating means for forwarding said light-pen
signal from a terminal to a control channel only while that control
channel is seized by that terminal.
7. The system defined in claim 1, wherein each respective one of
said n control channels is connected to all of said m terminals by
its own set of common conductors, whereby the total number of
common conductors is equal to the product of the number of members
of each set multiplied by the number of control channels and is
independent of the number of terminals.
Description
BACKGROUND OF THE INVENTION
U.S. Pat. No. 3,426,344, issued Feb. 4, 1969 for "Character
Generator for Simultaneous Display of Separate Character Patterns
on a Plurality of Display Devices" by the present inventor and
assigned to the same assignee as the present application, describes
a multiple terminal display system. A typical such system,
illustrated in FIG. 1 of the present application includes a digital
data processing center 8 which comprises a digital computer,
magnetic tapes, disks or other mass storage files, keyboards, and
other peripheral equipment. The center transmits digital coded
messages via control channels to the terminals and receives digital
coded messages via these channels from the terminals. Each message
- a block of digital data, corresponds to a page of television
picture information.
A digital-to-video converter (DIVCON) 10, connected to the computer
by a telephone line, includes a plurality of channels, one per
video terminal. Each channel includes a refresh memory for storing
a computer originated digital coded message and a digital-to-video
converter such as 13 for translating that message into a frame of
video signals for display on its video terminal. The latter may
include, for example, a television type monitor.
Each DIVCON refresh channel includes keyboard and light probe
(sometimes known as "light pen") data input control logic circuits
6 and 7, respectively, as shown in FIG. 1. It is possible for the
terminal operator to write information on the TV screen of his
terminal by typing on his keyboard and having the keyboard control
logic circuits 6 in the channel connected to his terminal write
this data into the refresh memory of the same channel. It is also
possible, using the light probe, to write check marks into
questionnaires displayed on the TV screen and to select information
indices displayed on the TV screen. In both cases this is
accomplished by pressing the light probe against the face of the TV
screen at the desired display position, as discussed shortly.
Two coaxial cables connect each terminal to a DIVCON channel as
indicated in FIG. 1. The TV video and keyboard data are multiplexed
onto one cable by the mixers 5 in FIG. 1, in well understood
fashion, to save one cable per channel. The light probe position
indicating signal, which may be produced in the manner described in
U.S. Pat. No. 3,579,225, issued May 18, 1971, for "Light Probe for
Persistent Screen Display System" to the present inventor, is
transmitted on the second coaxial cable. It is also possible, as an
alternative, to convert the light probe signal into X-Y position
codes which can be transmitted with the keyboard data on the same
cable 4 as the video signals. This would reduce the number of
cables between each DIVCON channel and each terminal from two to
one.
When a terminal operator desires to display information, the
terminal is turned on and logic circuits in the terminal transmit a
"Request for Index" code to the data communications control unit 11
shown in FIG. 1 via the control channel for that terminal. This
code may be one which is permanently stored in the terminal and
which is transmitted in response to the closing of a switch or the
code may be transmitted in response to the depression of one or a
combination of keys on the keyboard (other alternatives also are
possible). The data communications control unit 11, which may be
any one of a number of commercially available "mini computers," in
turn, transmits an index request message to the data processing
center 8. In response to the index request message, the data
processing center retrieves the index page from its mass storage
files and transmits it to the required terminal.
The connections from the unit 11 to the various channels is
typically via a multiple conductor cable, shown as a single line in
FIG. 1. In one system, by way of example, this cable may include
from 34-37 lines; 8 for data in, 8 for data out, 10 for memory
addresses, 5-8 for channel addresses, 1 for read/write control, 1
for cycle initiate, 1 for a data available indication. These are
just representative as alternatives are available depending upon
the machine and the particular installation.
The index page sent to a terminal may be a page of letters or a
page of words, each letter or word identifying a different subject.
