Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROVND OF THE INVENTION
In the field of digital data transmission, there has
existed a need for simple but reliable apparatus for connecting
a pair of data sources or one source and one utilization device
together along with a high degree of control and error detec-
tion capabilities. Most interconnecting devices for data sets
are designed for either short (a few feet) direct wire con-
nection or switchable or multiplexed operation over telephone
lines or networks. There has been a real need which has been
largely unrecognized for an interconnecting device for data
sets employing dedicated lines which are non switched and un-
loaded. Given such parameters the transmission characteristics
of the line are the controlling limitation.
The need for complex coding of control information
or separate control channels is unneeded in such system,
however, control for establishing the desired status between
the data devices is essential, such as Carrier On, Direction of
Data Transmission, and mode such as half or full duplex. De-
tection of busy, open circuit or other fault conditions is
also essential. Heretofore, the classic solution to line length
limitation for speech has been to add inductive loading to
balance the distributed capacitance of the line. Loading tends
however, to distort data signals and therefore is not a satis-
factory solution for data channels.
BRIEF STATE~ENT OF THE INVENTION
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I have developed data transmission apparatus filling
these needs and one which generates and transmits control in-
formation which is easily segregated from data and accomplishes
the necessary control functions without the need for data
storage devices.
I have also developed apparatus which utilizes the
transmission capability of unloaded twisted pairs far superior
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;-~ than heretofore by a balanced D.C. system of signalling. I
further have provided for discrimination against transients
without degradation of the signal as is common employing ~
filter techniques. ;
One feature of this invention resides in the genera-
tion of all control information codes as bit sequences
which are invalid data codes.
Another feature of this invention resides in the gen-
eration of various duration "O" bit trains for instutiting
the required control functions.
One further feature of this invention resides in the
data transmitter employing a constant current generator
capable of applying equal and opposite currents to each of
two conductors and for reversing the current indicative of
a change of state of the data.
Still another feature of this invention involves the
discrimination against any noise spikes which might appear
on the line. A discriminator is included which prevents
any pulses of less than a predetermined length from reaching
any of the control circuitry without distorting any received
data.
REFERENCE TO RELATED INVENTIONS
An improved constant current generator for use in the
transmitter portion of this invention is disclosed in my
copending application, Serial No. 403,998, filed October 5,
1973, now U. S. Patent 3,864,623.
In one aspect of the invention, there is provided
apparatus for interconnecting digital equipment to a direct
current passing data transmission loop having at least a
pair of line conductors comprising a constant line current
generator connected to each of said conductors for intro-
ducing substantially equal and opposite currents into
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respective line conductors forming said loop; means connect-
ing a source of digital data to control the direction of
current flow from said constant current generator to line
conductors; means for generating a code of alternate marks
and spaces; means for generating a plurality of codes of
predetermined lengths of spaces indicative of a plurality
of control functions; and means for introducing said codes
into said connecting means to control the direction of flow
of current from said constant current generator to said
line conductors.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of this invention may be
obtained by the following brief description of the drawings
in which:
Fig. 1 is a block diagram of the apparatus of this
invention
Fig. 2 is an electrical s~hematic diagram of the
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analog receiver of the apparatus of Fig. l;
Fig. 3 is an electrical schematic of the digital -
receiver of the apparatus of this invention; -
Fig. 4 is an electrical schematic diagram of the
noise rejection circuit of the apparatus of Fig. l;
Fig. 5 is an electrical schematic diagram of the
analog send circuit of the apparatus of Fig. l; and -
Fig. 6 is an electrical schematic diagram of the -
connect logic of the apparatus of Fig. 1.
10 DETAILED DESCRIPTION OF THE INVENTION ~ -
Now referring to Fig. 1, the signal direction appara- -
tus of this invention is illustrated as conneçtable to two :
pair of transmission wires, a first pair identified as
Receive Data RD+ and RD-, and a second pair identified as ''
Send Data SD+ and SD-. These pairs, RD and SD constitute the
interconnection to similar such apparatus located at a distance
eg. 1 to 10 miles on a dedicated non loaded non polarity
switched transmission line. Baud rates of 0-2400 are normal
depending upon line length.
