Two wire bi-directional digital telephone link

This invention details a baseband bipolar pulse signaling technique employing only two wires for simultaneous bi-directional communications. A first polarity of pulses is utilized for communication of traffic in one direction; traffic in the return direction utilizes pulses of the opposite polarity. Each end of the communication link has both a sender and a receiver. One sender arbitrarily transmits only positive pulses; the receiver at this end of the system will recognize only negative pulses. The inverse set of conditions is enforced at the opposite end of the link. Means are included for synchronizing the application of pulses to the link at the other end in response to the receipt of pulses at that end to avoid overlap of receive pulses at the originating end of the line.

BACKGROND OF THE INVENTION 
The present invention relates generally to signal transmission systems and 
particularly to pulse transmission systems employed in telephone 
communications technology, currently termed digital telephone 
communication. 
Prior Art 
A great variety of digital communication systems and schemes exist. One 
such scheme under current industry consideration for bi-directional 
digital telephone communication utilizes 8,000 samples of incoming signal 
per second. Eight bits of digital information are extracted for each 
sample or frame which yields a basic 64 kilobit per second transmission 
rate. Two additional bits per frame are specified for controlling and 
signaling purposes which make a grand total of 10 bits per frame and an 80 
kilobit per second transmission rate. The 80 kilobit per second 
transmission is required for each direction since, in a telephone system, 
both talking and listening are simultaneous. The general approach to this 
situation in the prior art has been known as a four-wire connection with 
one pair of wires carrying an outgoing 80 kilobit per second data signal 
and the other pair carrying an incoming 80 kilobit per second data signal. 
This presents a significant problem, for most telephone instruments today 
are analog devices. These transmit and receive analog signals and are 
connected to one another and to the external systems over a single pair of 
wires. Inside of the telephone itself, a device known as a hybrid circuit 
separates the analog signal to connect them respectively to the microphone 
and ear phone. If such telephones are to be converted to digital telephone 
use (i.e., those in which the voice signal is digitized for transmission 
and reception) some method must be found for transmitting and receiving 
the 80 kilobit per second data signals simultaneously over a single pair 
of wires as used by the existing telephone wiring. Rewiring of existing 
facilities to convert from two wire to four wire circuits would be 
extremely time consuming and expensive. This invention describes a means 
and method by which the system can transmit and receive 80 kilobit per 
second data signals over a single pair of wires simultaneously so that 
normal telephone conversations can be carried on as well as digital 
two-way data exchange. 
U.S. Pat. No. 3,721,763 illustrates some of the prior art in this area. Two 
separate unipolar transmitters which transmit pulses of the same polarity 
are utilized with a two-wire connecting system. Inverting transformers are 
interposed between the transmitter and the receiver at each end so that 
the received pulses are inverted for recognition and to avoid interference 
with positive pulses being simultaneously transmitted at the same end. 
However, a ground connection is also shown and can be inferred as being a 
three-wire connection between the transmitter and receiver and this ground 
connection introduces the possibility of ground loop interference which is 
most undesirable. 
Other illustrative patents showing two-wire simultaneous bi-directional 
communication are U.S. Pat. Nos. 4,012,590, 4,112,253 and 4,117,277. These 
patents show alternative approaches to the basic problem but utilize other 
communication techniques in which the current or voltage levels are 
compared or differentiated or in which special encoding and decoding 
operations are conducted to accommodate the bi-directional simultaneous 
communication. These systems and techniques are generally more complex and 
expensive than the currently contemplated one as will be readily 
appreciated by those of skill in the art. 
Objects 
In view of the foregoing difficulties of expense and complexity in the 
known prior art systems contemplated for a similar purpose to the present 
invention, it is the object of the present invention to provide an 
improved two-wire digital data communications system compatible with 
normal telephone installations in existence and which does not require a 
ground connection from station to station. 
A further object of the present invention is to provide an improved 
communication technique. 
Summary 
The foregoing and still other unenumerated objects of the present invention 
are met by providing at opposite ends of a two-wire communication link, at 
least two stations. Each of the stations has both a sender and a receiver. 
