Abstract:
A station in a communication bus system is connected to a signal transmission line. The station contains a wave splitter coupled to the transmission line and a transmission section coupled to an input of the wave splitter for transmitting an outgoing wave signal to travel out over the transmission line from the wave splitter. The station contains a reception section coupled to an output of the wave splitter for receiving an incoming wave signal that travels into the wave splitter from the connector. The station has a control unit being arranged to operate in different control modes, according to the presence or absence of a further apparatus connected to the transmission line, dependent on whether the reception section does not detect or does detect a reflection of a wave transmitted by the transmission section, respectively.

Description:
FIELD OF TECHNOLOGY 
     The invention relates to a communication bus system in which a plurality of stations is connected by signal transmission lines to enable communication between the stations using a bus like the USB bus. 
     BACKGROUND AND SUMMARY 
     A USB system has connectors, which the user may use to connect and disconnect stations at will, even when the system is running. The system detects whether a station has been connected to a connector and, if so, it logically incorporates the connected station into the system, so that communication with the station becomes possible. The system also detects when a station has been disconnected and; if so, logically disincorporates the station from the system, so that no more communication with the station is performed or expected. This automatic incorporation and disincorporation is an important feature to make the system easy to use for non-specialist consumers. 
     In a USB system one station, for example a personal computer, is at the root of the bus structure. This station is the bus master. This station has one or more connectors, which can be used to connect the root to “downstream” stations, each via its own cable connected to a connector. In the USB system the downstream stations in turn may have connectors that may be used to connect them to further downstream stations and so on in a tree structure. The interconnected stations, which are thus directly or indirectly coupled to the root, together form the bus system. 
     It is not necessary that all connectors are permanently connected to downstream stations. The user of the system may connect or disconnect a downstream station or leave the connector unused as desired. The system detects whether or not a downstream station is connected to or disconnected from the connector and operates accordingly: no messages commands etc. are transmitted via connectors to which no downstream station is connected. In the USB system detection of the presence of a station is performed by means of current drawn through resistors. Each downstream station contains a resistor connected to the cable that connects the downstream station to the connector via which it is connected to the bus system. When a downstream station is connected to the bus, its resistor draws current via the cable. This current causes a change in the voltage on the cable and this change is detected by another “upstream” station to whose connector the downstream station is connected. 
     Normally, detection of disconnection takes a period of “silence” in the communication on the bus. In the case of the USB bus, a time-slot is reserved for this purpose, during which no transmission is possible. This may cause a delay in disconnection and reduced transmission capacity. Furthermore it is desirable to reduce the voltage swing on the cables, both for reasons of increasing the speed and for reducing the power consumption of the bus system. However, a reduction of the voltage swing makes the detection of the voltage change due to connection of a station harder to detect. 
     Amongst others, it is an object of the invention to reduce the time that the bus needs to reserve to detect connection and/or disconnection. 
     An apparatus according to the invention is set forth in claim  1 . According to the invention wave reflection is used to detect the presence or absence of stations coupled to the bus system. The time dependent voltages and currents in an electrical transmission line for example can be described by two vectors, each representing the phase and amplitude of a respective one of two traveling waves that travel through the transmission line in mutually opposite directions. At an end of the transmission line, where an impedance is connected to the transmission line, the ratio between the two wave vectors is equal to the reflection coefficient associated with the impedance. The reflection coefficient is zero when the impedance is equal to the transmission line impedance of the transmission line. As a result there will be a wave traveling in only one direction when an impedance equal to the characteristic impedance of the transmission line is connected to the transmission line. 
     According to the invention this effect is used to control the incorporation and especially disincorporation of downstream stations into and from the system. Stations are designed to apply the characteristic impedance of the transmission line to the transmission line. An upstream station in the system located at the other end of the transmission line transmits a wave signal via the transmission line and spits off the returning wave (if any) from the transmitted wave. If during transmission the amplitude of the returning wave is not substantially zero or substantially increases from zero, it is concluded that the transmission line is no longer terminated and therefore that a downstream station is disconnected from the transmission line. Thereupon the upstream station takes the appropriate actions to remove the downstream station logically from the system. Similarly, to enter a station logically into the system, if the amplitude of the returning wave is substantially zero, it is concluded that the transmission line is terminated and therefore that a downstream station is connected to the transmission line. Thereupon the upstream station takes the appropriate actions to incorporate the downstream station into the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     These and other advantageous aspects of the system, apparatus, device and method according to the invention will be described in more detail using the following figures, of which 