The apparatus is also connected to a data source/sink
which has the following input or output signals to the direc-
tion apparatus~
INPUTS FROM DATA SOURCE OUTPUTS TO DATA SINK
Busy
Data SD Data RD
Data Terminal Data Set
Ready DTR Ready DSR
Ready to Clear to
Send RTS Send CTS
Carrier On CO
Ring RI
CO~TROLS
Considering first the controls of the apparatus, it
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includes in addition to normal power switches, unshown, a basic
function ANS/ORIG/Switch 10 which forms a part of the connection
logic circuitry 11 of Figs. 1 and 6. The switch 10, when in
the ANS position, conditions the apparatus for receive only
operation and when in the ORIG position, the station is the
originating or sending station. A similar switch 12 selects
the mode of transmission, namely, H (half duplex) or F (full
duplex). If the switch 12 is in the H position, the signal
direction apparatus of this invention can either transmit or
receive data in mutually exclusive time periods. When the
switch 12 is set in the F position, it can process, send and
receive data simultaneously. The H position of switch 12 can
also be used to loop data from send to receive at the local
station. -
A ring switch 13 is operated to originate a call.
It controls a character generator 14 through NAND logic 15
to produce a predetermined signal which is conducted through
the communications system, e.g. line conductors SD+ and SD-
to the remote station. 'rhe preferred code is a character
constituting two spaces separated by a mark 010. When the
remote computer responds, the corresponding direction apparatus `~
generates a similar character 010 which is detected in the
connect logic 11 and via lead 16, illuminates a "connectedn - ;
lamp 20.
A disconnect switch 21 is used to disconnect the
local station from the remote computer. When this switch 21 is
operated, a two second space character is produced in genera-
tor 22 and introduced via lead 23 to an analog (ie. trans-
mitted form) send transmitter 24. This signal is also sent via
lead 25 to the connect logic circuit 11 where it resets the
connect logic and disables the connect light 20.
~rwO pair of loop switches 30 and 31 interconnect the
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apparatus to the transmission lines RD and SD respectively.
The switch 30 constitutes a double pole double throw switch
which alternately connects an analog receive circuit 32 to
conductors RD+ and RD- when in the receive condition or to a -~
pair of cross connections 33 and 34 to the analog send cir-
cuit 24. When in this alternate position, the apparatus is
connected to loop data from a local source back to itself for
local printout of its own data. The normal position of
switch 30 is as shown connecting the analog receiver circuit to
the receive conductors RD+ and RD-.
Other controls include a test switch 35 used to test
not only the local station but the loop as well. When actuated
and held in the operated condition, a pair of indicator lamps, ~-
the busy lamp 36 and the fault lamp 40, are illuminated. The
first occurs by applying a ground to a busy error circuit 41
which in turn, through NAND logic circuit, illuminates lamp 40.
The same ground signal from switch 35 over lead 43 and NAND :~
logic circuit 44 illuminates busy lamp 36. At the remote
station a five second timer is triggered by the test signal on 1-
lead 45 to analog send circuit 24 and the loop which lights the
remote busy indicator and generates a code 010 which is returned ,;
over the line to the local station which passes the character
through the analog receive circuit 32, a noise discriminator
circuit 46 described in Fig. 4, a digital receiver circuit 50,
the busy error circuit 41 and NAND logic circuit 42, to de-
energize the fault lamp 40. The busy lamp 36, under the
direct control of test switch 35, remains illuminated as long
as the switch 35 remains operated. If the preceding sequence
all occurs as described, the transmission loop is operative.
SYSTEM OPE~ATION
The apparatus of this invention is basically an
asynchronous DC data set which operates at varying baud rates
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and over ranges of from one to ten miles of unloaded twistedpair common to the telephone plant as dedicated non switched
lines. -
Operation of this apparatus is best described by
following operation sequence while referring to the drawing,
Fig. 1. The operator depresses the ring switch 13 which
; through NAND gate 15, enables character generator 14 to pro- ;
duce a character of two spaces separated by 1 mark (010) which
is introduced into the analog send circuit 24 and then via
switch 31, to the conductors SD+ and SD-. The comparable ;; ;~
apparatus at the remote end illuminates its connected lamp
(comparable to 20) after its digital receive circuit (50)
detects the code and sends a ring signal RI to its associated
computer by operation of its ring logic circuit (52) and ring
oscillator (53). The ring signal traverses lead (5~ and its
driver amplifier (55) to the ring terminal (RI) connected to -;~
its associated computer. If the computer is ready, it applies '-
a high potential to the DTR terminal which in turn causes the
Data Set Ready DSR, Carrier On CO and Clear To Send CTS lines
... ...
to the computer to become "high" or on.
