Two-way signal transmission and one-way DC power supply using a single line pair

A signal transmission system which perform two-way signal transmission between a first transmitter-receiver and a second transmitter-receiver via a pair of signal lines and supplies a DC current from the first transmitter-receiver to the second transmitter-receiver. In each of the first transmitter-receiver and the second transmitter-receiver, there are provided a current varying circuit for varying a signal line current by a transmission pulse signal so that the signal line current may assume a value in a predetermined current region lower than a minimum level of a load current to the other transmitter-receiver during sending out a signal pulse, a current detector for detecting a current or voltage variation in the signal line current, and a gate circuit for blocking the detected output from the current detector while the transmission pulse is sending out to the current varying circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to a signal transmission system which 
performs DC power supply and signal transmission and reception through the 
use of a pair of signal lines. 
In conventional systems of this kind in which signals are transmitted in 
two directions between first and second equipments via a pair of signal 
lines and a DC current is supplied from the first equipment to the second 
one, a constant-current circuit must be provided in the second equipment 
in order for the first equipment to make distinction between a load 
fluctuation current and a signal current of the second equipment. A fixed 
load current and a signal current flow in the pair of signal lines, and a 
change in the signal current is detected as a signal by a current detector 
of the first equipment; accordingly, the constant-current circuit is 
required to be capable of consuming power that a difference in the load 
fluctuation current of the second equipment is multiplied by the DC input 
voltage of the second equipment, and this leads to a considerable power 
loss, resulting in inefficient DC supply. 
BRIEF SUMMARY OF THE INVENTION 
An object of the present invention is to provide a signal transmission 
system which permits signal transmission between two equipments of simple 
circuit arrangements via a pair of signal lines and highly efficient DC 
supply from one of the two equipments to the other, thereby to reduce the 
manufacturing costs and decrease the number of pairs of lines in a cable 
used. 
To attain the above object of the invention, there is provided a signal 
transmission system which performs two-way signal transmission between a 
first transmitter-receiver and a second transmitter-receiver 
interconnected via a pair of signal lines and supplies a DC current from 
the first transmitter-receiver to the second transmitter-receiver. The 
first transmitter-receiver is provided with a first current varying 
circuit for varying a signal line current by a first transmission pulse 
signal so that the signal line current may assume a value in a 
predetermined current region lower than a minimum level of a load current 
to the second transmitter-receiver during sending out a signal pulse from 
the first transmitter-receiver. 
The second transmitter-receiver is provided with a second detector for 
detecting a current or voltage variation in the signal line current, a 
second current varying circuit for varying the signal line current by a 
second transmission pulse signal so that the signal line current may 
assume a value in the predetermined current region during sending out the 
signal pulse, and a second gate circuit for blocking the detected output 
from the second detector while the second transmission pulse signal is 
sent out by the second current varying circuit. The first 
transmitter-receiver is further provided with a first current detector for 
detecting a current variation in the signal line current by the second 
transmission pulse, and a first gate circuit for blocking the detected 
output from the first current detector while the first transmission pulse 
signal is sent out.

DETAILED DESCRIPTION 
With reference to FIG. 1, a first operational principle will first be 
described. In a first transmitter-receiver, reference numeral 10 indicates 
a power source input terminal; 11 designates a signal output terminal; 12 
identifies a signal input terminal; 13 denotes a first current detector; 
14 represents a first current varying circuit; 15 shows a gate circuit; 
and 16 refers to a first gate input terminal. In a second 
transmitter-receiver, reference numeral 20 indicates a power source output 
terminal; 21 designates a signal output terminal; 22 identifies a signal 
input terminal; 23 denotes a second current detector; 24 represents a 
second current varying circuit; 25 shows a diode; 26 refers to a 
capacitor; 27 indicates a second gate circuit; and 28 designates a gate 
input terminal. The first and second transmitter-receivers are 
interconnected via a pair of signal lines 30 indicated by a single line. 
FIG. 3 is a diagram showing the principle of signal transmission according 
to the present invention. Reference character i indicates a load current 
available from the power source output terminal 20; t designates time; and 
i.sub.0 identifies a minimum level of the load current i. Let it be 
assumed that a predetermined, hatched current region lower than the 
minimum level i.sub.0 is handled as a signal. 