     FIG. 1 shows a topology of a bus communication system 
     FIG. 2 shows a station for use in a bus communication system 
     FIG. 3 shows an example of a wave splitter. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a topology of a bus communication system. The topology contains a master station  10 . The master station  10  has a number of connectors  12   a-g.  A number of slave stations  14   a-c  is connected to the connectors  12   a-e  via respective cables  16   a-c.  A number of connectors  12   d,e  is not connected to anything, other connectors  12   f,g  are connected to cables  16   d,e,  which, however, are not connected to any stations. A standard definition for the communication bus system prescribes the type of cable  16   a-e  and the characteristic impedance of the cable  16   a-e  that should be used, for example a symmetric cable with characteristic impedance 300 Ohm. The slave stations  14   a-c  contain termination impedances (not shown) that form terminations of the cables  16   a-c , each by its characteristic impedance. 
     Preferably, in case of slave stations  14   a-c  that need their own power supply independent of their connection to the bus system, or slave stations that are able to choose whether or not to operate as a slave station in the bus system, the termination impedances are dynamical impedances, which take their operational value only when power is supplied to the slave station  14   a-c , another impedance (like an open circuit) being applied to the cable  16   a-c  when the slave station  14   a-c  is not powered up or not prepared to operate as a slave station in the bus system. 
     FIG. 1 shows a “flat” topology, with only one master station  10  and a number of slave stations  14   a-c , but the invention applies equally well to more hierarchical topologies, in which the master station  10  in turn functions as a slave station of a higher part of the topology (not shown). 
     FIG. 2 shows a station  10  for use as master station in a bus system. The station  10  contains a processor  20 , a connector control circuit  22  and a wave splitter  24  for one of the connectors  12   a . The processor  10  is coupled to the connector control circuit  22  with an interface for sending information, for receiving information and for receiving a detection of connection or disconnection of a slave station to the connector  12   a . The connector control circuit  22  has an output coupled to an input of wave splitter  24  and an input connected to an output of wave splitter  24 . Wave splitter  24  has a transmission line output coupled to the connector  12   a . The master station  10  may contain further connector control circuits and further wave splitters connected between the processor  10  and respective ones of the other connectors  12   b-g.    
     In operation, processor  20  keeps a record indicating which of the connectors  12   a-g  are connected to slave stations  14   a-c . The slave stations  14   a-c  may be for example printers, cameras, storage devices or display devices etc. The processor  20  functions, amongst others, as a control unit for the bus. When a program running in the processor  20  needs a slave station  14   a-c  with a specific function, processor  20  checks whether such a station is connected and to which connector  12   a-g  it is connected. Thereupon commands and/or data may be transmitted between the master station  20  and the slave station  14   a-c  involved. Information that represents whether a station is connected may be kept for example in records in a status table in a memory (not shown) of the station, so that the processor  20  can consult the memory to determine whether or not a required station is connected. 
     When a connection is made between slave station  14   a-c  and one of the connectors  12   a-c , this is detected by the connector control circuit  22  and signaled to the processor  20 . In response, an initialization protocol is executed, wherein the processor  20  queries for example what type of function is available from the connected slave station  14   a-c , assigns a station identification to the slave station  14   a-c , initializes the slave station  14   a-c  and updates the record in the processor  20  to indicate the presence of the slave station  14   a-c  of a particular type with an assigned identification at the specific connector  12   a-g . The initialization protocol may follow for example the corresponding steps of incorporating a station in a USB bus system. 
     When connector control circuit  22  detects from the reflected wave that a slave station  14   a-c  is disconnected from one of the connectors  12   a-g  or one of the cables  16   a-e  connected to these connectors  12   a-g , the connector control circuit  22  also signals this to the processor  20 . In response, the processor  20  removes the slave station  14   a-c  from its record and terminates or throws exceptions to any processes that use the slave station  14   a-c.    
     The connector control circuit  22  transmits signals to the connector  12   a  via wave splitter  24  and detects connection and disconnection by comparing the amplitudes of wave signals transmitted and received by the wave splitter  24  to and from the connector  12   a . Thus, connector control circuit operates both as receiving and transmitting section for the connector  12   a,b.  On a cable with characteristic impedance R, for example, the voltage “V(x)” and current “I(x)” as a function of position “x” along the cable can be described by two wave vectors, A 1 , A 2 : 
     