The digital signals making up the index page are stored in the
refresh memory in the channel for a terminal, and the corresponding
video signals produced by the digital-to-video converter 13,
sometimes termed a character generator, in the same channel
thereafter are repeatedly applied to that terminal for continuously
refreshing the displayed information. In the case of a terminal
having a cathode-ray tube type television raster display, as shown,
the signals are read from the memory synchronously with the
horizontal and vertical scanning of the electron beam, in well
understood fashion.
Each terminal may include either push buttons or the like rather
than a light probe for information selection, and also may include
a keyboard and/or a printer (in FIG. 1, terminal 1 is shown to
include a keyboard, light probe and TV monitor, by way of example)
although it is not essential that each terminal include all of this
equipment. Considering only the light probe for the moment, when
the terminal user wishes a page of information identified by a
certain letter or topic on the index page to be displayed, he
points the light probe at that topic or letter and actuates the
light probe. In response to the signal produced by the light probe,
the logic circuits in the channel for that terminal will transmit
the X-Y position coordinates of the topic or letter pointed to by
the light pen, to the data processing center. Using the X-Y
position coordinates, the data processing center determines which
topic or letter the terminal operator has pointed to on the index,
retrives the requested data from its mass storage files, and
transmits the selected data back to the required terminal.
As mentioned above, in the system described in the patents, there
is one control channel required per terminal. Each control channel
duplicates or substantially duplicates the equipment in each other
control channel. Each includes, for example, a refresh memory,
logic circuits 6 and 7, converter circuit 13 and so on.
In certain applications where it is desirable to have a terminal
present at many different locations but where each terminal is used
only a relatively small fraction of the time, the cost for the
system is relatively high considering the amount of time each
terminal actually is in use. An example of this type of use is in a
reasonably large sized hospital. Here it would be advantageous to
be able to have a terminal in each patient's room as well as
terminals in the doctor's offices, the supply rooms, the various
administrative offices and so on. However, the terminals in the
patients' rooms would be used only when the nurse or doctor wished,
for example, to display a patient's record, to enter new
information into that record or for certain other uses occupying
only limited time intervals. It is estimated, for example, that a
system of the type shown in FIG. 1 for 200 cathode ray tube
terminals would cost about $3,000 per terminal ($500 for the
terminal itself and $2500 for the channel for that terminal).
SUMMARY OF THE INVENTION
In the system of the present application there are far fewer
control channels than there are display terminals. A data channel
exchange couples the channels to the terminals. In response to a
request for service by a terminal, the exchange finds an available
channel and connects it to that terminal. The system includes
features such as the use of logic signals on the same line as video
signals to indicate channel busy status; the use of a common line
to carry video signals from a control channel to a display terminal
and a request for service signal from the display terminal to the
control channel, and others discussed in detail below.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of the known multiple terminal display
system discussed above;
FIG. 2 is a block diagram of a multiple terminal display system
according to an embodient of the invention;
FIG. 3 is a more detailed block diagram of a single one of the
modules of the data channel exchange shown in FIG. 2;
FIG. 4 is a block diagram of the circuit in one of the terminals
for producing the idle signal;
FIG. 5 is a block diagram of the circuits in the DIVCON for
producing the TV line pulses and the channel sample signals and the
circuits in the control channels for producing the not busy
signals;
FIG. 6 is a drawing of waveforms to help explain the operation of
the system of FIGS. 2-5; and
FIG. 7 is a block diagram of the idle detect pulse generator of
FIG. 3.
DETAILED DESCRIPTION
The embodiment of the invention shown in FIG. 2 includes a
digital-to-video converter (DIVCON) system 10 similar to the system
of FIG. 1. The DIVCON 10 is connected to the digital computer
system 8 just as in FIG. 1.
