Abstract:
The inventive data exchange device comprises a transmitter (SA 4 ) fed by a power supply (VDDA), an electric cable (C 1 ) whose first conducting wire is connected to a fixed potential point (GNDA) of the transmitter and second conducting wire is connected to a variable potential point of the transmitter and a receiver (SB 4 ). Said receiver (SB 4 ) comprises a component (DZB 4 ) which defines a voltage threshold opposite to the direction of electric current in the cable (C 1 ). Said device is embodied in such a way that it is simple and low-cost in the production thereof. The device makes it possible to interconnect a plurality of transmitters and receivers and is low sensitive with respect to voltage and parasite currents.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to an installation for exchanging information comprising a transmitter supplied from a power supply, an electric cable of which a first conductor is connected to a point of fixed potential of the transmitter and of which a second conductor is connected to a point of variable potential of the transmitter and at least one receiver.  
         [0002]     Such installations are widely used for exchanging information. They require, on the one hand, the use of shielded cables or pairs of twisted wires, protected against electromagnetic radiations and, on the other hand, the use of circuits for generating signals constituting the information and for shaping these signals. Data transmission installations using the EIB (registered trademark), LONWORKS (registered trademark) or RS485 standards, for example, are known. Such systems are very competitive and make it possible to transmit information with a high bit rate. However, these installations are overdimensioned for certain applications in which, in particular, a high bit rate is not an important criterion.  
       PRIOR ART  
       [0003]     Simpler installations are known from the prior art. For example, an assembly such as that represented in  FIG. 1  is known. This assembly comprises a transmitter SA 1  and a receiver SB 1  linked to one another by an electric cable C 1  with two conductors whose electrical resistances are symbolized by the resistors RL 1  and RL 2 . The transmitter mainly comprises a controlled switch consisting of a transistor TA 1  operating in switch mode and making it possible to connect together or not the two ends of the conductors of the electric cable. The receiver SB 1  itself comprises a power supply providing a voltage VDDB linked to the end of one of the conductors of the electric cable via a resistor RB 1 . A voltage Us is measured between the ends of the conductors of the electric cable. This voltage Us varies according to the state of the transistor of the transmitter SA 1 . Thus, an item of information is coded as a succession of states of the transistor TA 1  at the level of the transmitter and decoded by measuring the variations of the voltage Us at the level of the receiver SB 1 . When the transistor TA 1  is on, the current intensity in the electric cable linking the transmitter and the receiver is mainly limited by the resistor RB 1 . The information bit rate being fairly low, it is unnecessary to represent on this diagram the capacitive and inductive effects of such an arrangement.  
         [0004]     Such an installation has drawbacks. Specifically, if one envisages connecting 100 receivers with the transmitter SA 1  on the same line, the current limiting resistors are arranged in parallel and their value then equals RB 1 /100. In order to avoid causing overly large currents to flow through the transistor TA, it is then necessary to limit the number of elements that can intercommunicate or to choose a large resistance RB 1 , for example 100 times the value causing the maximum current allowable by the transistor TA.  
         [0005]     It is known that in such installations, common-mode voltages and differential-mode voltages appear at the level of the transmitters and receivers, in particular when the latter are distantly separated.  
         [0006]     The common-mode voltages are represented by the arrows Ucm. On account of the resistance RB 1 , a common-mode current necessarily causes a modification of the voltage Us.  
         [0007]     The differential-mode voltages are caused by currents Idm flowing around the loop formed by the two conductors of the electric cable between the transmitter and the receiver. These currents passing through the resistors RL 1  and RL 2  likewise contribute to modifying the voltage Us.  
         [0008]     To reduce the effects of the interference induced currents Idm, shielded or twisted electric cables are used. In addition, conductors exhibiting a very small resistance are used and the allowable distance separating the various elements of the installation is restricted.  
         [0009]     A compromise must be found regarding the value of the resistor RB 1 . Its value must be high so as to allow communication between a maximum of elements and to maintain, when the transistor TA 1  is on, a voltage Us below the upper threshold of the low logic value of any logic circuit using this voltage. Conversely, its value must be small so as to limit the effects of the induced currents.  