The remote apparatus then generates a character (010) ¦`
which is transmitted to the originating station, illustrated
in Fig. 1. The character received over leads RD+ and RD- and
switch 30 is passed through analog receiver 32, noise rejection
circuit 46 and detected in the digital receive circuit 50. The
detection of a character in digital receive circuit so results
in the enablement of connect logic 11 over lead 60. Logic
circuit 11 in turn illuminates the CONNECTED lamp 20 over
lead 16.
If the busy line 61 from the local data device is
raised, the busy and fault indicators 36 and 40 are illuminated
and a continuous space 0 is applied to the analog send
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circuit 24 via lead 62 from the test switch 35 and receiver 63.
If the remote data apparatus raises a busy line, the remote or !:
ensuing apparatus generates a train of "0"'s of five seconds
duration. The local apparatus in the digital receive circuit 50,
as described more fully, detects a space of five seconds or -
more and produces a pulse on lead 64 which enables NAND gate 44,
illuminating the local busy lamp 36. An enabling pulse is
also conducted via leads 64 and 65 to NAND logic 15 trigger
character generator 14 to repeat the ring sequence 010. In-
terrogation will continue until the computer is available.
DATA TRANSMISSION
When the appar~tus of this invention! its local
data set, the transmission line, the remote apparatus and its
computer are operatively connected, data from the data set from
terminal SD is conducted via switch 51, a data receiver ampli-
fier 66 and lead 70 to the analog send circuit 24 and to the 1 -
line conductors SD~ and SD-. Data is transmitted in the form
of polarity reversals or more correctly, as reversals in current
dlrection on leads SD+ an~ SD- with a mark 1 being one direction
of current flow and a space 0 the opposite direction of current
flow. This is reliably acheived employing the analog send
circuit of Fig. 5 which is more fully described in my U.S. Patent
3,909,510 referenced above. Suffice it to say, equal and op-
posite currents are applied to each conductor SD+ and SD-
durinq a mark 1 and equal and opposite currents are applied to
opposite conductors during a space 0. i~
Employing the improved transmission capability
afforded by the analog send circuit 24 of Fig. 5, reliable
transmission is easily accomplished reliably over circuits of
twisted pair of 19 gauge conductor at the following rates:
DATA RATE RANGE ~
300 baud 22 miles -
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600 baud 15 miles
1200 " 11 "
2400 " 7 " -
For smaller gauge cable, the range is reduced because
of the wire limitations. ~ ;
~ALOG RECEIVER
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The receiver 32 as shown in Fig. 2 employs basically
a balanced resistance input circuit 80 driving a dual sense
amplifier 81, diode 82 and NAND gate 83 which look for the
presence of line current. In the absence of line current, a
signal on leads 84 and 85 illuminates fault lamp 40. Where
line current is present, its direction is determined by a diode
network 86 cooperating with amplifier 90, providing an output
signal for each mark (1). This signal on lead is applied with
the analog error signal on lead 84 to a NAND gate 92 which pro- '-
vides receiver data RD on lead 93 and receive data RD through
inverter 94 on lead 95.
NOISE REJECTION
Both received data RD and received data RD on their
respective leads 93 and 95 are applied to noise rejection cir- '
cuit 46 illustrated in Fig. 4. This circuit employs an OR
gate 95 as the trigger input to a retriggerable monostable
multivibrator 100 producing an inhibit signal to a quad two
input NAND gate 101. If the input signal RD lasts less then
a predetermined length of time, eg. 200 microseconds, the multi-
vibrator 100 will be reset and one enabling input to NAND
gate 101 removed and the further advance of the pulse on
lead 102 is prevented. Short pulses, ie. noise, are therefore
isolated from the control apparatus. This circuit rejects noise
spikes which may appear on the line RD+ and RD- but does not
degrade the quality of valid data signals which are passed
through without the distortion common to filters.
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DIGITAL RECEIVER
Received data RD distinguished from noise spi~es is
applied on lead 102 to digital receiver 50 shown in detail in ~ -
Fig. 3. The circuit detects:
a) connect code 010
b) disconnect code 2 second space 0---0
c3 busy code 5 second space 0-~ 0.
