Abstract:
Each node (N1) of the network is connected to each of the subscriber sets (P1-1) that are associated with the node via a single optical fiber (FP1-1) on two different wavelengths. The node is connected to each of certain other nodes via a single fiber (FN1-2) on a single wavelength. In the node, the electrical signals are supplied by two optical receivers (2, 6) and are transmitted via amplifiers (8, 10) to a single transmitter (4). A passive directional coupler (22) and a wavelength separator (20) provide the necessary optical connections. Passive optical dividers (24, 26) form two interfaces with the sets and with the other nodes. The invention is applicable in particular to industrial transmission networks.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a local area network with optical transmission. It is known that, in an industrial installation, a local area network provides links between sensors and/or actuators and automation systems such as controllers, programmable or otherwise, or regulators, etc. Such sensors, actuators, and systems constitute subscribers to the network. The data transmitted by a subscriber is typically transmitted to all of the other subscribers via optical fibers and via active optical star couplers performing optical coupling and constituting nodes of the network. 
     Frames carrying the data are transmitted at a data rate that is typically equal to 2.5 Mbit/s. Consideration is being given to raising the data rate to 5 or 10 Mbit/s. 
     A family of known local area networks is described in French Standard UTE C 46-607: 
     &#34;FIP bus for performing data interchange between transmitters, actuators, and controllers--Base-band physical layer on optical fiber&#34;. 
     A local area network of this known family includes the following elements which, with respect to their functions indicated below, are common to a local area network of the present invention: 
     Terminal sets capable of transmitting and receiving optical signals. Such sets are in particular subscriber sets, each of which is connected, e.g. electrically, and assigned to a subscriber. 
     Nodes constituted by active star couplers. 
     Primary fibers optically connecting each of the nodes to sets associated with the node. 
     Secondary fibers optically connecting each of the nodes to other ones of the nodes which are thus connected directly to the node. By means of the secondary fibers, each of the nodes is connected at least indirectly to all of the others. Via the fibers to which it is connected, each node is capable of receiving an incoming optical signal carrying data, and of responding by transmitting corresponding outgoing optical signals carrying the data to all of the associated sets and to all of the nodes connected directly to the node. For that purpose, the node includes optoelectrical transducers and electrical amplifiers. More precisely, for each association of an incoming optical signal and of a corresponding outgoing optical signal, the node includes: 
     a receive transducer for transforming the incoming optical signal into an electrical reception signal; 
     an amplifier for amplifying the electrical signal; and 
     a transmit transducer for transforming the amplified electrical signal into an outgoing optical signal. 
     Each receive transducer or transmit transducer is referred to below as either a &#34;primary&#34; transducer if the optical signal is transmitted via a primary fiber, or a &#34;secondary&#34; transducer if the signal is transmitted via a secondary fiber. 
     Within the above known family, a local area network is characterized in particular by a wavelength which constitutes the wavelength of the network and to which all of the transducers of the network are tuned. Each node is connected to each of the sets that are associated with it via two optical fibers connected to respective ones of two primary transducers of the node, one of which transducers is a transmit transducer, the other transducer being a receive transmitter, and both transducers being assigned exclusively to the set. 
     For that purpose, the node includes a primary transmit transducer and a primary receive transducer for each of the sets that are associated with the node. 
     Each direct connection between two nodes of the network is provided by two secondary fibers which are assigned exclusively to interconnecting the two nodes. A first fiber guides the incoming signals of a first node and the outgoing signals of the second node. The second fiber guides the outgoing signals of the first node and the incoming signals of the second node. The outgoing signals of the first node (or of the second node) are transmitted via a secondary transmit transducer of the first (or of the second) node, and are received via a secondary receive transducer of the second (or of the first) node, the two transducers being assigned exclusively to interconnecting the two nodes. For that purpose, each node includes a secondary transmit transducer, and a secondary receive transducer for each other node connected directly to the node. 
     Such a known network suffers from the drawback that, even if the elements used to make up the network are as well chosen as possible, the number of subscribers that can be interconnected appears too limited to satisfy the needs of industry. Moreover, its cost is high. 
     SUMMARY OF THE INVENTION 
     Particular objects of the present invention are to increase the number of subscribers connected to a local area network, and/or to reduce the cost of such a network. 
     To these ends, the receive primary transducers and the transmit primary transducers of a network of the invention are tuned to two different respective wavelengths; 
     a single primary fiber connects each set to the associated node; 
     the receive secondary transducers and the transmit secondary transducers are tuned to one of the two wavelengths constituting a common wavelength; 
     a single secondary fiber interconnects two nodes when the two nodes are connected together directly; and 
     means are provided so that, when a first node transmits an optical signal over the secondary fibers, an optical signal transmitted in response by a second node cannot interfere with the operation of the first node. 
     For example, such means include buffer memories disposed in each node so that, when the node receives a signal, it can wait before re-transmitting the signal for long enough so that the re-transmitted signal cannot interfere with the received signal. 