In the system of FIG. 1 there are as many channels as there are
terminals. As explained in the background section, this makes the
system relatively expensive. In the system of FIG. 2 there are only
a fraction of the number of channels as there are terminals. In the
system chosen by way of example for illustration, there are only 32
channels, each including a refresh memory (either a memory
individual to that channel or a specific group of locations in a
larger memory, which larger memory may be common to serial
terminals), logic circuits and so on, but there are 200 terminals.
These 32 channels can handle only 32 terminals at any one time;
however, in practice, no more than this number is expected very
often to be demanding service at the same time. The actual number
of channels required per given number of terminals is determined on
the basis of statistical calculations similar to those employed by
a telephone company in determining the number of trunk lines
required for a building's switchboard. These calculations take into
account the total number of video terminals; the average time each
terminal will be in use; the peak number of terminals requesting
access to a channel per hour; the channel availability factor
(there should be a channel available about 99 percent of the time);
and so on.
The 32 channels of FIG. 2 are coupled to the terminals by modules,
there being one module per terminal. Each module is connected to
its terminal by two coaxial cable wires and each module connects to
all channels. At each module, there may be three wires per channel
at the interface between the module and the 32 channels in which
case each module has 96 lines at this interface. Operated in this
way, the mixers 5 of FIG. 1 are relocated next to the DCX module as
shown in FIG. 2. In addition, each module connects directly to the
DIVCON 10 via two wires one carrying the c signal and the other a b
signal as will be shown in more detail in connection with FIG.
3.
In operation, when a user wishes to place a terminal in operation
he may, for example, have a key (or card or the like) that he
inserts into the terminal. In response to the insertion of the key,
means may be provided for determining whether that user is
authorized to have access to the terminal and, if not, means may be
provided for sounding an alarm either locally or at the computer,
or at some other location. However, as none of this is of direct
interest in the present application, it will not be discussed
further here.
Assume that the user is an authorized user. Upon insertion of his
key into a terminal, a signal is transmitted to the module for that
terminal requesting access to the DIVCON and computer system. The
module, upon receipt of this signal, searches from
channel-to-channel until it finds one which is not busy. Upon
locating that channel, the module connects its terminal to that
channel and thereafter the terminal can communicate via its channel
to the DIVCON and computer in ways similar to those discussed in
the co-pending applications.
In brief, upon obtaining access to a channel, the terminal
automatically may request the processing center 8 to retrieve from
its mass memory a block of digital data corresponding to the frame
of video information for an index page and transmit that block of
data to the channel's refresh memory. The digital-to-video
converter 13 in the channel thereafter translates the digital data
to a frame of video signals and these are applied in repetitive
fashion, to the terminal via the mixer 5 for display on its cathode
ray tube screen.
Thereafter, by means of a light probe or other selection device,
the user may select additional frames of information and by means
of a keyboard, if one is present at that terminal, enter
information into the displayed data. As a simple example, if the
user is a nurse, she may enter onto the patient's record his
present temperature, blood pressure and so on. This displayed
record (stored in binary form in the refresh memory) subsequently
may be transmitted to the computer for storage in its memory and/or
printout. If the terminal includes a printer, that printer may
receive, via the channel, from the computer, hard copy instructions
or, if desired, may be instructed by the user to print out at the
terminal, the record being displayed on the screen of the cathode
ray tube.
One of the modules, legended the "i' th module" for the sake of
generality, where i is any number from 1 to 200 in this example, is
illustrated in FIG. 3. There are 200 modules such as shown in FIG.
3 and all may be identical.
The module of FIG. 3 receives a TV line pulse b.sub.i from the
DIVCON. This pulse is applied to normally disabled AND gate 20. The
gate connects to a five bit counter 22 and the latter connects to a
decoder 24. When the AND gate is primed and the counter 22 is
counting, the decoder (enabled by the b.sub.i pulse) produces
during 32 different time intervals, 32 different channel select
pulses, each on a different output lead 26-1, 26-2 . . . 26-32.
