Device for the transmission of push-pull signals across a two-wire line in full duplex operation

A device for the transmission of push-pull signals by way of a two-wire line in full duplex operation has a push-pull transmitter for the transmission of the push-pull signals to an opposite station and a push-pull receiver for the simultaneous reception of the push-pull signals transmitted from the opposite station, and a compensation circuit for compensating the push-pull signals transmitted from the associated push-pull transmitter in relation to the inputs of the push-pull receiver. The push-pull transmitter and the push-pull receiver are designed as symmetrical differential amplifiers composed of emitter-coupled transistors and are provided with a constant current feed. The compensation circuit is of symmetrical construction and inputs of the push-pull receiver are connected to the wires of the two-wire line by way of two identical decoupling resistors which form parts of the compensation circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for the transmission of push-pull 
signals, and more particularly to such a device in which a compensation 
circuit is provided for the compensation of push-pull signals transmitted 
by a push-pull transmitter in relation to the inputs of an associated 
push-pull receiver. 
2. Description of the Prior Art 
The transmission of dc data signals between remote opposite stations for 
which identical ground potentials are not assured, is primarily carried 
out with the aid of push-pull signals across two-wire transmission lines. 
Differences and mutual fluctuations in the ground potentials then 
influence the pair of lines as push-pull disturbances, and can be 
neutralized by a receiver having push-pull suppression facilities. A 
disadvantage of such an arrangement resides in the fact that two lines, in 
respect of each transmission direction, are required for every signal. 
Since it is necessary to transmit a plurality of data signals in parallel, 
in particular between data processing systems or between the central unit 
and the periphery of the system, a considerable expense is involved in 
respect of lines and connecting plugs. 
The line requirement can be reduced with the aid of devices which 
facilitate data transmission in both directions across a two-wire 
transmission line, i.e. full duplex operation. In this case it is 
necessary to prevent the output signal of a transmitter from reaching the 
assoicated receiver. It has long been known to interpose the transmitter 
and receiver of every opposite station into the diagonals of a bridge 
circuit, and to arrange the two-wire transmission line and a balancing 
line into two adjacent bridge arms. However, this type of bridge circuit 
has the disadvantage that only a small part of the transmitter power is 
applied to the transmission line, and likewise only a small portion of the 
power of the incoming signal is available at the receiver input. 
The German published application No. 2,045,654 discloses a circuit 
arrangement for the full duplex transmission of dc data signals by way of 
a two-wire transmission line, wherein the transmitter, a compensation 
circuit, and the receiver are connected in series to the two-wire 
transmission line. The output signal of the compensation circuit 
compensates the signal from the associated transmitter which would 
otherwise be applied to the receiver. The compensation circuit has a 
comparatively complicated construction in order to be able to balance 
deformations of the transmitted signal, as a result of the generally 
complex input impedances of long, and therefore dissipative transmission 
lines. However, the compensation circuit can be considerably simplified if 
only lines whose input impedances are real, with good approximation, are 
used. 
The series arrangement of transmitter, receiver and compensation circuit in 
the known transmission device gives rise to asymmetry in respect of ground 
potentials, which is very harmful. This applies in particular to the 
two-wire transmission line. Fed-in interference voltages now become 
manifest as push-pull interferences, which can no longer be eliminated by 
a receiver which has good push-push suppression facilities. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to provide a device of 
the type referred to above, hereinafter simply referred to as a 
transmission device, which device is strictly symmetrical in respect of 
the assigned ground potential. Furthermore, the device is to be integrable 
and ECL compatible, i.e. it is to be able to be used in association with 
networks constructed in the ECL technique (emitter coupled logic 
technique) without altering the signal range of the signal which controls 
the push-pull transmitter, or the signal which is emitted from the 
push-pull receiver. The integration requirement gives rise to a power loss 
and a dispensation with transformers, inductances and capacitances, unless 
the inductances and capacitances are otherwise unavoidable as parasitic 
parameters. However, this results in the fact that the distances which can 
be bridged are limited to values at which the two-wire lines employed are 
still sufficiently loss-free, in order to be able to meet the requirement 
that their surge impedance should be, with good approximation, real. In 
this case, as is well known in the art, no reflections occur when the 
lines are terminated with real impedances of the value of the 
characteristic impedance. 
According to the invention, the foregoing object is realized, in a circuit 
of the type described above, in that the push-pull transmitter and the 
push-pull receiver are designed as symmetrical differential amplifiers 
composed of emitter-coupled transistors having a constant feed-in of 
current, and the compensation circuit is of symmetrical construction, and 
further that the inputs of the push-pull receiver are connected to the 
wires of the two-wire line by way of two identical decoupling resistors 
which form parts of the compensation circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a transmitter S is divided into two (keyed) sources of 
constant voltage, which are symmetrical with respect to ground potential, 
and which together supply a signal having an amplitude US. Two line 
terminating resistors R, whose values each correspond to half the 
characteristic impedance of the line L are also symmetrically arranged. 