FIG. 4 shows an operating waveform for the signal transmission. Reference 
character i indicates a current flowing in the pair of signal lines 30 and 
t designates time. Assuming that, from t.sub.1 to t.sub.6, pulses are sent 
out at regular intervals if the signal is to be transmitted at the time 
slot, it is possible to take out the signal by detecting its presence or 
absence on the basis whether or not the current varies down to i.sub.1 in 
a current region below the level i.sub.0. 
In FIG. 1, the gate circuit 15 is switched between transmission and 
reception modes by making the gate input terminal 16 a high-level and a 
low-level, respectively. In a case of transmitting a signal from the first 
transmitter-receiver to the second one, the gate input terminals 16 and 28 
are usually rendered the high-level and the low level respectively, 
thereby establishing the current varying circuits 14 and 24 in the states 
in which they permit the passage therethrough of a current. When a signal 
is applied to the signal input terminal 12, the current varying circuit 14 
is transferred to a state in which no current flows therethrough so that 
this current variation can be detected as a signal by the current detector 
23. At that time, the power source output terminal 20 is disconnected from 
the current varying circuit 24 by the diode 25 but, by the capacitor 26, 
the power source output terminal 20 can be held at a certain voltage. 
Next, in a case of transmitting a signal from the second transmitter 
receiver to the first one, the current varying circuits 14 and 24 are made 
to permit the passage of currents by rendering the gate input terminals 28 
and 16 the high-level and the low-level respectively, as is the case with 
the above. Upon application of a signal to the signal input terminal 22, 
the current varying circuit 24 is rendered into a state in which it does 
not flow therethrough the current so that this current variation can be 
detected as a signal by the current detector 13. In this case, by the 
capacitor 26, the power source output terminal 20 can be retained at a 
certain voltage, permitting the signal transmission and the DC supply in 
the two directions. 
In FIG. 2, the current detector 23 used in FIG. 1 is substituted by a 
voltage detector 23a to eliminate a voltage drop for the current 
detection, and the illustrated circuit is identical in other arrangements 
and in operation with the circuit of FIG. 1. 
FIGS. 5A and 5B illustrates an embodiment of the signal transmission system 
of the present invention employing the operational principle described in 
connection with FIG. 2. In the first transmitter-receiver shown in FIG. 
5A, reference numeral 10 indicates a power source input terminal; 11 
designates a signal output terminal; 12 identifies a signal input 
terminal; 13 denotes a current detector; 14 represents a current varying 
circuit; 15 shows a gate circuit; and 16 refers to a gate input terminal. 
In the second transmitter-receiver shown in FIG. 5B, reference numeral 20 
indicates a power source output terminal, which is connected via a voltage 
regulator to a load; 21 designates a signal output terminal; 22 identifies 
a signal input terminal; 23a denotes a voltage detector; 24 represents a 
current varying circuit; 25 shows a diode; 26 refers to a capacitor; 27 
indicates a gate circuit (usually in the reception mode when held at the 
low-level); and 28 designates a gate input. 
For activating them, the gate input terminal 16 and the signal input 
terminal 12 are made the low-level and a source power is applied from the 
power source input terminal 10. The source power is supplied to the signal 
lines 30 via a resistor 101 and a transistor 119 of the ON state. Since 
the gate input terminal 28 is held at the low-level, a transistor 207 is 
in the ON state so that the signal lines 30 are connected to a load via 
the transistor 207, the diode 25, the capacitor 26, the power source 
output terminal 20 and the voltage regulator. 
In a case of transmitting a signal from the first transmitter-receiver to 
the second one in such a state as mentioned above, the gate input terminal 
16 is held at the high-level to switch the first transmitter-receiver to 
the transmission mode and positive pulses are applied to the signal input 
terminal 12. In this case, the transistor 119 is turned-OFF via an AND 
gate 121 of the gate circuit 15, an open-collector inverter 111 of the 
current varying circuit 14, a pull-up resistor 112, a transistor 113, a 
collector resistor 114, a diode 116 and a base resistor 118, inhibiting 
the passage of a current through the transistor 119. As a result of this, 
a voltage drop occurs across the signal lines 30 to cut off the diode 25, 
resulting in no load current flowing. the voltage detector 23a of the 
second transmitter-receiver connected across the signal lines 30 becomes a 
state where no current flows in a resistor 211 and a diode of a photo 
coupler 212 so that high-level pulses are obtained at the output side of 
an inverter 215 via a speed-up resistor 213 and an emitter resistor 214 of 
the photo coupler 212. The resistor 114, the diode 116 and a capacitor 117 
of the current varying circuit 14 of the first transmitter-receiver are 
used to delay only the rise of the current in the signal lines 30, 
preventing noise generation in other signal lines. The capacitor 26 is 
provided for maintaining a certain voltage across the load when the diode 
25 is held at the OFF state. 
