Abstract:
A data transmission system including a pair of parallel transmission lines, applies data signals to the transmission lines through a transformer and an associated impedance connected across the lines. The transformer is magnetically coupled to at least one of the lines to apply a signal current thereto. The signal current is transferred to the other line through the impedance. The impedance is tuned to the transmission frequency so that it has a very low value during data transmission.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an improved data transmission system which is not prevented from performing accurate transmission by a load connected between both ends of transmission wirings or lines. 
     Generally data transmission wirings, particulary, using power wirings as service wirings are operated with a parallel signal input method. When a low impedance load is connected between both ends of power wirings, signals can not be transmitted because of a decrease in the receiving level. 
     FIG. 1 shows a circuit diagram of a general data transmission system using the parallel signal input method. The general transmission system comprises transmission wirings or lines 1, a transmitting station 2 (where E S  is a transmitting output voltage and Z S  is an output impedance of the transmitting station 2), a receiving station 3 (where Z R  is an input impedance of the receiving station 3), loads 4 and 5 (where Z LS  and Z LR  are impedances of loads 4 and 5, respectively). 
     A receiving voltage E R  can be expressed by the following equation. ##EQU1## Further, if Z LS  &gt;&gt;Z LR , the receiving voltage E R  is given by the following equation in view of equations (1) and (2). ##EQU2## Accordingly, signals can not receive on the receiving station by using the voltage E R  when Z LR  ≈0. But, because a transmission current I S  ≈0, a receiving current I R  is given as follows: ##EQU3## where the transmission current I R  is maximum. Accordingly, signals can receive on the receiving station by using the current I R . 
     But, when Z LS  ≈0, the receiving current I R  and the transmission current I S  and the receiving voltage E R  are given as follows: 
     
         I.sub.S =I.sub.R ≈0, E.sub.R ≈0            (4) 
    
     Effective signals for transmitting data are not applied on the transmitting wirings or lines 1 by using either current I R  or the voltage E R . 
     SUMMARY OF THE INVENTION 
     With the foregoing prior difficulty, it is an object of the present invention to provide an improved data transmission system which is not prevented from performing accurate transmission even by a load connected between both ends of two wirings or lines while the data are transmitted. 
     It is another object of the present invention to provide an improved data transmission system including two transmission wirings which can transmit data by using either the current or the voltage even when a low impedance load is connected between both ends of the wirings. 
     It is a further object of the present invention to provide an improved data transmission system including two transmission wirings, a magnetic coupling connected to one line, and an element connected between a pair of transmission wirings, the element showing a low impedance at a data transmitting frequency. 
     Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description of and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     According to an embodiment of the present invention, a data transmission system including two transmission wirings comprises magnetic coupling means connected to at least one of said two wirings and element means connected between a pair of transmission wirings, the element means showing a low impedance at a data transmitting frequency for transferring data in the data transmission system, so that the magnetic coupling means and the element means apply the data to be transferred in the data transmission system. 
     According to another embodiment of the present invention, a data transmission system including two transmission wirings comprises magnetic coupling means connected to at least one of said wirings and element means connected between a pair of transmission wirings at both sides of the magnetic coupling means, the element showing a low impedance at a data transmitting frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: 
     FIG. 1 shows a circuit diagram of a conventional data transmission system; 
     FIG. 2 shows a circuit diagram of a data transmission system according to an embodiment of the present invention; 
     FIG. 3 shows an equivalent circuit of a data transmission system of FIG. 2; 
     FIG. 4 shows a circuit diagram of a data transmission system of another embodiment of the present invention; and 
     FIG. 5 shows a circuit diagram of a data transmission system according to still another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a circuit diagram of a data transmission system according to an embodiment of the present invention. Components similar to those in FIG. 1 are represented by the same reference numbers. 
     A transformer 7 at the transmitting station 2 applies transmission signals upon a transmission wiring a by using a magnetic coupling. An impedance element 8 (where an impedance is Z A ) applies a current induced on the wiring a to the other transmission wiring b. The impedance element 8 shows a low impedance at the data transmission frequency. (the impedance element 8 shows a high impedance in response to the application of a power supply frequency while signals are transmitted by the power wirings.) For example, the impedance element 8 may be a capacitor or a series resonance circuit including a capacitor and a coil, so that the resonance frequency is made identical with the transmitting frequency. 
     A switching means 9 is switched ON when signals are transmitted only from the transmitting station 2 so that the impedance element 8 does not function as a load or another transmission system. 
     FIG. 3 shows an equivalent circuit of FIG. 2. In FIG. 3, there are connected a series impedance Z X , a signal detector impedance Z Y , a mutual inductance M, the coils L 1  and L 2  having the numbers of turns L 1  and L 2 . Using the symbols in FIG. 2, the impedances Z X  and Z Y  can be written as follows: ##EQU4## 
     A receiving current I R  as shown in FIG. 3 is expressed by the following equation (6). ##EQU5## where the signs ± are determined by the direction in which the coil is wound. Because the impedance Z A  is set equal nearly zero, the impedance Z X  becomes equal nearly zero. If L 1  is selected to be ωL 1  &gt;&gt;Z S , the receiving current I R  is expressed by the following equation (6&#39;). ##EQU6## 
     If the transformer 7 is high coupled, a mutual inductance M becomes. 
     