The channel select pulses are each applied to a different group of
selection gates. For example, channel select pulse 1 (the channel
select pulse appearing on line 26-1) is applied to the select gates
1--1, 1-2, 1-3 and 1-4 for channel 1. Select pulse 2 is applied to
the select gates (not shown) for channel 2 and so on, select pulse
32 being applied to the select gates 32-1 . . . 32-4 for channel
32. The i'th terminal is connected to the i'th DCX module via two
lines 28 and 29. Line 28 carries the data signals and the idle
signals from the terminal to the module. It also carries the video
signals from the selected channel through the video amplifier 30 to
the i'th terminal, as will be discussed in more detail shortly.
Line 29 carries a light pen signal from the i'th terminal to the
light pen signal detector 34 in the i'th module.
The data detector 36 receives the data signals and idle signals
from the video terminal. The idle signals e are amplified to
signals k at the desired logic level and applied to the idle detect
pulse generator 38. As is clear from FIG. 6 (which does not show
the difference in amplitude between signals e and k) both signals
occur at the same times. the same times.
The keyboard data signals received by detector 36 are applied to
AND gate 40. These keyboard data signals occur during the video
display time of each scan line and never occur during the same time
as the idle signals and channel sample pulses c. Therefore, as will
become clearer from the discussion of FIG. 7, which shows the
details of generator 38, the latter ignores the keyboard data
signals. In a similar way, the IDLE signal is ignored by the
keyboard data input control logic circuit (6 in FIG. 1) in the
DIVCON control channel. The idle detect pulse generator 38 is shown
in more detail in FIG. 7. It includes two inverters 37 and 39 and
two AND gates 41 and 43. Under one set of conditions, namely the
presence of both of the signals c and k, AND gate 43 applies a
reset signal f to the search channel enable flip-flop 44. Under
another set of conditions, namely the absence of a signal from
channel enable flip-flop 44, the absence of the signal k from the
data detector 36, and the presence of the channel sample pulse c
from the DIVCON, AND gate 41 produces a set signal g which it
applies to the search flip-flop 42. The channel enable flip-flop
(FIG. 3) when set, supplies a priming signal to AND gate 45 and to
AND gates 40, 48 and 50 (all in FIG. 3). AND gate 45 receives a
second input - the channel sample select pulse c, from the DIVCON
via line 46. The circuit for producing pulse c, shown in FIG. 5, is
discussed later.
In the operation of the system of FIG. 3, when the video terminal i
is not in use, it continuously applies an idle signal e.sub.i to
the data detector 36. This and the other waveforms to be discussed
are shown in FIG. 6. The circuit for producing the idle signal,
shown in FIG. 4 comprises an idle signal generator 37 which is
responsive to the horizontal synchronization pulses. It applies the
idle signal, which occurs once each horizontal scan line interval,
through the normally primed AND gate 52 (FIG. 4) to the line 28.
The data detector 36 (FIG. 3), in response to the IDLE signal
produces an output signal k which is applied to the idle detect
pulse generator 38. The channel enable flip-flop 44 and search
flip-flop 42 are in their reset condition at this time.
When an operator desires to use a terminal, he inserts a key into
that terminal. In response to the insertion of the key, the key
responsive circuit 54 of FIG. 4 supplies an inhibiting signal to
the inhibit terminal of AND gate 52. This removes the idle signal e
from line 28 of th module for that terminal.
In the absence of the idle signal e, the data detector 36 of FIG. 3
removes the inhibiting signal k from the idle detector pulse
generator 38. In response to the next occurring channel sample
pulse c from the DIVCON, the idle detector pulse generator 38 (see
FIG. 7) produces an output pulse g which it applies to the set
terminal S of the search flip-flop 42. (While in FIG. 6 pulses e, f
and g are all shown to be present concurrently, this is done merely
to show their relative time relationships. It can be appreciated by
referring to FIG. 7 that pulse g is generated in response to pulse
c in the absence of both the channel enable signal and the idle
signal e (which is concurrent with the signal k) and pulse f is
generated in response to the pulse c in the presence of the idle
signal e.)
