A circuit for ring-redundant communications

Abstract

There is provided a power-communication decoupler circuit (3) for a ring-redundant Ethernet network system that comprises one or more Ethernet switches. The power-communication decoupler circuit (3) comprises a trunk cable interface (8) for connecting a powered single-pair Ethernet trunk cable, a power output interface (12) for powering the one or more Ethernet switches, and a communications interface (20) for connecting a communications cable to one of the Ethernet switches. The power-communication decoupler circuit has a power supply (18) connected between the trunk cable interface and the power output interface, and an internal switch (17) connected between the trunk cable interface (8) and the communications interface (20).

Description

[0001] A CIRCUIT FOR RING-REDUNDANT COMMUNICATIONS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a circuit for enabling ring-redundant communications with one or more Ethernet switches that are connected to, and powered by, two powered single-pair Ethernet trunk cables.

[0004] BACKGROUND OF THE INVENTION

[0005] Network switches are commonly used to connect different portions or segments of a network to one another, to allow data to be passed between them. Historically most network switches have been powered from a power supply input that is separate from the connected network cables. To improve the resilience of the system to any faults occurring along the network cables, it is possible to provide a ring-redundant system such as shown in Fig. 1 , in which the network switches / devices are arranged in a ring and so can receive data from either direction around the ring to provide redundancy. For classic industrial Ethernet 4- wire or 8-wire applications, comprising multiple switches 41 with their own separate power supplies 43, ring-redundancy can be employed by providing switch ports 40 and 45 at either end of the network and also a ring completion segment 47 connecting the switch ports 40 and 45 together. In this case, if any cable segment 42 between one of the switches 41 and the switch port 40 is short- circuited, then communication to that switch 41 will be re-routed via the ring completion segment 47 and the switch port 45. Ring redundancy is simple to implement for conventional Ethernet because each switch or device is separately powered. However, providing separate power sources to supply the switches and devices is expensive and complex to install in an industrial environment, and more so when implemented in hazardous areas for instrumented process control purposes.

[0006] In recent years network standards have been developed that allow transmission of power along the network cables themselves, enabling switches to be powered from the network cables without the need for a separate power input. It is common for such switches to be integrated into larger devices, such as industrial control instruments, and for the devices to be powered by the power delivered through the network cables.

[0007] Standards such as PoE (Power over Ethernet) can be used to deliver power to devices, but cannot typically be used to deliver power to multiple devices spread over wide areas in a ring redundant network, as there is too much voltage drop due to the resistance of the CATx cables. The cable spans are limited to 100m, mechanically protected cable is expensive, and it is expensive to install in industrial and / or hazardous environments. There are a variety of SPE (powered single-pair Ethernet) standards such as Ethernet-APL or PoDL that allow power to be sent to multiple devices over wide areas, however these are not conventionally suitable for ring redundant networks because of the disruption to the data signals on the overall network that results from a short-circuit fault. If the switch ports 40 and 45 of Fig. 1 were modified to add power supplies for delivering power to the devices 44 alongside the data in accordance with powered single-pair Ethernet, then upon occurrence of a short-circuit the data being sent along the ring would become corrupted enough to cause prolonged communication failure, resulting in a reset (restart) of the whole system. This is not acceptable for more critical realtime (continuous) industrial control and measurement, where all participating devices, and therefore process control and monitoring functionality, can be lost for some time until the system recovers or ‘heals’. Also, the current can increase to values where the current limitation of the power sources is exceeded, tripping the power sources and causing the complete ring to lose power.

[0008] It is therefore an aim of the invention to provide a circuit enabling ring redundancy to be used in conjunction with powered single-pair Ethernet networks.

[0009] SUMMARY OF THE INVENTION

[0010] In accordance with the invention there is provided a power-communication decoupler circuit for a ring-redundant network system that comprises one or more Ethernet switches, wherein the power-communication decoupler circuit comprises: a trunk cable interface for connecting a powered single-pair Ethernet trunk cable; a power output interface for connecting one or more power supply cables to power the one or more Ethernet switches; a communications interface for connecting a communications cable to one of the network switches; a power supply connected between the trunk cable interface and the power output interface, the power supply comprising at least one series inductor; an internal switch connected between the trunk cable interface and the communications interface, wherein the internal switch comprises a first transceiver for communicating via the trunk cable interface and a second transceiver for communicating via the communications interface, the first transceiver being connected to the second transceiver for transferring communications therebetween.

