Broadcast serial bus termination

FIELD OF THE INVENTION

The invention relates to a broadcast serial bus termination circuit and to a subsea node comprising such circuit. Fur- thermore, a subsea broadcast serial bus system and a method of terminating a broadcast serial bus are provided.

BACKGROUND For enabling devices to communicate with each other, bus sys ^¬ tems are often used. A known system is for example the con ^¬ troller area network (CAN) bus which is frequently used in automotive applications, but also in industry automation. Several nodes that are capable of sending and receiving mes- sages are connected to the bus, such as controllers, sensors or actuators . A node connected to the CAN bus may for example comprise a processing unit processing e.g. sensor data, a CAN-controller which compiles messages to be transmitted on the bus or decodes received messages, and a transceiver which generates the electric signals by means of which the message is transmitted on the CAN bus.

Figure 1 illustrates a CAN bus 100 to which a number of N nodes 101, 102, ... are coupled. Each CAN node is locally pow- ered. The CAN bus 100 is to be operated with a predetermined termination impedance (R _T ) . To achieve this impedance on the bus, each node 101, 102, ... comprises a termination impedance 110 having a resistance value of N* R _T . If N nodes are con ^¬ nected to the bus, the total bus impedance will thus be R _T , as desired. A problem arises if nodes are to be added to or if nodes are to be removed from the bus . The bus impedance can change and an impedance mismatch on the CAN bus can result. This may be a particular problem in a subsea environment, in which the bus and the nodes connected thereto are inaccessi ^¬ ble. If a node is disconnected from the bus, e.g. due to failure of the node, the bus impedance will change, which may be detrimental to a reliable operation of the remaining nodes. Consequently, the communication between the different nodes may be disturbed and the whole system may need to be serviced, which is time and cost intensive.

It is thus desirable to enable the connecting/disconnecting of nodes to the bus without compromising the functionality of the bus. In particular, it is desirable to avoid impedance mismatches, which may result in signal reflections and may degrade system operability. SUMMARY

Accordingly, there is a need to obviate at least some of the drawbacks mentioned above and to improve the flexibility of a multidrop serial bus with respect to the number of connected nodes.

This need is met by the features of the independent claims. The dependent claims describe embodiments of the invention. An embodiment provides a broadcast serial bus termination circuit for a broadcast serial bus which is adapted to have a variable number of nodes connected thereto. The nodes can be connected in parallel to a first signal line and a second signal line of the broadcast serial bus, with each node con- necting the first signal line to the second signal line via a node impedance. The broadcast serial bus is to be operated with a predetermined bus impedance. The broadcast serial bus termination circuit comprises an adjustable impedance that is, in operation, connected between the first signal line and the second signal line of the broadcast serial bus. It fur ^¬ ther comprises an adjusting unit adapted to a just the ad ^¬ justable impedance. The adjusting unit is configured to en- able an adjustment of the adjustable impedance in dependence on the number of nodes connected to the broadcast serial bus. It is further configured to be capable of adjusting the ad ^¬ justable impedance such that for different numbers of nodes connected to the broadcast serial bus, the total impedance of the broadcast serial bus corresponds to the predetermined bus impedance .

If nodes are disconnected from or connected to the broadcast serial bus, the termination circuit may thus keep the total impedance of the broadcast serial bus substantially constant. If the node impedances are connected in parallel across the first and second signal lines of the broadcast serial bus, the removal of a node will increase the total bus impedance. Accordingly, the adjusting unit may be configured to decrease the value of the adjustable impedance in such case, so that the total impedance of the broadcast serial bus also de ^¬ creases, so that the predetermined bus impedance is reached. The termination circuit may thus make the connecting and dis- connecting of units from the broadcast serial bus more flexi ^¬ ble, which can be particularly advantageous in subsea environments where nodes of the broadcasts serial bus are not easily accessible. The adjustable impedance may be adjusted by setting a resistance value of the adjustable impedance. The resistance value may be controlled by the adjusting unit.