         [0010]     Installations such as that represented in  FIG. 2  are also known. This assembly comprises a transmitter SA 2  and a receiver SB 2  linked to one another by an electric cable C 1  with two conductors whose electrical resistances are symbolized by the resistors RL 1  and RL 2 . The transmitter SA 2  mainly comprises a power supply providing a voltage VDDA supplying a resistor RA and a transistor TA 2  arranged in series. The transistor TA 2  is controlled by a circuit (not represented) and operates in switch mode. The receiver SB 2  mainly exhibits a resistor RB 2  between the ends of the two conductors of the electric cable. The voltage Us is taken at the terminals of this resistor. Thus, an item of information is coded as a succession of states of the transistor TA 2  of the transmitter and decoded by measuring the variations of the voltage Us in the receiver SB 2 . When the transistor TA 2  is off, the current intensity in the electric cable linking the transmitter and the receiver is mainly limited by the resistor RA.  
         [0011]     In this installation, the value of the resistor RB 2  must, likewise, be large so as to allow the connection of a large number of elements, the value of RA being given. It must also be much greater than the values of the resistors RL 1  and RL 2 . However, the value of RB 2  must be as small as possible so as to reduce the effects of the common-mode and differential-mode induced currents.  
       SUMMARY OF THE INVENTION  
       [0012]     The aim of the invention is to produce an installation for transmitting information alleviating the drawbacks cited and improving the installations known from the prior art. In particular, the invention proposes to produce a simple installation whose manufacturing costs are low, making it possible to interconnect numerous transmitters and receivers that are insensitive to interference currents and voltages.  
         [0013]     The installation for exchanging information according to the invention is characterized in that the receiver or the receivers comprise a component defining a threshold voltage opposing the flow of the electric current through the cable. Thus, the interference voltages must be greater than this threshold voltage in order to bring about the flow of a current around the cable and be interpreted as information.  
         [0014]     The dependent claims  2  to  5  define various alternative embodiments of the installation according to the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The appended drawing represents, by way of examples, two embodiments of an installation for exchanging information according to the invention.  
         [0016]      FIGS. 1 and 2  represent transmitter-receiver assemblies known from the prior art, linked by electric cables allowing the exchange of information.  
         [0017]      FIG. 3  represents a first embodiment of an installation for exchanging information according to the invention, comprising a transmitter and a receiver linked by an electric cable.  
         [0018]      FIG. 4  represents a second embodiment of an installation for exchanging information according to the invention, comprising a transmitter and a receiver linked by an electric cable. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     The first embodiment of the installation for exchanging information according to the invention is represented in  FIG. 3  and comprises a transmitter SA 3  and a receiver SB 3  linked to one another by an electric cable with two conductors whose electric resistances are symbolized by the resistors RL 1  and RL 2 . The transmitter SA 3  comprises a transistor TA 3  and is identical to the transmitter SA 2  previously described. The receiver SB 3  comprises a voltage supply VDDB supplying a resistor RB 4  and a transistor TB 3  arranged in series. The base of the emitter TB 3  is linked to an end of a conductor of the electric cable via a DC voltage generator P 3  such as a dry-cell or an electric accumulator and a resistor RB 3 . The transistor TA 3  is controlled by a circuit (not represented) and operates in switch mode. The voltage Us is gathered between the emitter and the collector of the transistor TB 3 .  
         [0020]     Thus, an item of information is coded as a succession of states of the transistor TA 3  of the transmitter and decoded by measuring the variations in the voltage Us in the receiver SB 2 . When the transistor TA 3  is off, the current intensity in the electric cable linking the transmitter and the receiver is mainly limited by the resistor RB 3 .  
         [0021]     With such an arrangement it is possible to choose a large resistance RB 3  while being insensitive to the effects of the induced currents. The transistor TB 3  remains, in fact, off when the differential-mode voltage does not become greater than the voltage of the generator P 3  plus the voltage between the base and the emitter of the transistor TB 3 .  
         [0022]     For example, the voltages VDDA and VDDB of the power supplies of the transmitter SA 3  and of the receiver SB 3  may be equal to 12 V. The resistances RA and RB 4  may be taken equal to 1 kΩ and RB 3  equal to 50 kΩ so as to allow the interconnection of numerous elements. The voltage of the generator may be taken equal to 4.5 V and the base-emitter voltage of the transistor TB 3  equal to 0.6 V.  
         [0023]     Thus, the differential voltage allowing the change of state of the transistor equals substantially 5 V. This value gives a good safety margin making it possible to prevent the effects of the induced currents.  
         [0024]     Such an arrangement has the drawback of using a DC voltage generator such as an electric cell or an accumulator. In the latter case, it will be noted however that the accumulator is recharged continuously across the resistors RA, RL 1 , RB 3  and RL 2  when the transistor TA 3  is open, thereby compensating for autodischarge and giving the component a long lifetime.  