[0011] The trunk cable interface may be for connecting a powered single-pair Ethernet trunk cable in accordance with a first physical layer standard, for example, the IEEE 802.3 physical layer standards 10 BASE-T1 L including PoDL or Ethernet-APL. The communications interface may be for connecting a communications cable to one of the network switches in accordance with a second physical layer standard, for example the IEEE 802.3 physical layer standard for general Ethernet solutions using Cat5e cable or optical fibre, the first physical layer standard being different to the second physical layer standard. The first physical layer standard may define transmission of both power and data over the powered single-pair Ethernet trunk cable, and the second physical layer standard may define transmission of data but not power over the communications cable.

[0012] There is further provided a ring-redundant network system comprising two single-pair Ethernet power switches, two of the power-communication decoupler circuits, and one or more Ethernet switches that are connected in a daisy chain with a plurality of communications cables, wherein: each single-pair Ethernet power switch is connected to the trunk cable interface of a respective one of the power-communication decoupler circuits via one or more powered single-pair Ethernet trunk cables; the communications interface of each power-communication decoupler circuit is connected to a respective end of the daisy chain by a corresponding one of the communications cables; and the two single-pair Ethernet power switches are connected to communicate with one another via a ring completion path that is separate from the daisy chain.

[0013] Thus, a large area of the network may be constructed using powered single-pair Ethernet. A pair of the power-communication decoupler circuits can be located at opposing ends to one another of a daisy chain of one or more Ethernet switches and the communications cables, to decouple the power from the data communications within the daisy chain, and so to prevent a fault occurring in one of the communications cables of the daisy chain from corrupting the data signals being sent along other communications cables of the daisy chain. If one of the communications cables of the daisy chain fails, then the Ethernet switch(es) directly connected to the failed communications cable can still communicate with the rest of the network via the other communications cable that each Ethernet switch is connected to. The pair of power-communication decoupler circuits may be located together with one another, for example in a single enclosure or on a single circuit board.

[0014] Since the one or more Ethernet switches and the communications cables are connected in a daisy chain, each Ethernet switch comprises two communications interfaces each connected to a respective one of the communications cables. Each Ethernet switch may form part of an instrumentation device, and / or may comprise one or more further communications interfaces for connecting further cables, for example spur cables leading to further devices.

[0015] The one or more Ethernet switches are not typically powered via the plurality of communications cables forming the daisy chain, and instead may have separate power supply inputs. The power output interface of each powercommunication decoupler circuit may be connected to the power supply input of one or more of the Ethernet switches via at least one power cable separate from the communications cables. Since the power is sent separately from the data signals, a fault on one of the power cables does not risk corrupting the data signals being sent along the daisy chain.

[0016] Preferably, the power supply input of each Ethernet switch is connected to the power output interfaces of both of the power-communication decoupler circuits, for example the power supply input of each Ethernet switch may comprise a first power input port connected to the power output interface of one of the powercommunication decoupler circuits via one power cable and a second power input port connected to the power output interface of the other of the powercommunication decoupler circuits via another power cable. Then, if a fault occurs along the power cable connected to one of the power input ports, the Ethernet switch will still receive power from the power cable connected to the other power input port. To ensure that the faulted power cable does not draw current from the other power cable connected to the power supply input, the first and second power input ports may be connected to one another via two diodes arranged back-to- back in series, and the Ethernet switch may draw current from a circuit node between the back-to-back diodes. For example, the Ethernet switch may comprise a power supply that is connected between the back-to-back diodes.