In an embodiment, the adjustable impedance is adapted to be adjustable to integer fractions of the node impedance. The node impedance of each node may for example be N x R _T , wherein N is the maximum number of connectable nodes, and wherein R _T is the predetermined bus impedance. The adjustable impedance may thus be adjustable to resistance values of an integer fraction of N x R _T . The fraction may for example be the number of disconnected nodes + 1 (i.e. if no node is dis ^¬ connected, the adjustable impedance is N x R _T ) . The termina ^¬ tion circuit may thus be capable of adjusting the actual bus impedance to the predetermined bus impedance for any number of connected nodes up to the maximum node number N. The ter ^¬ mination circuit can be part of a node having the adjustable impedance as a node impedance.

In an embodiment, the adjustable impedance comprises a volt- age controlled resistance. The resistance value of the ad ^¬ justable impedance may thus be controlled by applying a cor ^¬ responding control voltage. An automatic adjustment of the adjustable impedance thus becomes feasible, although it is possible to manually adjust the control voltage, or to preset the control voltage, e.g. by means of a software user inter ^¬ face .

The adjustable impedance may comprise a field effect transis ^¬ tor. In particular, it may comprise a JFET (junction gate field effect transistor) . The field effect transistor may be operated as a voltage controlled resistance. The adjustable impedance can thus be adjusted to different resistance values in a simple way. In a further embodiment, the adjustable im ^¬ pedance may comprise a resistor ladder. As mentioned above, the resistances in the resistance ladder may have steps that correspond to an integer fraction of the node impedance, such that the adjustable impedance is adjustable in the corre ^¬ sponding steps. The resistance ladder network may have switches that may be manually switched or may be electroni- cally switched. In an embodiment, the adjusting unit may com ^¬ prise a controller that is adapted to control the value of the adjustable impedance, in particular its resistance value. Such controller may for example provide a control voltage to a voltage controlled resistance, or may switch the electronic switches of a resistor ladder. This may enable an automatic control of the adjustable impedance, or a remote manual con ^¬ trol, e.g. via a software interface from a topside control center . The controller may for example comprise a micro controller. It may be part of a node comprising the broadcast serial bus termination circuit.

In an embodiment, the adjusting unit may comprise mechanical switches configured such that by actuation of the mechanical switches, different values of the adjustable impedance are selectable. It may for example comprise dip switches, by which the adjustable impedance can, staring at a maximum value, stepwise be reduced. By means of the mechanical switches, the total bus impedance may be configured for a particular number of connected nodes. The mechanical switches may for example be used for switching the resistances of a resistor ladder. Note that mechanical switches may also be electronically controlled by means of a controller, such as mechanical relays. In such a configuration, galvanic isola ^¬ tion maybe achieved, while enabling a precise selecting of the different resistance values of the adjustable impedance.

In an embodiment, the broadcast serial bus termination cir ^¬ cuit is configured to be capable of adjusting the total im ^¬ pedance of the broadcast serial bus to match the predeter ^¬ mined bus impedance for any number of nodes connected to the broadcast serial bus up to a predetermined maximum number (N) of nodes. The broadcast serial bus termination circuit may thus be constructed for a particular maximum node number, which can be selected high enough so as to an enable a flexi ^¬ ble broadcast serial bus configuration.

According to another embodiment, a subsea node for a broad ^¬ cast serial bus is provided, wherein broadcast serial bus has a variable number of nodes connected thereto, the nodes being connected in parallel to a first signal line and a second signal line of the broadcast serial bus. Each node connects the first signal line to the second signal line via a node impedance. The broadcast serial bus is to be operated with a predetermined bus impedance. The subsea node comprises a broadcast serial bus termination circuit according to any of the above mentioned configurations. The broadcast serial bus termination circuit is, in operation, connected to the first and second signal lines.

Such subsea node may enable a flexible and adjustable termi ^¬ nation of the broadcast serial bus. In particular, the subsea node may be a terminating node which terminates the broadcast serial bus. It may achieve similar advantageous as the ones outlined further above.

In an embodiment, the subsea node is adapted to automatically determine the number of further nodes connected to the broad ^¬ cast serial bus. It may further be configured to adjust the adjustable impedance by means of the adjusting unit in accor ^¬ dance with the determined number of connected nodes. Thus an automatic compensation of the total bus impedance for missing or disconnected nodes can be achieved. The subsea node may for example be a master node located in a subsea canister. The remaining nodes may be sensor nodes external to the sub ^¬ sea canister. In case of failure of one of these external nodes, the node maybe disconnected, which can be compensated by the subsea node. The external nodes may certainly also be provided in a subsea canister, e.g. of the equipment which makes use of them, or in their own canister.