     Preferably, each node includes a common transducer tuned to the common wavelength, and constituting both a primary transducer and a secondary transducer. 
     A description is given below with reference to the accompanying drawings of how the present invention may be implemented, it being understood that the elements and dispositions mentioned and shown are given by way of non-limiting example only. When the same element is shown in more than one figure, it is given the same reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary view of a first network of the invention; 
     FIG. 2 is a view of a node and of two subscriber sets of the network; 
     FIG. 3 is a simplified view of the node shown in FIG. 2; and 
     FIG. 4 is a simplified view of a node of a second network of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIGS. 1 and 2, a network of the invention has a known basic disposition which is described firstly below: 
     In compliance with this disposition, the network includes a plurality of terminal sets P1-1 . . . P4-16 each provided with an optical connection terminal 16. Each of the sets includes: 
     a transmitter 12 capable of receiving an input signal S1 carrying data and of responding by supplying a message S2 including the data and transmitted by the set; and 
     a receiver 14 for processing a message S8, T10 including data and received by the set, the receiver being capable of responding by supplying an output signal S9, T11 including the data. 
     The sets are distributed in a plurality of groups of sets, e.g. such as groups P1-1 . . . P1-16 . . . P2-1 . . . P2-16, each group comprising at least one set, e.g. 16 sets. Most of the sets are subscriber sets P1-1 . . . P1-16, P2-2 . . . P4-16 connected to respective subscribers Al-1 . . . Al-16, A2-2 . . . A4-16 outside the network. Each subscriber A1-1 connected to a set P1-1 can include data to be transmitted to the other subscribers in an input signal S1 of the set. Each subscriber connected to a set can also receive the output signals S9, T11 of the set so as to use the data therefrom. The input signals and the output signals of the subscriber sets respectively constitute input signals and output signals of the network. 
     A management member A2-1 is connected to the subscriber sets so that, at any given time, only one of the sets is allowed to transmit a message. For example, the management member may constitute one of the sets of the network. It is then connected to the subscriber sets in the same manner as the subscriber sets are interconnected. This manner is indicated below. 
     A plurality of nodes N1 . . . N4 are associated with respective ones of said groups of sets, each node N1 associated with a group of sets P1-1 . . . P1-16 also being associated with the sets in the group. Such a node includes the following elements: 
     Primary connection terminals B1 . . . B16 for enabling messages to be interchanged between the node and the associated sets. These messages are referred to as &#34;primary&#34; messages, and the primary messages are further referred to either as &#34;incoming&#34; messages, such as message S2, or as &#34;outgoing&#34; messages, such as messages S8 and T10, depending on whether they are received or transmitted by the node. 
     At least two secondary connection terminals B17 . . . B20 for enabling messages to be interchanged between the node and other nodes N2 . . . N4. These messages are referred to as &#34;secondary&#34; messages, and either as &#34;incoming&#34; messages, such as message T1, or as &#34;outgoing&#34; messages, such as messages S11 and T8 depending on whether they are received or transmitted by the node. The primary and secondary, incoming and outgoing messages are in the form of optical signals constituted by modulations in optical carrier waves, and the wavelengths of the carrier waves constituting the respective wavelengths of the messages. 
     Amplifiers 8, 10 for receiving electrical reception signals S4, T4, and for responding by supplying amplified electrical signals S5, T5. 
     Optoelectronic transducers. Each of the transducers may be a receive transducer. Such is the case for transducers 2 and 6, each of which transfers data from an optical signal S3 or T3 coming from a terminal B1 or B17 to an electrical reception signal S4 or T4 supplied to an amplifier 8 or 10. Each of the transducers may also be a transmit transducer. Such is the case for transducer 4 which transfers data from an amplified electrical signal S5 or T5 coming from an amplifier 8 or 10 to an optical signal S6 or T6 supplied to a terminal B1, B16, or B17. The transducer is at the same time either a primary transducer or a secondary transducer depending on whether the terminal is a primary terminal or a secondary terminal. All of the transducers enable the outgoing messages to be formed from the incoming messages. 
     Each transmitter or receiver of a set and each transducer of a node is tuned to a wavelength which is that of the optical signals whereby it transfers the data. 
     Primary optical fibers FP1-1 . . . FP1-16 connect the optical terminals 16 of the sets P1-1 . . . P1-16 of each group of sets to respective ones of the primary terminals B1 . . . B16 of the node N1 associated with the group of sets, so that the messages transmitted by the sets constitute the incoming primary messages of the node and so that the outgoing primary messages of the node are received by the sets. 
     Each one of secondary optical fibers such as FN1-2, or FN1-4 interconnects two secondary terminals belonging to respective ones of two nodes such as N1 and N2 or N4 so that the outgoing secondary messages of one of the nodes constitute the incoming secondary messages of the other node. 