[0017] The power output interface may comprise a plurality of power output ports for connecting corresponding power cables to the power output interface. Then, the power supply input of each Ethernet switch may be directly connected to a respective one of the power output ports of the power output interface by a corresponding one of the power cables. In other words, the power supply inputs of the Ethernet switches may be connected to the power output interface of the power-communication decoupler circuit in a star configuration, so that if one of the power cables to one of the Ethernet switches fails then the failure does not have any impact on the delivery of power from the power-communication decoupler circuit to the other Ethernet switches. Preferably, the Ethernet switch connected to the failed power cable will draw current via another power cable connected to the other power-communication decoupler circuit.

[0018] The internal switch of the power-communication decoupler circuit may convert data signals between the first physical layer standard and the second physical layer standard, so that the communications being sent are transferred between the powered single-pair Ethernet trunk cable connected to the trunk cable interface and the communications cable connected to the communications interface of the power-communication decoupler circuit. The first physical layer standard typically defines transmission of both power and data over the powered single-pair Ethernet trunk cable whereas the second physical layer standard typically defines transmission of data but not power over the communications cable.

[0019] The internal switch may be a two-port switch, or in other words a repeater, a switch ASIC or a back-to-back coupler, or the internal switch may provide more than two ports. In the latter case the power-communication decoupler circuit comprises further communications interface(s) connected to respective ports of the internal switch. The communications interfaces may be in accordance with different standards to one another, although typically none deliver power in addition to data. The further communications interface(s) are for connecting further communications cable(s) to further Ethernet switch(es). For example, one of the further communications interfaces may be connected to a further daisy chain of the further Ethernet switches to help define a further redundant ring.

[0020] Powered single-pair Ethernet carries both power and data along a single pair of conductors, as will be apparent to those skilled in the art. The pair of conductors consists of a positive conductor and a negative conductor, and the data signals are modulated onto a DC voltage differential between the positive and negative conductors. Thus, power can be drawn based on the DC voltage differential and data can be communicated based on high frequency voltage components that are modulated onto the DC voltage differential. To aid in decoupling the power from the communications, the power-communication decoupler circuit preferably comprises a DC blocking circuit that connects the trunk cable interface to the first transceiver. Thus, the first transceiver may only receive the high frequency voltage components, which carry the data, and not the DC voltage. The DC blocking circuit may be constructed from various components, for example a series capacitor may be used, or an inductive coupling arrangement, as will be apparent to the skilled person.

[0021] The power supply of the power-communication decoupler circuit comprises at least one series inductor. The high frequencies present at the trunk cable interface which correspond to the data signals, are blocked from reaching the power output interface by the high impedance that the inductor presents to those high frequencies. Accordingly, the power supply separates the DC voltage from the data signals, and makes the DC voltage available at the power output interface.

[0022] The power supply typically comprises positive and negative supply lines, connected to positive and negative conductors at the power output interface. Preferably, to provide a balanced system the at least one series inductor comprises a positive side inductor connected in series within the positive supply line and a negative side inductor connected in series within the negative supply line.

[0023] The power supply may comprise at least one capacitor that is connected between the positive and negative supply lines, at a point between the power output interface and the at least one series inductor, to smooth the voltage presented at the power output interface.

[0024] The power supply may comprise at least one series rectifier diode, for example a positive side rectifier diode connected in series within the positive supply line and a negative side rectifier diode connected in series within the negative supply line. The rectifier diodes may be connected between the inductors and the capacitor to further smooth the voltage presented at the power output interface.

[0025] Advantageously, the power supply may be configured to power the internal switch so that the power-communication decoupler circuit does not require any separate power input but takes its power from the single-pair Ethernet connection to the trunk cable interface.

[0026] Whilst the invention has been described using Ethernet power switches, Ethernet switches and powered single-pair Ethernet cables, in other embodiments the Ethernet power switches, Ethernet switches and powered single-pair Ethernet cables may be respectively substituted for power switches, network switches and powered single-pair cables in general which do not have to be Ethernet compliant.