In an embodiment, the subsea node and the further nodes con ^¬ nected to the broadcast serial bus are fed with power from the same power bus. The subsea node is adapted to measure the power consumption on the power bus in order to determine the number of further nodes connected to the broadcast serial bus. As an example, if nodes are disconnected, the power con ^¬ sumption will be reduced, which can be used by the subsea node to determine the reduced number of nodes connected to the broadcast serial bus. The nodes connected to the broad ^¬ cast serial bus may for example be of the same type (e.g. sensor nodes) and may thus consume the same amount of elec ^¬ tric power. The current on the power bus may then be measured by the subsea node in order to determine the power consump ^¬ tion and thus the number of connected nodes. If the number of connected nodes was determined, the subsea node may automati ^¬ cally set the adjustable impedance to a resistance value which ensures that the total bus impedance of the broadcast serial bus matches the predetermined bus impedance.

In a further embodiment, a subsea broadcast serial bus system is provided. The subsea broadcast serial bus system comprises a broadcast serial bus having a first signal line and a sec ^¬ ond signal line, one or more nodes connected in parallel to the first signal line and the second signal line of the broadcast serial bus, wherein each node connects the first signal line to the second signal line via a node impedance, and a subsea node in any of the configurations mentioned above, which is connected to the first signal line and the second signal line and terminates the broadcast serial bus.

Such subsea broadcast serial bus system is flexible with re- spect to the connection and disconnection of nodes. Furthermore, it may achieve any of the advantages outlined further above .

In an embodiment, the subsea broadcast serial bus system fur- ther comprises a subsea canister. The subsea node may be a master node located inside the subsea canister. The further nodes connected to the broadcast serial bus may sensor nodes located outside the subsea canister. In such configuration, further nodes may be connected to or may be removed from the broadcast serial bus, e.g. when installing new equipment or removing equipment from a subsea installation. The subsea canister comprising the master node may thus not need to be removed for adjusting the broadcast serial bus termination, as the adjustable impedance may be remotely set or may be automatically set.

In a further embodiment, the subsea broadcast serial bus sys ^¬ tem further comprises a power bus, wherein the subsea node and the further nodes connected to the broadcast serial bus are supplied with electric power by the power bus. Besides having a common electric reference, the subsea node may thus be enabled to automatically determine the number of nodes connected to the power bus, e.g. as mentioned above by meas ^¬ uring a current on the power bus. In the configuration in which the system comprises a subsea canister in which the subsea node is located, the power bus may be supplied with electric power inside the subsea canister. The power bus may for example be coupled inside the canister to a power supply network, or to a transformer or the like.

The broadcast serial bus may be a serial based multidrop bus, and it may in particular be a controller area network (CAN) bus or a Profibus.

A further embodiment provides a method for terminating a broadcast serial bus, the method comprising the steps of de ^¬ termining a number of nodes connected to the broadcast serial bus, adjusting an impedance of a node connected between a first signal line and a second signal line of the broadcast serial bus in accordance with the determined number of con ^¬ nected nodes, wherein the step of adjusting the impedance is performed such that the total impedance on the broadcast se ^¬ rial bus corresponds to a predetermined bus impedance at which the broadcast serial bus is to be operated.

With this method, advantages similar to the ones outlined further above may be achieved.

In embodiments, the method may make use of any of the fea ^¬ tures mentioned further above. The impedance may be adjusted automatically or by user input. It may for example be per ^¬ formed by an adjusting unit, which may be implemented as out- lined above. The method may be performed by the above men ^¬ tioned broadcast serial bus termination circuit, subsea node or subsea broadcast serial bus system. The features of the embodiments of the invention mentioned above and those yet to be explained below can be combined with each other unless noted to the contrary. BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing and other features and advantages of the inven ^¬ tion will become further apparent from the following detailed description read in conjunction with the accompanying drawings. In the drawings, like reference numerals refer to like elements .