       
           I ( x )=( A   1  exp( ikx )− A   2  exp(− ikx ))/ R   
       
     
     
       
           V ( x )= A   1  exp( ikx )+ A   2  exp(− ikx ) 
       
     
     When the cable is terminated with a passive impedance Z, their is a fixed ratio G(x) between the two wave vectors A 1 , A 2  at any position x along the cable. This ratio G is called the reflection coefficient, as it represents the extent to which the wave vector Al of the wave travelling in one direction results in a wave vector A 2  of a wave travelling in the opposite direction. When the cable is terminated with an impedance Z, the relation between the wave vectors at the point of termination “p” (x=p) is 
     
       
           A   2 = G ( p )* A   1   
       
     
     where the reflection coefficient G(p) is given by: 
     
       
           G ( p )=( Z−R )/( Z+R ) 
       
     
     Hence if the cable is open ended at the point of termination (Z infinite) G=1, if the cable is short circuited (Z=0) G=−1 and if the cable is terminated by the characteristic impedance R of the cable (Z=R) G=0. The reflection coefficients G(x 1 ), G(x 2 ) at different positions x 1 , x 2  along the cable differ from each other by a phase factor exp(−2ik(x 1 −x 2 )), which represents the phase shift due travel of the wave with wave vector Al from x 1  to x 2  and the phase shift of the wave with wave vector A 2  back from x 2  to x 1 . As a result, since the amplitude of exp(−2ik(x 1 −x 2 ) equals 1, the amplitude of the reflection coefficient G(x), remains constant along the cable. 
     Wave splitter  24  sends data and/or commands to the slave station  14   a . These data and or commands control one of the wave vectors A 1 , A 2  as a function of time. The other one of the wave vectors A 1 , A 2  is a result of reflection by whatever is present at the connector  12   a  or the end of the cable  16   a  connected to the connector  12   a , be it a slave station  14   a  or an open end. This other one of the wave vectors A 1 , A 2  is equal to a reflection coefficient G times the wave vector that is determined by the data and or commands. 
     Wave splitter  24  splits off this other one of the waves, that is, it determines its wave vector and it applies this wave vector to connector control circuit  22 . Connector control circuit  22  uses this reflected wave vector to decide whether a slave station  14   a  is connected to the connector  12   a  via the cable  16   a.    
     The slave station  14   a , if present, applies substantially the characteristic impedance R of the cable  16   a  to the end of the cable  16   a , so that the reflection coefficient G substantially equals zero if the slave station is connected to the cable  16   a . If no slave station  14   a  is connected to the connector  12   a  via the cable  16   a , the amplitude of the reflection coefficient is substantially 1. Hence, connector control circuit  22  decides that a slave station  14   a  is present if the ratio between amplitudes of the wave vectors A 1 , A 2  of the reflected wave and the transmitted wave is equal to or below a threshold value between 1 and 0. Connector control circuit  22  decides that no slave station  14   a  is present if the ratio is above the threshold. Under ideal conditions, the threshold value might be taken anywhere in the interval from 0 to 1, but preferably the threshold value is not in an interval close to 0 that corresponds to non-zero reflection coefficients due to spread of the impedance of the slave station  14   a  within a tolerance range, nor in an interval close to 1 that corresponds to possible losses in the cable  16   a  etc. Of course, when the wave splitter applies an outgoing wave with a fixed average amplitude to the cable  16   a , it is not necessary to determine the ratio between the wave vectors explicitly and the amplitude of the reflected wave may be compared with a threshold value. 
     The conventional method of detecting stations in the USB system may be used in addition to the present method. This is advantageous, because the conventional method provides for detection of more different states of connection, albeit at the expense of a slower response time. 
     FIG. 3 gives an example of a possible circuit implementation of wave splitter  24  for a cable  16   a  with symmetrical signal conductors. It will be clear that this circuit is but one example of many circuits for splitting off reflected waves known, per se, in the art. The circuit of FIG. 3 drives voltage and current on the connector  12   a  with a circuit that can be represented by an equivalent circuit that corresponds to a voltage source with output voltage “e” in series with an output impedance equal to the prescribed characteristic impedance of the cable  16   a . As a result, the output voltage “e”, the voltage V at the output of the wave splitter that is connected to the connector  12   a  and the current at that output are related by 
     
       
         
           V=e−IR 
         
       
     
     As a result, the reflected wave vector is determined by 
     
       
           A   2 = V−e/ 2 
       
     
     In the circuit shown in FIG. 3 contains a primary driver  30   a,b  that drives the voltage at the output that is connected to the connector  12   a . This primary driver  30   a,b  is driven by the data and/or commands that are to be sent to the slave station  14   a  (not shown). The circuit contains a secondary driver  32   a,b  that forms e/2, by loading its output twice as heavily as the primary driver  30   a,b  (in proportion to the relative drive powers of the primary and secondary driver  30   a,b ,  32   a,b;  this is realized for example by using an NMOS driver transistor and a PMOS load in both primary and secondary drivers, where a ratio between the W/L of the NMOS driver transistor and the W/L of the PMOS load transistor in the secondary driver is half the corresponding ratio in the primary driver). The circuit contains a subtraction circuit  34  that subtracts the output voltage of the secondary driver  32   a,b  from the output voltage of the primary driver  30   a,b . The output of the substraction circuit is proportional to the reflected wave vector A 2  and is supplied to the connector control circuit  22  (not shown) for detection of a connected slave station.