[0027] DETAILED DESCRIPTION

[0028] Embodiments of the invention will now be described by way of non-limiting example only and with reference to the accompanying drawings, in which:

[0029] Fig. 1 shows a schematic diagram of a known ring-redundancy system incorporating separately powered network switches;

[0030] Fig. 2 shows a schematic diagram of a power-communication decoupler circuit in accordance with an embodiment of the invention;

[0031] Fig. 3 shows a schematic diagram of a power-communication decoupler circuit in accordance with a second embodiment of the invention;

[0032] Fig. 4 shows a schematic diagram of a power-communication decoupler circuit in accordance with a third embodiment of the invention;

[0033] Fig. 5 shows a schematic diagram of a power-communication decoupler circuit in accordance with a fourth embodiment of the invention;

[0034] Fig. 6 shows a schematic diagram of a ring redundant network system incorporating a power-communication decoupler circuit in accordance with an embodiment of the invention.

[0035] The figures are not to scale, and same or similar reference signs denote same or similar features.

[0036] The schematic diagram of Fig. 2 shows a power-communication decoupler circuit 3 in accordance with an embodiment of the invention. The circuit 3 comprises a trunk cable interface 8, a power output interface 12 and a communications interface 20. The circuit comprises a power supply 18 that is connected between the trunk cable interface 8 and the power output interface 12, and an internal switch 17 that is connected between the trunk cable interface 8 and the communications interface 12. The power supply 18 and internal switch 17 decouple the power and communications received at the trunk cable interface 8, and transfer them to the power output interface 12 and the communications interface 20 respectively.

[0037] The trunk cable interface may comprise positive and negative conductors 8a and 8b respectively, and the conductors 8a and 8b are for connecting to the positive and negative conductors of a single-pair Ethernet trunk cable. The power output interface 12 may comprise positive and negative conductors 12a and 12b respectively, and the conductors 12a and 12b are for connecting to one or more power cables to deliver power. The communications interface 12 may comprises at least one pair of conductors for connecting to a communications cable over which data communications may be sent.

[0038] The power supply 18 may comprise a positive supply line 19a that connects from the positive conductor 8a of the trunk cable interface to the positive conductor 12a of the power output interface, and a negative supply line 19b that connects from the negative conductor 8b of the trunk cable interface to the negative conductor 12b of the power output interface. The power supply may comprises positive and negative side inductors 9 that are connected in series within the positive and negative supply lines 19a and 19b. Since both the positive and the negative supply lines have a corresponding inductor 9 connected in series within the supply line, the inductors present a balanced impedance to the differential voltage between the positive and negative conductors 8a and 8b. The inductors 9 present a low impedance to the DC voltage across the positive and negative conductors 8a and 8b, and present a high impedance to the high-frequency voltage (data) signals modulated on the DC voltage. Thus, the DC voltage can pass towards the power output interface 12 whereas the high-frequency voltage (data) signals are greatly attenuated.

[0039] To further attenuate the high-frequency voltage (data) signals the power supply 18 may comprise one or more capacitors 10 and 13 that are connected in between the positive and negative supply lines 19a and 19b. The capacitor(s) are connected to the positive and negative supply lines at points between the series inductors 9 and the power output interface 12. The power supply 18 may also comprise one or more series rectification diodes to block any reverse currents from flowing and smooth the DC voltage provided by the power supply output. In this embodiment, there are two capacitors 10 and 13, a positive side rectification diode 11 a connected in series within the positive supply line 19a between the connections of the capacitors 10, 13 to the positive supply line, and a negative side rectification diode 11 b connected in series within the negative supply line 19b between the connections of the capacitors 10, 13 to the negative supply line. The capacitor 10 may primarily form a low-pass filter together with the inductors 9 and the capacitor 13 may primarily be a smoothing capacitor at the outputs of the series rectification diodes.

[0040] The internal switch 17 comprises a first transceiver (PHY) 17 and a second transceiver (PHY) 16 that are connected to one another for transferring data communications between them. The internal switch may be powered by a power input 14, and the power input 14 may be connected (connection not shown in Figs for clarity) to the power output interface 12. Thus, the circuit 3 may not require a separate power supply in order for the internal switch to function. The internal switch 17 is a two-port switch in this embodiment, or in other words a repeater, a switch ASIC or a back-to-back coupler.