One of the senders is selected to transmit only positive pulses and the 
receiver located at this station is selected to recognize only negative 
pulses. An inverse situation is enforced at the opposite end of the 
communication link. Means are further included for synchronizing the 
application of return pulses in response to the receipt of pulses from the 
opposite end so that the pulses returned will not overlap with those being 
transmitted when they are sensed at the receiver. A phase locked loop at 
each receiver extracts the clocking information from the transmitted 
signals for synchronizing the return transmission to avoid overlap of the 
pulses and for determining whether a received bit is a 1 or a 0. Means are 
also provided for extracting framing information for determining the 
significance of the detected bits.

DETAILED SPECIFICATION 
Turning to FIG. 1, the overall block schematic diagram of a communications 
system employing the method and apparatus of the preferred embodiment of 
the present invention is briefly shown. Station 1 and station 2 are 
connected to one another by a two-wire communication line 3. Both stations 
have a transmitter or driver 4 and a detector or receiver 5 connected as 
shown to the communication link 3 via transformers 6. It is to be noted 
that inverted polarity transformers are utilized for one of the driver 
receiver pairs connected by communication link 3. A terminating resistor 7 
appears at each end of the communication line 3 at the input/output 
connection for each station 1 and 2. A single positive power supply 8 is 
indicated for each station 1 and 2. The detector and driver circuits 5 and 
4 respectively, are shown in greater detail later. 
Turning to FIG. 2, the overall scheme of transmission in the present system 
is described with relationship to the timing charts showing the framing 
and synchronization of the transmission and receive operations. 
In FIG. 2, an 80 kilobit per second transmission system is described which 
is divided into 8,000 frames per second with 10 bits in each frame. Each 
frame thus comprises 125 microseconds and each bit is 12.5 microseconds in 
duration. Each bit time of 12.5 microseconds is further subdivided into 10 
segments of 1.25 microseconds each. The basic system clock rate for 
generating these divisions is thus 800 kilohertz. Line A of FIG. 2 shows 
the basic frame rate of an 8 kilohertz framing of 125 microseconds per 
frame. Line B shows the details of the first three bits of 12.5 
microseconds each on an expanded scale. 
It will be noted in line B that each bit includes at least one pulse at the 
start of the baud or bit time and may include another pulse located at the 
third subdivision or time slot within the baud. This secondary pulse, when 
employed, occupies 0.625 microseconds so it is 1/2 of a time slot in 
width. There is always at least one pulse at the start of each baud to 
facilitate the extraction of the basic 800 kilohertz clock signal at each 
receiver. The clock extraction is done by means of a phase locked loop 
circuit described in detail later. Additionally, the first pulse at the 
start of each baud occupies a full 1.25 microsecond time slot so that it 
is twice as wide as any secondary data pulse within the baud. The 
secondary pulse is the data pulse and its presence gives meaning to the 
content of the baud. The wide pulse at the start of each baud in a frame 
indicates the start of frame synchronization and also indicates that the 
most significant bit of a 10 bit word follows it. 
The pulses applied to the transmission line 3 of FIG. 1 will be delayed due 
to the basic transmission delay in the system and because a pair of wires 
are normally twisted to reduce outside interference. The time delay 
involved for a pulse propagation on a twisted wire pair is approximately 5 
microseconds per kilometer. Line C of FIG. 2 illustrates the effect on the 
pulse delay after approximately 750 feet of twisted wire has been 
traversed. The pulses are idealized and the effects of resistance, 
conductance and capacitance which would distort the pulse shape are 
ignored for this discussion. 
In Line C, the pulses which were assumed to be transmitted at the timing 
shown in Line B, are showed delayed in time being received after 
approximately 750 feet of propagation on the twisted two-wire line. At 
this point in the system, the clocking and frame synchronization signals 
would be extracted and any return data would be encoded. Line D 
illustrates the pulse train at the receiver which would be transmitted 
back to the originating station. In line D, the returning pulses are timed 
for transmission with respect to the incoming pulse train to be sent 
following receipt of the secondary pulse in each baud (if present). The 
return signals are negative pulses. It is important to note that positive 
pulses for transmission in one direction and negative pulses for 
transmission in the other lie at the heart of this scheme. The composite 
signal on the wire at the receiver which is then going to respond to the 
original station therefor looks like that in Line E of FIG. 2. 
Line F of FIG. 2 shows the wave train as it would appear as received at the 
original transmitter. This is a combination of the positive pulse trains 
of line B combined with the negative pulse train of line D with the pulses 
from line D delayed by the same amount as those in line C of the positive 
nature. It may be seen that the delay between sending of positive pulses 
and the return of negative pulses will be twice the delay of the 
transmission link. 