         [0025]     In such a circuit it is not possible to replace this generator with a Zener diode of Zener voltage equal to 4.4 V in order to circumvent the interference voltages.  
         [0026]     Specifically, if the generator P 3  is replaced with a Zener diode, when the transistor TA 3  is off, the current is mainly limited by the resistor RB 3  and equals substantially: (12−5)/50=0.140 mA. Such a low value has the consequence that the voltage across the terminals of the Zener diode is very different from the Zener voltage and is in this case substantially zero. As a result, the circuit is sensitive to the induced currents.  
         [0027]     A second embodiment of an installation, represented in  FIG. 4 , makes it possible to solve this problem. This installation comprises a transmitter SA 4  and a receiver SB 4  linked together by an electric cable C 1  with two conductors whose electrical resistors are symbolized by the resistors RL 1  and RL 2 . Information consisting of electric signals sent over the electric cable may be transmitted by the transmitter SA 4  and be received by the receiver SB 4 . A single transmitter and a single receiver have been represented in  FIG. 4  with the aim of simplification and clarity. However, it is obvious that the installation can comprise several command transmitters and several command receivers linked in parallel on the electric cable. For example, in a home automation network, such an installation allows communication between control devices, electrical equipment and sensors. Each of its elements can comprise a transmitter and a receiver so as to be able to carry out bidirectional communications between them.  
         [0028]     The transmitter SA 4  mainly comprises a power supply providing a voltage VDDA supplying a resistor RA and a transistor TA 4  arranged in series. The transistor TA 4  is controlled by a circuit (not represented) and operates in switch mode. The receiver SB 4  comprises a power supply providing a voltage VDDB and supplying a resistor RB 6  and a Zener diode DZB 4  arranged in series with two parallel branches comprising respectively, a resistor RB 7 , and a transistor TB 4  and a resistor RB 8  arranged in series. One of the two ends of the conductors of the electric cable is connected between the resistor RB 6  and the Zener diode, the other is connected to the base of the transistor TB 4  by way of a resistor RB 5 .  
         [0029]     The installation may be embodied, for example, with the following values:  
         [0000]     VDDA=VDDB=12 V  
         [0000]     RA=RB 6 =1 kΩ 
         [0000]     UZ=3.9 V  
         [0000]     RB 5 −47 kΩ 
         [0000]     RB 7 =4.7 kΩ 
         [0000]     RB 8 =100 kΩ 
         [0030]     An item of information which is to be sent from the transmitter SA 4  to the receiver SB 4  is coded as a temporal succession of off and on states of the transistor TA 4 . It is decoded in the receiver SB 4  by analyzing the variations in the voltage Us that is measured across the terminals of the resistor RB 8 .  
         [0031]     When the transistor TA 4  is off, a voltage is present between the collector and the emitter of the transistor TA 4 . This voltage equals substantially some 12 volts. It causes the flow of a current passing through the resistor RL 1 , the Zener diode DZB 4 , the transistor TB 4 , the resistor RB 5  and the resistor RL 2 . The Zener voltage of the diode DZB 4  is maintained by a current flowing across the resistors RB 6  and RB 7  wired between the power supply terminals of the receiver SB 4 . This Zener voltage and the emitter-base voltage of the transistor TB 4  oppose the flow of the current through the cable. When a sufficient current flows through this cable, the transistor TB 4  is on and the voltage Us then equals some 10 volts and is interpreted as a high state by a logic circuit. The large value of the resistor RB 5  allows limitation of the current and the possibility of connecting a transmitter with numerous receivers.  
         [0032]     When the transistor TA 4  is on, the voltage between its collector and its emitter is substantially zero. This has the consequence that no current flows around the loop. The transistor TB 4  is consequently off and the voltage Us is substantially zero. This is interpreted as a low state by a logic circuit. The current passing through the Zener diode and making it possible to maintain the Zener voltage at its terminals is chosen to be around 10 times greater than the induced currents that may be encountered in the cable. This makes it possible to ensure that induced interference currents cannot make the transistor TB 4  switch into an on state.  
         [0033]     Such an installation comprising some 100 receivers and a length of connection cable of 1000 m operates perfectly.  
         [0034]     The transmitters and the receivers may of course comprise other elements such as capacitors. The transistor TA 4  may, for example, be controlled by a microcontroller.  
         [0035]     To allow the connection of an even larger number of elements in the installation, the resistor RA can also be replaced by a transistor.