[0041] The first transceiver 15 may be connected to the positive and negative conductors 8a and 8b of the trunk cable interface, and send and receive data via high frequency differential voltage signals applied across those conductors. The first transceiver 15 is in accordance with a powered single-pair Ethernet physical layer specification, for example the IEEE 802.3 physical layer standard for any powered single-pair Ethernet application The data is passed to / from the second transceiver 16 via a repeater RPTR, and the second transceiver 16 is connected to the communications interface 20 for sending / receiving the data via the communications interface 20. The second transceiver 20 is not typically in accordance with a powered single-pair Ethernet physical layer specification, but is typically in accordance with a physical layer specification that only carries data not power along the connected network cabling, for example the IEEE 802.3 physical layer standard for multi-pair copper or optical fibre. The second transceiver 20 may be in accordance with any IEEE 802.3 based Ethernet specification.

[0042] Thus, Ethernet communications can pass in either direction between the trunk cable interface 8 and the communications interface 20, without passing the DC voltage present between the positive and negative conductors 8a and 8b to the communications interface 20. The first transceiver 15 may for example comprise at least one capacitor in series with the connections to the positive and negative conductors 8a and 8b of the trunk cable interface, filtering (blocking) the DC voltage component between the conductors 8a and 8b. Terminators may also be included between the positive and negative conductors 8a and 8b, as will be apparent to those skilled in the art, but are not shown for clarity.

[0043] The schematic diagram of Fig. 3 shows a power-communication decoupler circuit 3 in accordance with a second embodiment of the invention. The second embodiment is the same as the first embodiment except for that the two-port internal switch 17 has been replaced by a three-port switch 17a. The first and second transceivers 15 and 16 are still present, but a further transceiver 16n has been added to provide a further communications interface 20a for the connection of a further network cable. The further transceiver 16n may be the same type of transceiver as the second transceiver 16, however it could be in accordance with a different physical layer specification to the second transceiver 16 if desired. The further communications interface 20a may be used to connect an end of a further daisy chain of further Ethernet switches if desired, for example to help form a further redundant ring.

[0044] The schematic diagram of Fig. 4 shows a power-communication decoupler circuit 3 in accordance with a third embodiment of the invention. The third embodiment is the same as the first or second embodiment except for that the first transceiver 15 is connected to the trunk cable interface 8 via a DC blocking circuit in the form of two coupled inductors 9a. The internal switch of the third embodiment is the same as in the first or second embodiments, but only the first transceiver 15 is shown in Fig. 4 for clarity. The DC blocking circuit comprises a positive side coupling inductor 9a that is magnetically coupled with the positive side inductor 9, and also a negative side coupling inductor 9a that is magnetically coupled with the negative side inductor 9. The two coupling inductors 9a are connected in series with one another between positive and negative connections to the first transceiver 15, and so high frequency voltages are coupled between the trunk cable interface 8 and the first transceiver 15.

[0045] The schematic diagram of Fig. 5 shows a power-communication decoupler circuit 3 in accordance with a fourth embodiment of the invention. The fourth embodiment is the same as the first or second embodiment except for that the first transceiver 15 is connected to the trunk cable interface 8 via a DC blocking circuit comprising a capacitor 10a and an inductor 10b that are connected in series with one another between the positive and negative conductors 8a and 8b of the trunk cable interface.

[0046] The DC blocking circuit also comprises an inductor 10c that is magnetically coupled with the inductor 10b, and the inductor 10c is connected between the positive and negative connections to the first transceiver 15. Thus, high frequency voltages are coupled between the trunk cable interface 8 and the first transceiver 15. The internal switch of the fourth embodiment is the same as in the first or second embodiments, but only the first transceiver 15 is shown in Fig. 4 for clarity.