Figure 1 illustrates a prior art CAN bus.

Figure 2 is a schematic diagram illustrating a subsea

broadcast serial bus system comprising a subsea node in accordance with an embodiment of the in ^¬ vention .

Figure 3 is a schematic diagram illustrating the implementation of an adjustable impedance in accordance with an embodiment of the invention.

Figure 4 is a schematic diagram illustrating the implementation of an adjustable impedance in accordance with a further embodiment of the invention.

Figure 5 is a schematic diagram illustrating a subsea node in accordance with an embodiment of the inven ^¬ tion.

Figure 6 is a schematic flow diagram illustrating a method of adjusting the termination impedance of a broadcast serial bus according to an embodiment of the invention.

DETAILED DESCRIPTION

In the following, embodiments of the invention will be described in detail with reference to the accompanying draw ^¬ ings. It is to be understood that the following description of the embodiments is given only for the purpose of illustra tion and is not to be taken in a limiting sense. It should be know that the drawings are to be regarded as be ^¬ ing schematic representations only, and that elements in the drawings are not necessarily to scale with each other.

Rather, the representation of the various elements is chosen such that their function and general purpose becomes apparent to a person skilled in the art. The skilled person will ap ^¬ preciate that the physical or functional units illustrated and described herein with respect to the different embodi- ments do not necessarily need to be implemented as physically separate units. One or more physical or functional blocks or units may be implemented in a common circuit, chip, circuit element or unit, while other physical or functional blocks or units may be implemented in separate circuits, chips, circuit elements or units.

It should be understood that while the following description may be given with reference to a controller area network (CAN) bus and may refer to sensor nodes connected thereto, the teachings of the disclosure are similarly applicable to other types of broadcast serial busses and other types of nodes connected thereto.

Figure 2 schematically illustrates a broadcast serial bus termination circuit 10 which is coupled to a broadcast serial bus 30, a CAN bus in the present example. The broadcast se ^¬ rial bus 30 has a first signal line 31 (CANH) and a second signal line 32 (CANL) . The broadcast serial bus termination circuit 10 comprises an adjustable impedance 11, which is connected between the first a second signal lines 31, 32 of the broadcast serial bus 30 in operation. The broadcast se ^¬ rial bus termination circuit further comprises an adjusting unit, which is not explicitly shown in figure 2, but will be explained later with respect to figures 3 and 4.

Figure 2 further illustrates a subsea node 21 according to an embodiment of the invention, which comprises the broadcast serial bus termination circuit 10. Subsea node 21 may be a master node, e.g. a node having a high priority, in particu ^¬ lar the highest priority of the nodes connected to bus 30. Subsea node 21 is coupled to the lines 36 and 37 of the power bus 35, from which it obtains the supply voltage required for operation.

Figure 2 further illustrates a subsea broadcast serial bus system 40 which comprises the subsea node 21, the broadcast serial bus 30, the power bus 35 and further nodes 22, 23, ... connected to the two busses 30, 35. This system may be part of a subsea based sensor network. Subsea node 21 can be a master node located in a subsea canister 50, wherein the re ^¬ maining nodes 22, 23, ... are external nodes, for example sub ^¬ sea sensor nodes. The power to each sensor node 22, 23, ... is supplied from the subsea canister 50 on the power bus 35. The nodes are connected in parallel to the broadcast serial bus 30, meaning that each node connects the two signal lines 31, 32, and that the removal of a node from bus 30 does not in ^¬ terrupt the connection between the remaining connected nodes.

A node 21, 22, 23 coupled to the broadcast serial bus 30 may have a configuration that may be described as three levels. On an application level, a microcontroller may communicate with a sensor or other equipment such a controller, an actua- tor or the like. The microcontroller can transmit commands or data to the equipment or may receive data or commands from the equipment, which are to be transported on the broadcast serial bus. The micro processor may hand down the user data that is to be send to a next level, which comprises a bus controller, e.g. a CAN controller. The bus controller may for example compile a frame or a message in accordance with to the protocol used for data transmission on the broadcast se ^¬ rial bus 30, it may further provide a check sum and may ini ^¬ tiate the sending of the frame/message. It may further decode received messages and provide the user data extracted from the received message to the microcontroller. On the lowest level, a transceiver may perform the actual physical generation of signals to be transmitted on signal lines 31, 32 of the broadcast serial bus 30. The transceiver may comprise a driver which generates the