Because of the similar termination of the line at the opposite station, 
the signal amplitude UL on the two-wire line L amounts to half the signal 
amplitude US. The two inputs of the push-pull receiver E, which has an 
output EA, are connected by way of decoupling resistors r1 to the two 
wires of the two-wire line L. The values of the decoupling resistors r1 
are approximately 5-10 times as high as the values of the line terminating 
resistors R. As the input impedance of the push-pull receiver E can again 
be assumed to be extremely high-ohmic in comparison to the values of the 
resistors r1, without the provision of additional measures the signal 
fed-in from the associated transmitter to the two-wire line L would be 
connected, in full, to the push-pull receiver, and thus disturb the 
reception of the signal transmitted from the opposite station. In order to 
prevent this undesirable situation, two sources Q are provided which are 
impressed with a current, and which alternately transmit a current I.sub.k 
through the resistors r1, and are switched over with the same timing as 
the transmitted signals. The switch-on sequence of the currents I.sub.K 
and the magnitude of these currents are selected to be such that across 
one of the series resistors r1 there occurs, in each case, a voltage drop 
which is equal in magnitude, but oppositely directed to the signal 
amplitude UL on the line L. In this manner the signal produced by the 
associated transmitter S is eliminated in respect of the inputs of the 
push-pull receiver E. 
FIG. 2 illustrates an exemplary embodiment of the transmission device, 
which is governed by the basic principle described above with respect to 
the apparatus of FIG. 1. The circuit arrangement in FIG. 2 is divided into 
three sections by two perpendicular, broken lines. The left-hand section 
illustrates the push-pull transmitter S, which contains a differential 
amplifier comprising two emitter-coupled transistors T1 and T2. The 
emitters of the two transistors are connected to the collector of a 
further tansistor T3 having a base which is connected to a fixed auxiliary 
potential VS. The transistor T3 acts as a source of an impressed current 
I, the magnitude of which is determined by the value of its emitter 
resistor R3. Following a level shift by the emitter follower comprising a 
transistor T4 and an emitter resistor R4, the transmitter input signal 
which is connected to the terminal SE controls the differential amplifier 
of the transmitter by way of the base of the transistor T1. The base of 
the other transistor T2 is connected to a further auxiliary potential VR, 
the magnitude of which is approximately equal to the mean value of the two 
binary signal values of the control signal connected to the base of the 
transistor T1. The emitter follower, comprising the transistor T4 and the 
resistor R4, can also be omitted if the magnitude of the auxiliary 
potential VR is adapted accordingly, in which case the input terminal SE 
is then directly connected to the base of the transistor T1 of the 
differenital amplifier. 
The collectors of the transistors T1 and T2 are connected to further 
transistors T5 and T6 in an emitter follower circuit. The emitters of the 
transistors T5 and T6 are connected by way of line terminating resistors R 
having a value which corresponds at least approximately to half the 
characteristic impedance of the two-wire line L, to the wires of the line 
L. 
The push-pull receiver E comprises, in a well known manner, a differential 
amplifier which includes a transistor T7 and a transistor T8, whose 
emitters are connected to one another and are supplied with an impressed 
current which is formed with the aid of a transistor T9 and a resistor R9. 
The voltage which occurs across the resistor R8, which is interposed as a 
load resistance in the collector circuit of the transistor T8, controls 
the transistor T10 which is connected as an emitter follower. The emitter 
of the transistor T10 is connected to the output terminal EA, from which 
the received signals can be withdrawn as push-push signals. The inputs of 
the push-pull receiver E, which are identical to the base terminals of the 
transistors T7 and T8, are connected to the two-wire line L by way of a 
pair of decoupling resistors r1. The value of the decoupling resistors r1 
is approximately 5-10 times that of the characteristic impedance of the 
two-wire line L. 
The decoupling resistors r1 form part of a compensation circuit K which 
contains a further differential amplifier composed of emitter-coupled 
transistors T11 and T12. The base of the transistor T12 is connected to 
the auxiliary voltage VR. The base of the transistor T11, like the base of 
the transistor T1 of the differential amplifier of the push-pull 
transmitter E, is connected to the emitter of the transistor T4. If the 
emitter follower circuit with the transistor T4 is omitted, the input 
signal of the push-pull transmitter is also directly connected to the base 
of the transistor T11. The particular conductive transistor T11 or T12 of 
the differential amplifier in the compensation circuit K switches through 
the impressed current I.sub.K of a current source which comprises a 
transistor T13 and a resistor R13, so that a voltage drop is produced in 
each case across one of the two decoupling resistors r1. By means of 
suitable adaption of the values of the decoupling resistors r1 and the 
magnitude of the impressed current I.sub.K, it can be ensured that the 
voltage drop across the particular current-carrying decoupling resistor r1 
is exactly equal to the amplitude UL of the transmitted signal fed-in to 
the two-wire line L from the associated push-pull transmitter S. If, by 
correct connection of the collectors of the transistors T11 and T12 to the 
inputs of the push-pull receiver E, it is now ensured that the voltage 
drop across the relevant decoupling resistors r1 is in fact opposite to 
the simultaneously occurring signal value of the transmitted signals of 
the associated push-pull transmitter, a complete compensation can be 
effected in respect of the input of the push-pull receiver E. Assuming 
that the two-wire line L is likewise terminated in accordance with its 
characteristic impedance at the other end, this may be carried out for the 
relationship 
EQU I.sub.K = I (Ra/2r1) 
where Ra is the value of the load resistances Ra1 and Ra2 in the collector 
circuits of the transistors T1 and T2 in a differential amplifier of the 
push-pull transmitter S, and I is the collector currents of the 
transistors T1 and T2. 