[0047] The schematic diagram of Fig. 6 shows a ring redundant network system incorporating two of the power-communication decoupler circuits of any of the first and fourth embodiments. The two power-communication decoupler circuits are labelled as 3a and 3b, and each one has the trunk cable interface 8, the power output interface 12 and the communications interface 20. The power output interface includes two power output ports 21 a and 21 b which both provide connections to the positive and negative conductors 12a and 12b of the power output interface 12.

[0048] The ring redundant network system may also comprise two single-pair Ethernet power switches 1 a and 1 b, two Ethernet switches 6a and 6n, and two groups of Ethernet devices 26a and 26n. The two Ethernet switches 6a and 6n are connected in a daisy chain with a plurality of communications cables 4a, 4b and 4c. The communications interface 20 of the power-communication decoupler circuit 3a is connected to one end of the daisy chain by the communications cable 4a, and the communications interface 20 of the power-communication decoupler circuit 3b is connected to the other end of the daisy chain by the communications cable 4b. The communications cable 4a is also connected to a first communications interface 27a of the Ethernet switch 6n, the communications cable 4b is also connected to a second communications interface 27b of the Ethernet switch 6a, and the communications cable 4c connects between a second communications interface 27b of the Ethernet switch 6n and a first communications interface 27a of the Ethernet switch 6a. The daisy chain may be lengthened with more Ethernet switches and more communications cables if desired.

[0049] The power output interface of the power-communication decoupler circuit 3a may be connected to both the Ethernet switches 6a and 6n, specifically the power output port 21a may be connected to a power input port 7a of the Ethernet switch 6a via a power cable 5a, and the power output port 21 b may be connected to a power input port 7a of the Ethernet switch 6n via a further power cable 5a. Further power output ports of the power output interface may be provided to connect to further Ethernet switches, if present, by further power cables. The power output interface is connected to the power input ports of the Ethernet switches in a star configuration, so that failure of one power cable will not prevent power from reaching the Ethernet switches connected to the other power cables.

[0050] In addition, the power output interface of the power-communication decoupler circuit 3b may be connected to both the Ethernet switches 6a and 6n. Specifically, the power output ports of the power output interface may be connected to power input ports 7b of the Ethernet switches 6a and 6n, via power cables 5b, also in a star configuration. If one of the power cables 5a to one of the Ethernet switches fails, then the Ethernet switch can still receive power via the power cable 5b that is connected to the Ethernet switch. Accordingly, each Ethernet switch has a power supply input comprising a first power input port 7a and a second power input port 7b. The first and second power input ports of each Ethernet switch may be connected to one another via two diodes D1 and D2 arranged back-to-back in series, and the Ethernet switch may comprise a power supply 30 that is connected between the back-to-back diodes. The diodes mean that current flowing into the power supply input from one of the power cables 5a and 5b will not be able to flow back out of the power supply input along the other of the power cables 5a or 5b. Accordingly, if a short-circuit fault occurs on a power cable then current will not be delivered into the fault from one of the Ethernet switches, but will instead by blocked by one of the diodes D1 or D2. The power supply 30 may power the Ethernet switch, avoiding the need for a separate power supply.

[0051] Each Ethernet switch comprises the first and second communications interfaces 27a and 27b for connecting the Ethernet switch in the daisy chain. Each communications interface 27a and 27b is duplex and so allows communications in either direction through the communications interface. Thus, communications can be sent in either direction along the daisy chain.

[0052] Each Ethernet switch may also comprise spur cable interfaces 28, which may be connected to corresponding devices 26 by spur cables 29. Each Ethernet switch is illustrated as having three spur cable interfaces 28 connected to three corresponding devices 26 via three single-pair Ethernet cables 29 that are bundled together. The devices 26 may for example be instrumentation devices for industrial process control. In this embodiment the spur cable interfaces carry both power and data power in accordance with a powered single-pair Ethernet physical layer standard. The power is supplied from the power supply 30 and the data is routed between the communications interfaces 27a, 27b and the spur cable interfaces 28. Accordingly, the devices 28 may be powered from the spur cables 29 rather than requiring a separate power supply.