CAN_H and CAN_L signals on the signal lines of the bus 30. It may further comprise a receiver, e.g. in form of a compara ^¬ tor, which reads voltage signals of the signal lines. To achieve high noise immunity, the broadcast serial bus 30 is maintained at a low differential bus impedance. This may be achieved by using a low value terminating resistance, which may have a value in a range of about 50 - 1000 Ohms, e.g. 100 or 120 Ohms. A dominant state may be set on the bus by the transceiver applying +5 volt to CAN_H and zero volt to CAN_L . A recessive state may be present on the bus if non of the transceivers of the nodes connected to the bus is assert ^¬ ing a dominant state. Both signal lines may then be at a po ^¬ tential of about 2.5 V, e.g. 2.6 V and 2.4 V for the CANH and CANL signal lines, respectively.

Note that the above values in the description of the opera ^¬ tion of the bus system are only given for the purpose of il ^¬ lustration and are not to be taken in a limiting sense.

Each node has a node impedance 13 in form of e.g. a termina ^¬ tion resistance which is connected across the bus lines 31, 32 when the node is connected thereto. To achieve a total bus impedance of R _T , the value of the termination resistance 13 in each node connected to the bus 30 can be set to N-R _T , wherein N is the total number of the nodes connected to a bus (including the master node 21 located in the subsea canis ^¬ ter) . N-l external sensor nodes 22, 23, ... can then be connected to the bus 30. If fewer than N-l external nodes are connected, the total bus impedance is raised, which may re ^¬ sult in a degraded operation of broadcast serial bus 30, in particular in a loss of signal quality, increased signal re ^¬ flections and there like. To enable a more flexible connection of nodes to the broad ^¬ cast serial bus 30 without degrading the quality of signal transmission on the bus, e.g. to enable the connection of a variable number of external nodes to the bus 30, the subsea node 21 comprises the termination circuit 10 in which an ad ^¬ justable impedance 11 (R _TSC ) is used. Adjustable impedance 11 may be implemented by a termination resistor having an adjustable resistance value. It is in particular adjustable in dependence on the number of connected external nodes 22, 23, .... In order to adjust the total impedance of broadcast serial bus 30 to the desired bus impedance, the resistance value of the adjustable impedance 11 can be determined as follows. The total impedance R _T on the broadcast serial bus 30 can be cal ^¬ culated as:

R,

wherein N is the maximum total number of nodes connected to broadcast serial bus 30 (including subsea node 21), n is the number of currently connected external nodes, and R _T sc is the resistance value of the adjustable impedance of subsea node 21 in subsea canister 50. As mentioned above, R _T sc should be set so that the desired bus impedance R _T is reached, e.g. R _T 100 Ohm or 120 Ohm.

Solving this equation, R _TSC can be determined to for any number of connected external (sensor) nodes between 1 and N-l . To reach the desired total bus impedance R _T , the adjustable impedance can be implemented with selectable resistance val ^¬ ues R _T sc for n = 1 to n = N-l, and the selection of the resis ^¬ tance value R _T sc can be done with respect to the currently connected number of nodes n. In such set up, the total imped ^¬ ance of broadcast serial bus 30 can be adjusted to have the desired value of R _T .

As an example, if the maximum number of nodes (N) is con- nected to broadcast serial bus 30, the adjustable impedance

11 will have resistance value of R _T sc = N * R _T , similar to all remaining nodes connected to bus 30. If nodes are now remove from the bus, the number n of external nodes connected to the bus decreases. According to equation 2, if one, two or three nodes are removed from broadcast serial bus 30, the resis ^¬ tance R _TSC can be determined to N * R _T /2, N * R _T /3 and N * R _T /4, respectively. As can be seen, the resistance value is an integer fraction of the node impedance N * R _T , wherein the denominator corresponds to the number of removed external nodes + 1.