It is apparent from the discussion that if a pulse train of line F is 
viewed, that the longer the wire linking of the two stations together 
becomes, the longer the delays will be and that at some length a returned 
negative pulse will interfere at the originating transmitter with the 
sending of positive pulses since they will overlap in time. 
To illustrate this notion, FIG. 2, line G shows the same transmitted pulse 
train as line B and line H shows the effect of assuming a longer wire 
connecting the two stations so that the pulses will be delayed by 3.125 
microseconds. Line J shows negative pulses being returned following the 
receipt of delayed pulses on line H. Line K shows the composite pulse 
train as it would be seen at the original receiver and line L shows the 
composite as it would appear at the original transmitter including the 
additional delay for the return of negative pulses. It is well to note 
that the return pulses just barely miss interfering with positive pulses 
being transmitted. Thus, if the communication link between the two 
stations were any longer or their propagation delay any greater, negative 
pulses would interfere in part with the transmission of positive pulses 
and would render one or the other or both undetectable at their respective 
receivers. 
A delay of approximately 3.125 microseconds in propagation time represents 
a length of transmission wire of about 2,000 ft. Hence, given the scheme 
described so far with the timing of the return pulses following the 
receipt of incoming ones, a transmission length of approximately 2,000 
feet is maximum. Any length of wire that results in a transmission delay 
of more than 3.125 microseconds would cause some interference at the 
transmission end in the example given. There is no specific problem at the 
receiver end in this example since negative pulses can be placed anywhere 
in the baud following the receipt or prior to the receipt of positive 
pulses. A solution to the problem which permits wire of theoretically 
unlimited length is described in lines M through P of FIG. 2. 
Line M shows a negative pulse train equivalent to that shown in line J 
except for the fact that the pulse train in line M is assumed to be sent 
at the end of the baud rather than immediately after the receipt of 
positive pulses which were described in line K. A new composite at the 
receiver end is shown in line N. The positive pulses still arrive as shown 
in lines H and K. Now adding the delay of the line for the negative 
pulses, the composite pulse train appearing at the original transmitter is 
shown in line P. In line P, it can be seen that the return negative pulses 
also just barely miss interfering with the transmitted positive pulses, 
but this time the negative pusles appear after the positive pulses. 
Therefore, any greater delay will further separate the pulses and avoid 
interference. 
Comparing the pulse train in line P with that in line L, it can be seen 
that both pulse trains just barely miss interfering with the transmitted 
pulses. But it is apparent that the pulse train of line L would interfere 
if the wire were longer and the pulse train of line P would interfere if 
the wire were shorter. Since a wire length which produces a delay of 3.125 
microseconds permits either pulse position for timing the return negative 
pulses to be acceptably received, it is obvious for any length of wire, 
one of the two pulse timing positions for transmission of negative pulses 
will be satisfactory and will not interfere with the originating positive 
pulses. Thus, any length of wire can be accommodated in this scheme by an 
appropriate selection of the timing point for returned negative pulses 
relative to the incoming positive pulses or to the beginning or end of a 
baud time. 
Returning to FIG. 1, the basic scheme described in FIG. 2 can be further 
described. As noted above, the basic scheme consists of transmitting 
positive pulses from one facility, conveniently called a central facility 
but obviously capable of being a single station, and of transmitting 
negative pulses from remote terminals or telephones, each pair being 
connected on a two-way path consisting of one twisted pair of wires. 
Starting with the positive pulses generated in station 1 of FIG. 1, a 
driver circuit 4 drives pulses into the primary circuit of a pulse 
transformer 6. The secondary of transformer 6 is connected to the 
communication link through a PNP transistor. Thus, when a positive pulse 
appears at the output of driver 4 in station 1, it causes a base emitter 
current to flow, thus turning on the entire transistor 9. 
Positive pulses will therefore appear across communication line 3 and the 
terminating resistor 7 (typically a 100 ohm resistance) at the other end 
of the communication link 3 another terminating resistor 7 with another 
PNP transistor 9 connected to the transformer winding for transformer 6 as 
shown. As on the transmission station 1 end, positive pulses go through 
PNP transistor 9 and are picked up by the other winding of transformer 6. 