         M.sup.2 =L.sub.1 ·L.sub.2 
    
     By simplifying the equation (6&#39;), the result can be written as: ##EQU7## and a receiving voltage E R  becomes: ##EQU8## 
     Accordingly, when Z Y  →∞, equations (7) and (8) are expressed as follows: ##EQU9## When Z Y  →0, the equations (7) and (8) are expressed as follows: ##EQU10## Signals can be applied to the wirings by the current or voltage in spite of the value of the impedance Z LR  and the impedance Z Y . 
     When another transmission system is present, a series circuit including the impedance 8 and the switching means 9 as shown in FIG. 2 may be set opposite to the transformer 7. FIG. 4 shows a circuit diagram of a transmission system which comprises the above series circuits connected to both sides of the transformer 7 according to another embodiment of the present invention. 
     Operation modes of the data transmission system as shown FIG. 4 can be set as shown in the following table 1 when the switching means 9 and 9&#39; are ON and OFF. Accordingly,signals can transmitted in a desired direction in response to the connection of the switching means 9 and 9&#39;. 
     
                       TABLE 1______________________________________switch 9  switch 9&#39;           Operation of System______________________________________OFF    OFF      The signals are not transmitted from station           C.           The signals can be transmitted between           stations A and B.OFF    ON       When the signals are transmitted between           stations A and C, the signals are transmitted           between neither stations A and B, or           A and C.           (The station A is isolated from the station           B.)ON     OFF      When the signals are transmitted between           stations B and C, the signals are transmitted           between neither stations A and           B, or stations A and C.           (the station A is isolated from the station B.)ON     ON       The signals are not transmitted from station           C, and the station A is isolated from the           station B.______________________________________ 
    
     In the present invention, receiving is executed by both a current coupling used as a magnetic coupling of the coils and a voltage coupling connected to two transmitting wirings. If the transmission system comprises a receiving circuit having a simple structure such as the embodiment of the present invention, receiving can be excuted easily. 
     FIG. 5 shows a circuit diagram of the data transmission system including a transmission/receiving circuit using a core according to another embodiment of the present invention. As shown in FIG. 5, a coil 10 induces a current for flowing in a wiring a by using the magnetic flux coupling because the coil 10 is combined with the core of the transformer 7. Further, the coil 10 induces a voltage by connecting itself to wirings a and b. A capacitor 11 forms a series resonanse circuit with the coil 10, to induce only the signals corresponding to the transmitting frequency. An input impedance 12 is part of the receiving station 13. When the switching means 9 and 9&#39; are ON, a voltage at both ends of the impedance 12 in the receiving station 13 becomes nearly zero, so that no signals are received at the receiving station. 
     In the above description, the data transmission system includes two transmission wirings, in which one of the transmission wirings is magnetically coupled a transmitter and, an element is provided for serving as a low impedance at the data transmission frequency, the element being connected between the two wirings. In the case where low impedance loads are connected between both ends of two wirings, data signals can be transmitted by using either current or voltage. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.