In this respect it should be noted that when configuring the subsea broadcast serial bus system 40, N may be set to a num ^¬ ber higher than the number of external nodes that are actu- ally supposed to be installed, so that the possibility to in ^¬ stall further external nodes exists.

As can be seen, by making use of the broadcast serial bus termination circuit including the adjustable impedance 11, the total impedance on broadcast serial bus 30 can be main ^¬ tained constant even when external nodes 22, 23, ... are dis ^¬ connected from bus 30 or when additional nodes are connected to the bus . It should be clear that subsea node 21 and subsea broadcast serial bus system 40 which is schematically illustrated in figure 2 may comprise further components, or may be provided in different configurations. As an example, subsea node 21 may be provided with a body in which the adjustable resis ^¬ tances 11 are incorporated, and which may be provided with connectors for connecting to the broadcast serial bus 30. Further impedances, in particular resistors, can be provided, they may for example be switched parallel or in series with adjustable impedance 11.

Note that each node comprises two termination impedances. One impedance may for example be connected between the high sig- nal line (CAN_H) and the ground rail (0V) and the other be ^¬ tween the low signal line (CAN_L) and the high voltage rail (e.g. +5V) in operation. Note that this is just one possible configuration of the nodes 21, 22, 23, and that other configurations are also conceivable. Although illustrates as separate units for reasons of comprehensiveness, it should be clear that adjustable impedance 11 is part of subsea node 21, and that the node impedances 13 are part of the respective node 22, 23, ... . The detailed and general explanations given above also apply to the further examples described herein be- low.

Figures 3 and 4 illustrate schematically different configura ^¬ tions of the broadcast serial bus termination circuit 10 which may be implemented in the subsea node 21 of figure 2. In figure 3, the adjustable impedance 11 of the broadcast se ^¬ rial bus termination circuit 10 comprises a resistor ladder 15. The adjustable impedance 11 can be the terminating resis ^¬ tor for the high (H) or the low (L) signal line, and as indi ^¬ cated, it may thus be connected to either CAN_H or CAN_L . The adjustable impedance 11 comprises an adjusting unit 12, which in the present example is implemented as a number of switches using which the resistance value of the adjustable impedance 11 can be selected. The switches of the adjusting unit 12 may be mechanical switches or electronic switches. The resistors in the resis ^¬ tor ladder 15 can be configured such that by closing the switches of adjusting unit 12, the total resistance of the adjustable impedance 11 can be set in the above mentioned steps, e.g. to integer fractions of N * R _T . The resistors of the resistor ladder 15 may for example be configured to give a maximum resistance of NR _T , which can be set by closing the top switch.

The switches of adjusting unit 12 may be mechanical switch ^¬ ers, such as dip switches. In other configurations, they may be electronic switches. A micro controller may be provided in the subsea node, which controls the electronic switches. The micro controller may receive a command, e.g. over a communi ^¬ cation connection, to set the appropriate resistance, or a software running on the micro controller may set the resis ^¬ tance. The resistance value of the adjustable impedance 11 may thus be set by controlling the software or by sending a command from the remote location.

In the example of figure 4, the adjustable impedance 11 com ^¬ prises a voltage controlled resistance 17, which can be im- plemented as a field effect transistor (FET) in particular a junction gate field effect transistor (JFET) . By providing a corresponding control voltage at the gate of the voltage con ^¬ trolled resistance 17, the resistance value of the adjustable impedance 11 can be set. For this purpose, adjusting unit 12 may be provided, which may comprise a controller, such as a micro processor, digital signal processor or the like. The adjusting unit 12 provides the control voltage to the voltage controlled resistance 17, thereby adjusting the resistance value of the adjustable impedance 11 in the above outlined manner, e.g. in integer fraction of the node impedance NR _T . The controller or adjusting unit 12 may again use a precon- figured software to select the appropriate control voltage, or may receive a command from a remote location, which is for example input by a user, to just the control voltage.