     In a more particular first disposition of the present invention, described firstly below in general terms, the incoming primary messages S2 and the outgoing primary messages S8, T10 respectively have a first wavelength of the network and a second wavelength of the network. The two wavelengths are different and one of them constitutes a common wavelength. The incoming secondary messages T1 and the outgoing secondary messages S11, T8 have the common wavelength. 
     Each set such as P1-1 includes: 
     an optical terminal 16; and 
     a wavelength separator 18 connecting the terminal both to the transmitter 12 of the set for the first wavelength, and also to the receiver 14 of the set for the second wavelength. The set further includes electronic management circuits and memories for communicating with the subscriber which is connected to it, and for verifying, once it has transmitted a message, that it has in fact received a return message, correctly reproducing the transmitted message, from the associated node. If such a return message is not received, the set transmits the same message again. Other dispositions known per se and not described herein are used to limit the damaging consequences of a failure of an element of the network. 
     The node N1 associated with the set includes the following elements: 
     A primary terminal B1 associated with the set and connected to the optical terminal 16 of the set via one of said primary fibers FP1-1. 
     A wavelength separator 20. 
     A primary receive transducer 2 tuned to the first wavelength and connected to the primary terminal B1 via the separator 20 for the first wavelength. 
     A primary transmit transducer 4 tuned to the second wavelength and connected to the primary terminal via the separator for the second wavelength. That one of the two primary transducers which is tuned to the common wavelength is constituted by a &#34;common&#34; transducer 4 whose functions are specified below. The other of the two primary transducers constitutes an assigned transducer assigned to the primary messages 2. 
     A passive optical coupler 22. 
     A secondary receive transducer 6 tuned to the common wavelength and optically connected to a secondary terminal B17 via the passive coupler 22. 
     A secondary transmit transducer 6 tuned to the common wavelength and optically connected to a secondary terminal B17 via the passive coupler 22. The common transducer also constitutes one of the two secondary transducers. More precisely, the common transducer constitutes either both the primary and the secondary transmit transducers, which is the case for transducer 4 shown in FIGS. 2 and 3, or else both the primary and secondary receive transducers, which is the case for transducer 104 in FIG. 4. The other of the two secondary transducers constitutes an assigned transducer assigned to the secondary messages, such as 6 and 106. The common transducer 4 or 104 is electrically connected to the two assigned transducers 2 and 6, or 102 and 106 via respective ones of two amplifiers 8 and 10, or 108 and 110. 
     Other advantageous dispositions for the networks given by way of example are described below: 
     The passive coupler 22 has two pairs of terminals 30, 32 and 34, 36 (see FIG. 3) and is capable of receiving an optical signal at at least one terminal of each of the pairs, and of then splitting the signal so as to restore it in part at each of the two terminals of the other pair. The common transducer 4 is then optically connected to said primary terminal B1 of the node N1 via the passive coupler and the wavelength separator 20 in series so as to enable optical signals to be transmitted between the transmitter and the terminal. 
     The wavelength separator 20 connects the common transducer 4 and the assigned transducer assigned to the primary messages 2 to a plurality of primary terminals B1 . . . B16 of the node N1 simultaneously and preferably to all of the primary terminals thereof via a primary passive optical divider 24. 
     The passive coupler 22 connects the common transducer 4 and the assigned transducer assigned to the secondary messages 6 to a plurality of the secondary terminals B17 . . . B20 of said node N1 and preferably to all of the secondary terminals thereof via a secondary passive optical coupler 26. 
     The above three dispositions make it possible for the node N1 to include three transducers only: the common transducer 4, the assigned transducer assigned to the primary messages 2, and the assigned transducer assigned to the secondary messages 6. 
     In the typical case in which the secondary fibers FN1-2, FN1-10, F2-3, FN2-4 are longer than the primary fibers and must attenuate only as little as possible the secondary messages, the choice of the wavelengths and of the materials of the fibers is preferably such that the secondary fibers have a lower coefficient of absorption for said common wavelength than for the other wavelength of the network. For example, the common wavelength is 1,300 nm, with the other wavelength of the network being 800 nm. The primary fibers are chosen so as to have low coefficients of absorption at both of the wavelengths. 
     As shown in the simplified diagrams in FIGS. 3 and 4, the nodes of both the first and the second networks of the invention have analogous elements disposed in compliance with two identical architectures. Each element of the second network is referenced by a reference numeral which is the reference numeral of the analogous element of the first network increased by 100. 
     The differences between the two networks are as follows: 
     The common transducer is a transmit transducer 4 in the first network, whereas it is a receive transducer 104 in the second network. 
     The assigned transducers assigned to the primary and the secondary messages are receive transducers 2 and 6 in the first network, whereas they are transmit transducers 102 and 106 in the second network. 
     the propagation directions of the optical signals and of the electrical signals between the wavelength separator 20 of 120 and the passive coupler 22 or 122 are inverted from one network to the other, as are the connection directions of the amplifiers 8 or 108, and 10 or 110.