[0053] Alternative numbers of spur cable interfaces 28 may be provided in alternative embodiments. It is also possible for the spur cable interfaces 28 to be omitted entirely, and the Ethernet switch formed as part of one of the devices 26 instead. The single-pair Ethernet power switch 1a is connected to the trunk cable interface of the power-communication decoupler circuit 3a via a single-pair Ethernet trunk cable 2a, and the single-pair Ethernet power switch 1 b is connected to the trunk cable interface of the power-communication decoupler circuit 3b via a single-pair Ethernet trunk cable 2b. Each single-pair Ethernet power switch provides both power and data along the corresponding single-pair Ethernet trunk cable 2a or 2b, as will be apparent to those skilled in the art.

[0054] The single-pair Ethernet power switches are connected to communicate with one another via a ring completion path that is separate from the daisy chain. For example, the power switches may be connected by a ring completion link CL and / or by an Ethernet backbone 25. A short-circuit fault along the daisy chain will not prevent communications from reaching any of the Ethernet switches of the daisy chain since the communications can be routed via the ring completion path to bypass the short-circuit fault. For example, if a communication between the single-pair Ethernet power switch 1a and the Ethernet switch 6a fails due to a short-circuit fault on the communications cables 4a or 4c, then the communication will be re-routed to the Ethernet switch 6a via the ring completion path CL or 25, the single-pair Ethernet power switch 1 b and the communications cable 4b.

[0055] A short circuit fault on one of the communications cables 4a, 4b or 4c will not feed from the communications interfaces 20 through to the trunk cable interfaces 8, and subsequently risk disrupting the data at the trunk cable interfaces 8 or the power at the power output interfaces 12, in part due to the DC blocking circuit between the first transceivers 15 and the trunk cable interfaces 8. Similarly, a short-circuit at the trunk cable interface 8 will not negatively affect the communication interface 20, apart from the loss of communication with the trunk cable interface 8.

[0056] Accordingly, two of the power-communication decoupler circuits may be used to allow standard SPE power switches and standard network (e.g. field) switches to be used in a ring-redundancy solution for high integrity, high availability Ethernet-APL instrumented systems, without the need to connect the Ethernet switches to any local, redundant power source. The approach allows low power loss, long reach (up to 1 km), two-wire Type-A instrument / fieldbus / APL grade cables to be used for the dominant cabling infrastructure, which can use existing instrument / fieldbus cable typically found in process plants, without the need to install specialised Ethernet cable and / or repeaters. It also permits the use of lower cost standard Zone 2 or Safe Area Power and / or field (network) switches, which can be connected to IEC60079 2-WISE or FISCO instruments, for use in Zone 0, 1 and 2 areas. This will reduce the numbers of different types of Ethernet- APL components that are needed for spares-holding, when implementing ring redundancy alongside simplex solutions. The power-communication decoupler circuits can also be mounted within the same enclosure as the Ethernet switches, therefore only requiring short, standard, Ethernet Patch Cables / fibres to be used to connect each Ethernet switch, or to connect the power-communication decoupler circuits to the Ethernet switches.

[0057] Many other variations of the described embodiments falling within the scope of the invention will be apparent to those skilled in the art.

Claims

CLAIMS1 . A power-communication decoupler circuit for a ring-redundant network system that comprises one or more Ethernet switches, wherein the powercommunication decoupler circuit comprises: a trunk cable interface for connecting a powered single-pair Ethernet trunk cable; a power output interface for connecting one or more power supply cables to power the one or more Ethernet switches; a communications interface for connecting a communications cable to one of the Ethernet switches; a power supply connected between the trunk cable interface and the power output interface, the power supply comprising at least one series inductor; an internal switch connected between the trunk cable interface and the communications interface, wherein the internal switch comprises a first transceiver for communicating via the trunk cable interface and a second transceiver for communicating via the communications interface, the first transceiver being connected to the second transceiver for transferring communications therebetween.

2. The power-communication decoupler circuit of claim 1 , comprising at least one further communications interface for connecting at least one further communications cable to at least one further Ethernet switch, wherein the internal switch comprises at least one further transceiver connected to the first and second transceivers for transferring communications therebetween.