FIG. 3 illustrates a further exemplary embodiment of the transmission 
device for the transmission of push-pull signals corresponding to the 
present invention. As in FIG. 1, the push-pull transmitter is again formed 
by two alternatingly connected sources having an impressed current. They 
feed the transmitted signal, having the amplitude UL, to the two-wire line 
L by way of the line terminating resistors R, which are connected between 
the sources and the wires, and which have a value corresponding to half 
the characteristic impedance of the two-wire line. The two input terminals 
of the push-pull receiver E are connected by way of decoupling resistors 
r2 to the wires of the two-wire line. Furthermore, the input terminals of 
the push-pull receiver E are connected by way of further resistors r3 to 
the outputs of the push-pull transmitter S. When the series resistors r3 
are connected, a transposition is effected relative to the line of 
symmetry determined by the design of the push-pull transmitter S, the 
push-pull receiver E, and the two-wire line L. The values of the 
decoupling resistors r2 are again to be equal to approximately 5-10 times 
the characteristic impedance of the two-wire line L. If it is again 
assumed that the two-wire line L is terminated at the opposite station in 
accordance with its characteristic impedance, the output signals from the 
associated push-pull transmitter S do not reach the inputs of the 
push-pull receiver E, if the values of the resistors r3 are twice the 
values of the decoupling resistors r2. 
With the further assumption that the losses on the two-wire line L are 
negligible, the amplitude of the signals received from the opposite 
station at the input terminals of the push-pull receiver E amounts to 67% 
of the amplitude with which these signals are fed onto the two-wire line L 
at the opposite station. 
FIG. 4 illustrates a further exemplary embodiment of the compensation 
circuit based on the principle represented in FIG. 3. In this embodiment, 
the decoupling resistors r2 are themselves decoupled from the wires of the 
two-wire line L by a pair of transistors T14 and T15 which are operated as 
emitter followers. In order to compensate for the potential shift which is 
caused by the base emitter paths of these transistors, a pair of diodes D1 
and D2 are inserted between the corresponding connection points of the 
resistors r3 and the emitters of the transistors T5 and T6, which form the 
outputs of the push-pull transmitter. Because of the availability of the 
emitter followers which include the transistors T14 and T15, and the 
low-ohmic outputs of the push-pull transmitter, it is no longer necessary 
for the values of the resistors to be high in relation to the 
characteristic impedance of the two-wire line L. This results in shorter 
signal transit times between the two-wire line L and the input terminals 
of the push-pull receiver E. 
A further economy can be achieved, with respect to lines, by the formation 
of a so-called phantom four-wire element, which constitutes a measure 
generally known in data transmission technology. In this manner it is 
possible to connect three pairs of opposite stations to one another by way 
of two physical two-wire lines (four wires). As in the present situation 
all the devices which serve to transmit push-pull signals are to be 
integrable, which has already been referred to in the introductory portion 
of this paper, it is not possible to employ transformers to form the 
phantom four-wire unit. FIG. 5 illustrates a possibility of connecting a 
third transmission device, comprising a push-pull transmitter S3, a 
push-pull receiver E3, and a compensation circuit K3 to two physical 
two-wire lines L1 and L2, along with the first station having a push-pull 
transmitter S1, a push-pull receiver E1 and a compensation circuit K1, and 
a second station having a push-pull transmitter S2, a push-pull receiver 
E2 and a compensation circuit K2. The devices which are illustrated in 
FIG. 5 and serve for transmission and reception of, in each case, three 
independent signals in both directions are advantageously integrated in 
one module. 
It should be noted that the values of the balancing resistors R", which are 
necessary for the formation of the third transmission path, must be taken 
into consideration in dimensioning of the line terminating resistors, here 
reference R'. If the values of the balancing resistors R" and the values 
of the line terminating resistors R' are selected to be equal to one 
another, these must also be equal to the characteristic impedances of the 
two-wire lines. The line terminating resistors of the third transmission 
device S3, E3 and K3 can be omitted if the transmission device is 
constructed in the manner illustrated in FIGS. 1 and 2. In this case, the 
line terminating resistors fundamentally have no influence on the 
effectiveness of the compensation circuit. However, the impressed current 
I.sub.K should be increased to twice the former value. 
Although I have described my invention by reference to certain illustrative 
embodiments thereof, many changes and modificiations of the invention may 
become apparent to those skilled in the art without departing from the 
spirit and scope of the invention. I therefore intend to include within 
the patent warranted hereon, all such changes and modifications as may 
reasonably and properly be included within the scope of my contribution to 
the art.