This winding has one end grounded and one end connected to a signal 
detector circuit for detecting positive pulses. The signal detector 
circuit 5 amplifies and squares the signal received since it will be 
received in a distorted condition due to resistance, capacitance and 
inductance effects in the communication link 3. The signal is then 
presented to the phase locked loop and other circuitry not shown in FIG. 1 
for data and clock extraction. 
At the receiving station 2 the logic (not shown) is used to generate a 
return signal if any is to be presented which is generated by operating 
driver 4 connected to another pulse transformer 6. As the dots on the 
windings of pulse transformer 6 at station 2 connected to driver 4 show, 
the pulses are inverted and become negative pulses. The negative pulse is 
passed through an NPN transistor 10 onto the communication link 3 through 
the terminating resistor 7. Negative pulses cannot re-enter the detector 
circuit at station 2 because of the action of the PNP transistor 9. 
Incoming positive pulses from the transmission station 1 cannot enter into 
driver 4 at station 2 due to the action of the NPN transistor 10. 
Similarly, at the transmission end, incoming negative pulses pass through 
NPN transistor 10 into detector 5 through the pulse transformer 6. The 
pulse transformer 6 by its dot positions shows a re-inversion of the 
pulses so that detector 5 can be of the same form as that in the receiver 
station 2. Positive pulses are prevented from entering the detector 
circuit 5 by action of the NPN transistor 10 as with the receive station 
2. 
It is well to note that through the use of the transformer 6, the actual 
transmission wires 3 are never connected to either the local ground or the 
power supply. This provides several distinct advantages. First, there is 
no ground connection common to the transmitter and receiver stations 1 and 
2 so there is no ground loop interference presented and a true two-wire 
system exists. Secondly, common mode noise is effectively eliminated by 
the use of the transformers and twisted pair communication wires. Thirdly, 
the transformer inverting function permits the generation and detection of 
negative pulses using only a single positive power supply at each end of 
the transmission line. 
Applying this scheme to the overall system envisioned for use, the 
following overall system would exist. A central facility transmitting 
positive pulses over a twisted pair of wires to a receiver would be 
employed. The receiver would detect positive pulses, extract clocking and 
frame information (after appropriate wave shaping and squaring of received 
distorted pulses) and will extract any data present and the data and 
synchronization information so derived will be passedto a user facility. 
The clocking information derived at the receiver is used for encoding data 
if any to be returned. The return data is in the form of negative pulses 
placed on the same pair of wires for transmission back to the central 
facility. At the central facility, negative pulses are detected, decoded 
(after appropriate wave shaping, etc.) and passed on in similar fashion to 
another user facility. Thus, it may be seen that either a central 
communication facility communicating with single stations or a central 
station handling communications between multiple remote signal stations 
can be constructed. If two remote facilities such as telephones wish to 
communicate with each other, they each transmit negative pulses to a 
central facility which connects them together, for example, by a time 
division multiplex arrangement so that they communicate respectively to 
each other via the interface of positive pulses generated at the central 
station. 
FIG. 3 is a simplified block schematic diagram of a central facility or 
transmission station 1. Basically the transmitter section of the system is 
shown in FIG. 3. It is assumed that the transmitter obtains data from some 
source and encodes it into the format shown in FIG. 2. In FIG. 3, the 
basic elements of the transmitter include a crystal oscillator 11 
producing a basic 800 kilohertz signal. This basic frequency would be used 
by all of the transmitters at the central facility or PBX. It is also 
shown in a section in FIG. 3 labeled Logic and Control Block 12. This is 
intended to encode data into the format shown in FIG. 2. It produces an 
output to driver 4, a frame synchronization pulse train of 8 kilohertz on 
an output 13 and a bit synchronization output 14 operating at 80 
kilohertz. The output of driver 4 is connected through transmformer 6 to 
modulate power from supply 8 onto the transmission line 3 as previously 
described. Because the logic and control circuitry for receiving either 
parallel or serial input data and for the 800 kilohertz clocking frequency 
is well known and available and does not form a specific element of 
novelty in the present invention, it is not described further herein. 
The encoding function is described in FIG. 2 from which it is obvious to 
those of ordinary skill in the art how the gating and timing for 
controlling driver 4 are to be synchronized with the frame synchronization 
signals and bit synchronization signals derived by counting down the basic 
800 kilohertz input. 