One second or more space detection is accomplished employing
an OR gate 110, retriggerable monostable multivibrator 111,
NAND gate 112 and reset transistor 113. Five second spaces
are detected by OR gate 114, retriggerable monostable multi- ,
vibrator 115, NAND gate 116 and reset transistor 120. Three
NAND gates 121, 122 and 123 detect a series of two space~
separated by a mark (010) indicative of a received character
used to establi&h connection of the loop, via lead 124 and -
connect logic 11. Since most common data sources such as i~
a teletypewriter, include this sequence in common characters,
the commencement of any operation will, when this apparatus is -
on, send a ring up signal to the remote end and attempt con-
nection.
CONNECT LOGIC
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The basic switching functions under the control of
manual switches plus incoming characters or spaces are
carried out by the connect logic circuitry of Fig. 6. This
circuit includes a pair of NAND gates 130 and 131 with A~ALOG
ERROR on lead 84 and received data RD as well as Data Terminal
Ready signals DTR and Carrier On CO from the remote station.
The NAND gates 130 and 131 pass received data RD through to
the receiver and additionally, furnish the Data Set Ready DSR,
Clear To Send CTS and Carrier On CO signals to the local data
set. The connect logic circuit also includes a second pair of
~AND gates 132 and 133 under the control of a test switch 35,
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the original ANS function switch 10 to provide a xeset pulse to
a monostable multivibrator 135, which is operated when the
system connections are complete and ready for transmission.
ANALOG SEND CIRCUIT
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One of the features of this apparatus making maxi-
mum utilization of the transmission medium is the analog send
circuit 24 of Fig. 1 shown in detail in Fig. 5. It employs
basically an operational amplifier 160 having a pair of input
terminals 161 and 162 and an output terminal 163 and a pair
of opposite polarity voltage supplys 164 and 165. Equal
value resistances R2 are in series with each input signal path
and a resistance Rl shunts the input. Resistance Rl is in the
direct current path from a transistor switch which is connected
alternately applying positive or negative potentials from
sources 167 and 168 to a lead 170 under the control of NAND
logic includes gates 180-182. The voltage drop across the -
resistance Rl, is a function of the currènt from the
supplies 167 or 168 associated with the transistor switch 166.
The operational amplifier 16 includes two feedback
paths, one including a resistance R2 back to input terminal 161
and the other including resistance Rl plus a resistance R2 ~
to provide an unbalanced feedback proportional to resistance Rl. ~-
With matched resistances Rl at both the input and output, the
current on lead SD- is equal and opposite to the current on
lead SD+ regardless of loop impedance variations. This driving
of both conductors simultaneously provides balanced signals
applied to the line and enhanced direction at the remote
terminal.
Two pair of back to back Zener diodes 171-174 with
their center top grounded provide for shunt lightning or surge
protection for the operational amplifier 160.
The analog send circuit, in addition to the generation
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of line current, includes a three NAND gate 180, 181 and 182
logic circuit which combines control information with data on
lead 184. Data and control characters serve to forward or back-
ward bias transistor 166 to apply current to the lines SD+
and SD- from the sources 167 and 168 on lead 170. Source 168
is connected to the line 170 via a constant current source 169 ~ -
which in fact is a field effect transistor operated as a diode. -
SUMMARY
From the foregoing, it may be seen that I have
developed a DC data transmission set designed to interconnect
remote data devices by simple twisted pairs of conductors while
allowing full supervision and control as in complex data trans-
mission systems. ~ -
Data is transmitted as current reversals produced by
an analog data transmitter. The data transmitter produces
equal and opposite currents on the pair of conductors.
The data receiver includes a noise rejection circuit
which inhibits the processing of any pulse of length less than
one half the shortest pulse length, eg. 200 microseconds, for
2400 baud ~pulse ler.gth 400 microseconds). The noise rejection
responds only to the pulse length and does not affect the
amplitude of any valid pulses.
Signalling and the establishment of the connection
and disconnection functions is accomplished employing various
length spaces and the apparatus includes means for detecting
trains of spaces constituting control information. The appara-
tus provides means for testing both the local loop including
the apparatus and the loop including the remote station.
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