In response to the set flip-flop 42, gate 20 becomes primed. It now
passes the TV line pulses b.sub.i (see FIG. 5) produced by the
DIVCON.
The TV line pulse b.sub.i is applied to the i'th terminal once each
262 line times. These pulses may be generated in the manner shown
in FIG. 5. The horizontal synchronization pulses are applied to a 9
bit counter 62 and the latter connects to a decoder 64. The decoder
is capable of producing 2.sup.9 or 512 output pulses on 512
different output lines, however, as only 200 of them are of
interest (in view of the fact that there are only 200 terminals)
only 200 output lines are shown. (The nine bit counter is employed
rather than an eight bit counter as in some installations more than
256 terminals may be needed.) Each line from a decoder goes to a
different module. Accordingly, each module is clocked to a new
count during a different horizontal line time. This feature is
important as it prevents more than one module from selecting the
same not busy channel.
Returning to FIG. 2, when AND gate 20 is enabled, it applies the
successive TV line pulses b .sub.i it receives to the five bit
counter 22. Each such pulse advances the count produced by the
counter 22 by one. In response to each count and enable pulse
b.sub.i, the decoder 24 produces an output pulse on a different one
of its output lines. For example, in response to the count of one,
the decoder produces an output channel select pulse, having a
duration of one completed TV scan line time, on line 26-1. This
channel select pulse primes the select gates 1--1 . . . 1-4 for
channel 1. Assume now that channel 1 is not busy. When in this
condition, the channel applies to the vidoe terminal V a not busy
signal d.sub.1, once each horizontal line time. As shown in FIG. 5,
this pulse is produced in synchronism with the horizontal
synchronization pulses and is a relatively narrow pulse delayed in
time with respect to the horizontal synch pulse. The not busy
signal is narrower than the channel sample signal c and occurs
within the interval of the channel sample signal c. This signal d
actually is superimposed on the video signal (a) of FIG. 6 (the
video signal is not shown at d in FIG. 6 to avoid cluttering the
drawing) but occurs during the horizontal blanking interval.
The not busy signal d.sub.1 is applied through primed AND gate 1-2
to AND gates 48 and 70, priming AND gate 70. AND gate 48 is
disabled at this time as flip-flop 44 still is reset. In response
to the same channel sample signal c that started the pulse g and
the not busy signal d.sub.1, AND gate 70 becomes enabled and resets
the search flip-flop. This disables the AND gate 20 and the five
bit counter stops at the count of one.
The lagging edge of the pulse produced by the search flip-flop 42
sets the channel enable flip-flop 44. This primes AND gates 45 and
48 (and 40 and 50) and inhibits further g pulses. Each succeeding
channel sample c which now occurs causes AND gate 45 to produce an
inhibit not busy signal INB (of the same duration as pulse c) which
causes AND gate 1--1 to place terminal V for channel 1 at a
reference voltage level such as ground for a brief time interval
starting slightly before the d.sub.1 pulse and ending slightly
after the d.sub.1 pulse. This ground level indicates to all other
modules that channel 1 is busy. The INB signal does not pass
through any other of the first selection gates 2-1 (not shown) . .
. 32-1 because they are all disabled by the absence of signal on
leads 26-2 . . . 26-32.
Upon the termination of each INB pulse, the video signal from
channel 1 passes through AND gate 1-2 and through primed AND gate
48 to the video amplifier 30 and the latter applies this video
signal to the terminal itself, terminal i in the case of FIG.
3.
As mentioned above, the channel enable flip-flop 44 produces a
priming signal on its 1 output terminal when the i terminal has
obtained access to a channel in the manner described. This signal
primes the three control gates 48, 40 and 50. It is possible for
the terminal to communicate via these three gates to the channel
which has been selected, which is channel 1 in this example. As
already mentioned, AND gate 48 permits video to be applied to the
terminal via line 28.