In a case of the signal transmission from the second transmitter-receiver 
to the first one, the gate input terminal 28 is held at the high-level to 
switch the second transmitter-receiver to the transmission mode and, at 
the same time, the gate input terminal 16 of the first 
transmitter-receiver is held at the low-level to alter it to the reception 
mode, in which an AND gate 120 is opened and the transistor 119 assumes 
the ON state. Applying positive pulses to the signal input terminal 22 to 
the current varying circuit 24 of the second transmitter-receiver, the 
transistor 207 is turned-ON via an AND gate 220, an open-collector 
inverter 202, a pull-up resistor 203, a transistor 204, a collector 
resistor 205 and a base resistor 206, preventing the flow of the load 
current. This current variation is detected as a voltage drop (about 1 
volt) across the resistor 101 of the current detector 13 of the first 
transmitter-receiver by a base resistor 102 and a transistor 103, so that 
positive pulses are generated at the signal output terminal 11 via a 
collector resistor 104, a base resistor 105, a transistor 106, a collector 
resistor 107 and the AND gate 120. When the transistor 207 is held at the 
OFF state, charges stored in the capacitor 26 are discharged, by which the 
voltage across the load is held at a certain voltage. 
As described above, the signal transmission is possible between the first 
and second transmitter-receivers, and since the transistors all perform 
the switching operations, no appreciable heat is generated and the DC 
supply can be achieved at the high efficiency. The larger the ratio of the 
period to the pulse width of the signal to be transmitted is, the more 
efficiently the DC supply is performed. 
FIGS. 6A and 6B show signal waveforms transmitted in the embodiment of FIG. 
5, FIG. 6A showing the signal waveform transmitted from the first 
transmitter-receiver to the second one. In FIG. 5, the signal input 12 is 
held at the low-level at first and, when a positive pulse is applied to 
the signal input, a signal line current i of the signal line 30 (the line 
resistance of which is desired to be lower than 40 .OMEGA.) undergoes a 
current variation of a negative pulse in case of a load current change of 
50 to 150 mA. As a result of this, a signal line voltage V undergoes a 
variation of a negative pulse so that a voltage waveform of a positive 
pulse is generated at the signal output terminal 21 from the voltage 
detector 23a. A voltage E.sub.out at the power source output terminal 20 
is attenuated at a time constant corresponding to the load resistance 
(=E.sub.out /load current).times.the capacitance of the capacitor 26 upon 
each application of the signal. 
FIG. 6B shows waveforms of signals which are transmitted from the second 
transmitter-receiver to the first one. The signal input 22 is held at the 
low-level at first and when a positive pulse is applied thereto, a current 
variation of a negative pulse occurs in the signal line current i. Since 
the resistor 101 is connected in series to the signal line 30, the signal 
line voltage V is generated as shown and a positive pulse is provided at 
the signal output terminal 11 at the output side of the current detector 
13. The voltage E.sub.out at the power source output terminal 20 is the 
same as mentioned above. 
FIGS. 7A and 7B illustrate another embodiment of the signal transmission 
system of the present invention employing the operational principle 
described previously with respect to FIG. 1. This embodiment differs from 
the embodiment of FIGS. 5A and 5B in the provision of the current detector 
23, which comprises transistors 212 and 215 and resistors 210, 211, 213, 
214 and 216. That is, the resistor 210 is connected in series to the 
signal line 30 and, in the signal transmission from the first 
transmitter-receiver to the second one, a current variation in the signal 
line 30 is detected as a drop (about 1 volt) across the resistor 210 by 
the base resistor 211 and the transistor 212, and positive pulses are 
provided at the signal output terminal 21 via the collector resistor 213, 
the base resistor 214, the transistor 215, the collector resistor 216 and 
an AND gate 221. The other operations of this embodiment are similar to 
those of the embodiment of FIGS. 5A and 5B. 
As has been described in the foregoing, the present invention permits the 
signal transmission and reception between two equipments through the use 
of a pair of signal lines and enables the DC supply from the one equipment 
to the other; therefore, the present invention contributes to the 
reduction of the number of pairs of lines of a cable used and is of great 
utility from the economical point of view. The present invention is 
applicable, for example, to a key telephone system and terminal I/O 
equipments of computers.