In other configurations, the adjusting unit 12 may adjust the resistance value automatically, as will be explained with re ^¬ spect to figure 5. Figure 5 illustrates a possible implemen- tation of subsea node 21 in more detail, wherein the subsea node 21 can comprise a broadcast serial bus termination cir ^¬ cuit 10 having a configuration as outlined with respect to figure 2, figure 3 or figure 4. Subsea node 21 is connected to the power bus 35 and the broadcast serial bus 30. The ter ^¬ mination circuit 10 again comprises the adjustable impedances

11 and the adjusting unit 12, which is implemented as a con ^¬ troller adapted to automatically set the resistance value of each adjustable impedance 11. Adjustable impedance 11 can be a resistor ladder network having electronic switches, a volt ^¬ age controled resistance or there like. Adjusting unit 12 in ^¬ terfaces each adjustable impedance 11 to provide a corre ^¬ sponding control signal. Subsea node 21 further comprises a measuring unit 18. Measuring unit 18 interfaces the adjusting unit 12. By means of the measuring unit 18 the adjusting unit

12 determines the number of external nodes connected to the broadcast serial bus 30 and in accordance with the determined number, automatically sets the resistance value of the ad ^¬ justable impedance 11.

Each external node is supplied with power from the subsea canister 50 by the power bus 35. Using measuring unit 18, the power consumption of all connected nodes is measured by meas ^¬ uring the power consumption on the power bus 35. This is par- ticularly simple if all nodes connected to the power bus 35 are of the same type and thus have the same power consump ^¬ tion. Each node will then add the same current to the total current on the power bus 35. If each sensor uses a current I, than the total current on the power bus 35 will be m * I, with m being the total number of currently connected nodes

(including node 21) . The number of externally connected nodes n can thus be determined automatically, and the resistance value R _T sc of the adjustable impedance 11 can be set accord ^¬ ingly.

Measuring unit 18 may thus be configured so as to measure the current on power bus 35. Adjusting unit 12 may be preconfig- ured with the information on which current a single node re- quires, and may accordingly determined the number of con ^¬ nected nodes from the total current. It can set R _T sc to the appropriate value, e.g. as outlined above with respect to equation 2.

The full automatic adjustment of R _T sc thus becomes possible. Thereby, an automatic and dynamic termination of the broad ^¬ cast serial bus 30 can be realized, which results in the de ^¬ sired total bus impedance irrespective of the number of con- nected external nodes in a range of n = l to n = N - l.

Figure 6 is a flow diagram of a method according to an embodiment of the invention. The method may be performed by means of a broadcast serial bus termination circuit, subsea node and subsea broadcast serial bus system in any of the configurations outlined above. In step 61, a node, in par ^¬ ticular a sensor node is connected to or disconnected from the broadcast serial bus, in particular a serial based mul- tidropped bus. The current on the power bus 35 is measured in step 62. Based on the current measurement, the number of nodes connected to the broadcast serial bus is determined in step 63. In step 64 it is determined which termination imped ^¬ ance is required from subsea node 21 so that the total imped ^¬ ance of the broadcast serial bus matches the predetermined bus impedance. This determination can be based on the number of connected nodes and equation 2.

The adjustable impedance of the subsea node is then adjusted in step 65 to the determined required impedance in order to obtain the desired total bus impedance. This again may be achieved by providing a corresponding control signal to elec ^¬ tronic switches of a resistor ladder or by providing a control voltage to a voltage controlled resistor. Step 62 may be performed by measuring unit 18, while steps 63 to 65 may be performed by the adjusting unit 12, which can be implemented as a controller using a micro processor, a digi ^¬ tal signal processor, an ASIC, an FPGA or the like. With the above embodiments, a dynamic and adjustable termina ^¬ tion of the broadcast serial bus becomes possible. The broad ^¬ cast serial bus may in particular be a controller area net- work (CAN) bus, a Profibus or any other type of serial based multidrop bus. In particular it becomes possible to upgrade the number of nodes, in particular sensors, connected to an existing bus or network during its life cycle, while the de ^¬ sired total impedance on the broadcast serial bus can be achieved irrespective of the number of connected nodes. The maximum number of connecting nodes N for which the subsea node/the termination circuit can adjust the impedance of the broadcast serial bus can be selected before installation. By setting N to a higher number, it is possible to connect fur- ther nodes to the bus at a later stage, so that a future ex ^¬ pansion of the bus system, in particular a sensor network, is possible without the need to service the master node/bus ter ^¬ mination .