Turning to FIG. 4, the overall schematic for a receiver station 2 as shown 
in FIG. 1 is illustrated in greater detail. Positive incoming pulses pass 
through the PNP transistor 9 and transformer 6 to detector 5. In detector 
5, the incoming pulses will be amplified and squared to remove the effects 
of distortion previously noted. These pulses are then applied to a phase 
locked loop circuit 15 to extract the 800 kilohertz clock frequency in 
phase with received pulses. The phase locked loop operation is facilitated 
by the fact that each baud has at least one pulse at the start thereof and 
that each frame has a wide pulse at the start of the first baud as 
previously described. Other pulses within each pulse may be data pulses 
occurring at the third time slot within the baud and their presence or 
absence may be indicative of the presence of data. By means of simple 
logic, the data and character synchronization pulses as well as the data 
clocking function are extracted from the incoming signals and presented to 
an external time division multiplex circuit or other similar apparatus for 
use at the receiver station. The character synchronization pulse, i.e., 
the frame synch pulse is identified in the logic because it has a width of 
twice the other pulses and occurs at the start of each frame at the start 
of the first baud therein. 
Using this same extracted 800 kilohertz clocking frequency, a transmitter 
similar to that shown in FIG. 3 and having the same transmitter logic and 
control 12 as that shown in FIG. 3 may be employed. By means of the 
inverting transformer 6, the driver 4 connected to the transmitter logic 
and control 12 at the receiver station 2 illustrated in FIG. 4, negative 
pulses are placed on the communications line 3 for the return trip to the 
central station indicated in FIG. 1. It should be understood that at the 
central station 1 in FIG. 1, there would be placed another receiver next 
to the transmitter shown in FIG. 3 which is a duplicate receiver with its 
own phase locked loop to extract the 800 kilohertz clock required to 
detect any returning negative pulses. The receiver at the central station 
is the same as that shown in FIG. 4 and it should be noted that at the 
central station, the basic central station 800 kilohertz crystal clock 
frequency cannot be used for detection since, for extracting clocking 
information, the clock used must be in phase synchronization with the 
return data in order for detection to be performed. Due to the variable 
delay of different line lengths between stations, the central station 
cannot count upon the crystal clock at its site being in phase 
synchronization with any return pulses thus a separate phase locked loop 
clock circuit must be provided for each line connected to each remote 
terminal. 
As was noted earlier, the various received pulses at the end of long 
lengths of wire will be attenuated and distorted. As a result, some means 
is necessary to reconstruct and extract clean square edged pulses. Simple 
circuits for doing this have been devised and much more elaborate ones 
exist as will be understood by those of skill in the art. However, for 
purposes of demonstrating the preferred embodiment of the present 
invention, simple detector circuits were designed as shown in FIGS. 5A and 
5B, for detecting negative pulses and positive pulses, respectively. The 
transformers 6 used in these circuits have a one to three turn ratio step 
up resulting in a positive pulse of about 2 volts at the secondary in 
FIGS. 5A and 5B. (The primary is connected to the transmission line 3.) A 
2 volt input level is sufficient to open the diode 17 to cause the TTL 
inverter which may be a type 7404 module to change level cleanly to 
reconstruct an output pulse as shown. Before arrival of an incoming pulse, 
the TTL inverter 18 is held at ground through the diode 17 grounding 
through the coil of transformer 6. The 1K resistor 19 acts as a threshold 
setting means for the TTL inverter 18. 
FIG. 5A is the detector circuit for negative pulses while FIG. 5B showns 
the detector for positive pulses, the only difference being the 
transistors 9 and 10 and the inverting connection of one of the 
transformers 6 in the case of the negative detector circuit. The circuitry 
shown in FIGS. 5A and 5B is satisfactory for operation, but further 
refinement for high accuracy data transmission and detection would be 
desirable as will be understood by those skilled in the art. 
The system as described above with regard to the preferred embodiment has 
been constructed and tested with a multi-million bit transmission and 
reception test without error over a 4,000 foot wire. Thus, the feasibility 
of the basic concept of multiplexing pulses of different polarity 
traveling in two directions over the same pair of wires has been 
demonstrated. A range of at least 4,000 feet is clearly attainable and, 
with a better pulse detector circuit, an indefinite increase is possible 
especially with the use of intermediate repeaters.