3. The power-communication decoupler circuit of claim or 1 or 2, wherein the power supply comprises a positive supply line and a negative supply line, the at least one series inductor comprising a positive side inductor connected in series within the positive supply line and / or a negative side inductor connected in series within the negative supply line.

4. The power-communication decoupler circuit of claim 3, wherein the power supply comprises at least one capacitor connected between the positive andnegative supply lines, at a point between the power output interface and the at least one series inductor.

5. The power-communication decoupler circuit of claim 3 or 4, wherein the power supply comprises at least one series rectifier diode between the power output interface and the at least one series inductor, the at least one series rectifier diode comprising a positive side rectifier diode connected in series within the positive supply line and / or a negative side rectifier diode connected in series within the negative supply line.

6. The power-communication decoupler circuit of any preceding claim, wherein the internal switch is a repeater, a switch ASIC or a back-to-back coupler.

7. The power-communication decoupler circuit of any preceding claim, wherein the power supply is configured to power the internal switch.

8. The power-communication decoupler circuit of any preceding claim, wherein the first transceiver is connected to the trunk cable interface via a DC blocking circuit.

9. The power-communication decoupler circuit of any preceding claim, wherein the trunk cable interface and first transceiver are in accordance with a first physical layer standard, for example, a physical layer standard for any powered single-pair Ethernet application, the first physical layer standard defining transmission of both power and data over the powered single-pair Ethernet trunk cable.

10. The power-communication decoupler circuit of any preceding claim, wherein the second transceiver and communications interface are in accordance with a second physical layer standard, for example the IEEE 802.3 physical layer standard for multi-pair copper or optical fibre, the second physical layer standard defining transmission of data, but not power, over the communications cable.11 . The power-communication decoupler circuit of any preceding claim, wherein the power output interface comprises two or more power output ports for connecting corresponding power cables to two or more Ethernet Switches.

12. A pair of the power-communication decoupler circuits of any preceding claim both located within a single housing or on a single circuit board.

13. A ring redundant network system comprising two single-pair Ethernet power switches, two of the power-communication decoupler circuits of any preceding claim, and one or more Ethernet switches that are connected in a daisy chain with a plurality of communications cables, wherein: each single-pair Ethernet power switch is connected to the trunk cable interface of a respective one of the power-communication decoupler circuits via one or more powered single-pair Ethernet trunk cables; the communications interface of each power-communication decoupler circuit is connected to a respective end of the daisy chain by a corresponding one of the communications cables; and the two single-pair Ethernet power switches are connected to communicate with one another via a ring completion path that is separate from the daisy chain.

14. The ring redundant network system of claim 13, wherein the one or more Ethernet switches are not powered via the plurality of communications cables forming the daisy chain.

15. The ring redundant network system of claim 13 or 14, wherein the one or more Ethernet switches comprise power supply inputs and wherein the power output interfaces of the power-communication decoupler circuits are connected to the power supply inputs of the one or more Ethernet switches via at least one power cable separate from the communications cables.

16. The ring redundant network system of claim 15, wherein the power supply input of each Ethernet Switch is connected to the power output interfaces of both of the power-communication decoupler circuits.

17. The ring redundant network system of claim 16, wherein the power supply input of each Ethernet switch comprises a first power input port connected to the power output interface of one of the power-communication decoupler circuits and a second power input port connected to the power output interface of the other of the power-communication decoupler circuits.

18. The ring redundant network system of claim 17, wherein the first and second power input ports of each Ethernet switch are connected to one another via two diodes arranged back-to-back in series, and wherein the Ethernet switch comprises a power supply that is connected between the back-to-back diodes.

19. The ring redundant network system of any one of claims 13 to 18 when appended to claim 11 or any claim dependent thereon, wherein the power supply input of each Ethernet switch is directly connected to a respective one of the power output ports of the power-communication decoupler circuits by a corresponding one of the power cables.

20. The ring redundant network system of any one of claims 13 to 19, wherein each Ethernet switch comprises at least one spur cable interface for connecting a powered single-pair Ethernet spur cable to a device.

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