AND gate 50 permits the light probe signal to be applied from the
terminal via the light probe detector 34 and the gates 50 and 1-4
to the light probe terminal L of channel 1. In brief, when the
light probe receives a "hit" this hit is detected by the light
probe detector. The time at which this signal occurs is indicative
of the place on the television screen at which the light probe was
pointing when the hit was made. In more detail, the time at which
the photodetector in the light probe senses the electron beam
passing the light pen position, indicates the location on the
screen of the character, symbol or the like at which the probe is
pointing. This light probe signal indicates to the logic selection
circuits in channel 1 a course of action to be followed as, for
example, the new frame of information desired to be displayed.
The purpose of the data detector 36, aside from that of producing
the signal k in response to the idle signal, as already described,
is to transmit keyboard information to the channel. This
information flows from the keyboard through the detector and AND
gates 40 and 1-3 to the data (D) terminal of channel 1. This data
may be stored in the refresh memory of channel 1 and translated by
the character generator circuits (the digital-to-video converter
13) within channel 1 to the video signals necessary for displaying
a character on the cathode ray tube screen, as one example. The
place on the screen that the character or characters will be
located may be indicated by a cursor on the screen, in well-known
fashion.
Assume now that in response to a particular count, such as one,
produced by decoder 24 the channel corresponding to that count is
not available - is busy. In this event the not busy signal d.sub.1
will not be present. Accordingly, even though in response to the
count of 1 the selection gates 1--1 . . . 1-4 are primed, AND gate
1-2 will not produce an output during the interval of the channel
sample signal c when the not busy signal d.sub.1 for channel 1
should be present. In the absence of an output from AND gate 1-2,
AND gate 70 remains disabled and the search flip-flop does not
become reset. Accordingly, AND gate 20 remains enabled and the
counter 22 continues to count the TV line pulses b.sub.i.
Similarly, the channel enable flip-flop 44 remains in its reset
condition.
The process described above continues until the module finds a
channel which is not busy. It then connects that channel in the
manner described to the video terminal demanding access to a
channel.
Returning now to the first set of circumstances mentioned above,
that is, terminal i connected to channel 1, assume now that the
operator is done. He removes his key from the i terminal. In
response thereto, the circuit of FIG. 4 starts again to generate
the idle pulses e.sub.i. In response thereto, the data detector 36
produces the signals k applied to the idle detector pulse
generator. The next time the channel sample pulse c occurs, the
idle detector pulse generator 38 (see FIG. 7) produces an output
pulse f which resets the channel enable flip-flop 44. This disables
AND gate 45 removing the INB signal from AND gates 1--1, 2-1 (not
shown) . . . 32-1. This permits the not busy signal d.sub.1 which
previously had been inhibited again periodically to occur at the TV
horizontal line frequency indicating to all modules that channel 1
again is available.
The reset channel enable flip-flop 44 also disables control gates
48, 40 and 50. The search flip-flop 42 previously was reset so that
AND gate 20 also is disabled. Channel 1 is now available for other
modules and terminal i is not in use. The next time terminal i
comes into use, the five bit counter will start counting again,
starting at the last count stored in the counter. Thus, if the five
bit counter happens previously to have selected, say channel 24,
the first TV line pulse b.sub.i it receives will change the count
to 25. This is perfectly all right as it makes no difference in
which order the channels are queried.
In the background of the invention section of this application the
cost per terminal for the FIG. 1 system is given as roughly $3,000.
Using the same basis for calculations, the cost per terminal for
the FIG. 2 system is:
a. $500 for the terminal itself just as in FIG. 1.
b. There are 6 1/4 display terminals per channel making the channel
cost per terminal $400.
c. The cost per module for the FIG. 3 circuit is estimated to be
$50.
The figures above gives a cost of $950 per terminal a saving of
roughly $2,050 per terminal.
* * * * *