Abstract:
The invention describes a field bus system, in particular a field bus system ( 10 ), comprising at least one clocked transmitter ( 16 ) and one clocked receiver ( 17 ) for transmitting data signals to another field bus device ( 30 ) or for receiving data signals from the other field bus device ( 30 ). To allow interfering emissions to be reduced, a spread spectrum clock ( 40 ) is provided which supplies a local spread spectrum clock signal (SST 1 ). The spread spectrum clock signal is sent to the transmitter ( 16 ) and the receiver ( 17 ) to allow data signals (DO 1 , DI 1 ) to be transmitted and received synchronously with the local spread spectrum clock signal.

Description:
FIELD OF INVENTION 
     The invention relates to a field bus system comprising a plurality of users, each having at least one transmitter and one receiver which respectively operate using spread spectrum clock signals. The invention further relates to a field bus device for use in such a field bus system. 
     BACKGROUND OF THE INVENTION 
     In field bus systems heretofore, conventional clock sources are used for the clock supply for data transmission. For field bus systems having rapid binary data transmission it is difficult to maintain the tolerance limits for interfering electromagnetic radiation. Avoidance of analog signal forms requires expensive parts in the field bus system. Interfering radiation is reduced by the use of spread spectrum technology, which is based on varying the frequency of a signal and thus obtaining a data or clock signal with a varying bit length. 
     The use of spread spectrum clock signals in field bus devices is known from U.S. Pat. No. 7,010,621 B2 for at least partially limiting emissions of interfering electromagnetic radiation originating from a local oscillator. However, the linkage of adjacent field bus devices by returning spread spectrum clock signals to the local field bus device is not provided. 
     A network controller is known from U.S. 2002/0112070 A1 which directs messages between a plurality of remote users of a field bus. The bit rate of the messages may be modified by the network controller without using clock signals. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a field bus system and a field bus device which are designed for rapid binary data transmission while avoiding the fairly intense interfering emissions expected for such an application. 
     According to the invention, a field bus system having a plurality of users is provided, each having least one clocked transmitter and one clocked receiver for transmitting data signals to a first adjacent user or for receiving data signals from the first user. In addition, a spread spectrum clock is assigned to each user for providing a user-specific spread spectrum clock signal which is sent to the transmitter and the receiver to allow data signals to be transmitted and received synchronously with this spread spectrum clock signal. 
     A core concept of the invention is to assign a user-specific spread spectrum clock signal to each user, also referred to as a field bus device, in the field bus system for transmitting data signals and clock signals, using spread spectrum technology, to an adjacent user in the field bus system, the adjacent user using the received spread spectrum clock signal for returning signals to the transmitting user. 
     It is practical to transmit the user-specific spread spectrum clock signal to the respective first adjacent user via a separate clock line or by means of the transmitted data signal. 
     By using information about the particular user-specific spread spectrum clock signal it is possible to recover received, spectrally modified data signals in the receiver of the particular user. When the user-specific spread spectrum clock signal is transmitted together with the spectrally modified data signal, this clock signal in the receiver of the particular user is available, and may be used for data decoding when the transmitted data of the adjacent user are received. 
     To enable a bidirectional clocked data transmission, at least one of the users has an additional clocked transmitter and an additional clocked receiver for transmitting data signals to a second adjacent user and for receiving data signals from the second adjacent user, the user-specific spread spectrum clock signal of the second user being sent to the additional transmitter and the additional receiver. 
     To allow the specific spread spectrum clock signal of the second adjacent user to be received via a separate clock line, the at least one user has a correspondingly designed interface. 
     If the specific spread spectrum clock signal of the second adjacent user is transmitted by means of the transmitted data signal, not via a separate clock line, the at least one user has a clock recovery circuit for recovering the user-specific spread spectrum clock signal from the data signal coming from the second user. 
     This organization in the field bus system enables only one specific spread spectrum clock signal to be provided for each user, thereby greatly reducing the technical complexity. 
     To allow data that is to be transmitted to be suitably coded, and coded received signals to be decoded, the transmitters each have one coder, and the receivers each have one decoder. 
     At this point it is noted that the spread spectrum clock may provide a spread spectrum clock signal whose frequency varies within a spread period, so that the frequency of the data signal that is to be transmitted changes in accordance with the particular user-specific spread spectrum clock signal. 
     To enable the data signal to be coded or decoded in a phase-stable manner, the spectrally modified data signal must be scanned at the correct time. To this end, each user may have at least one phase control circuit, in particular a phase-locked loop circuit, which compares the phase position of the data signals to be transmitted and received with the phase position of the respective spread spectrum clock signal. 
     It is practical for each user to have a programmable control device and/or a data processing unit. 
     Accordingly, a field bus device having at least one clocked transmitter and one clocked receiver is provided for transmitting data signals to another field bus device or for receiving data signals from the other field bus device. The field bus device also has a spread spectrum clock for providing a user-specific spread spectrum clock signal which is sent to the transmitter and the receiver to allow data signals to be transmitted and received synchronously with the user-specific spread spectrum clock signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in greater detail with reference to one exemplary embodiment. The figures show the following: 
         FIG. 1  shows a block diagram of a field bus system in the region of a user, and 
         FIG. 2  shows signal forms for various switching points in the sending and return of signals of the ring-shaped field bus system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows by way of example a section of a ring-shaped field bus system, having a user  10  and two users  20  and  30  adjacent thereto which are connected via a field bus. Of course, the field bus system may also have more than three users. In the illustrated example the field bus comprises one clock line  50 ,  55  each in addition to data lines  60 ,  66  and  70 ,  77  for bidirectional data transmission between users  10 ,  20 , and  30 . The users are also referred to as field bus devices. At least user  10 , situated between users  20  and  30 , has an incoming interface and an outgoing interface. The incoming interface contains an interface  18 , a receiver  14  which may have a decoder, and a transmitter  12  which may have a coder. A spread spectrum clock signal, which is locally provided in user  20  and therefore is referred to as user-specific, arrives at interface  18 . The data DO 2  which are sent by user  20  in rhythm with the specific spread spectrum clock signal thereof are transmitted to the receiver  14  via data line  66  and decoded. For this purpose the spread spectrum clock signal of the receiver  14  received at interface  18  is preferably supplied via a PLL circuit  13 . Data DI 2  intended for user  20  are transmitted by the transmitter, synchronously with the specific spread spectrum clock signal of user  20 , via data line  77  to user  20 . For this purpose the spread spectrum clock signal received at interface  18  is supplied to the coder for the transmitter  12 , preferably via the PLL circuit  13 . 
     The outgoing interface of user  10  has a transmitter  16  and a receiver  17 . The transmitter  16  preferably has a coder, whereas the receiver  17  preferably has a corresponding decoder. The user  10  also has a spread spectrum clock  40  which provides a spread spectrum clock signal SST 1 , which is specific for the local user  10 , for the outgoing interface. The (local) spread spectrum clock signal provided by the spread spectrum clock  40  is preferably supplied via a PLL circuit  15  to the transmitter  16 , the receiver  17 , and, according to the embodiment shown by way of example in  FIG. 1 , to an output  50  of user  10 . At this point it is noted that user  20  has at least one outgoing interface and a spread spectrum clock which are at least similar to the outgoing interface and the spread spectrum clock of user  10 . Subscriber  30  has at least one incoming interface which is at least similar to the incoming interface of user  10 . 
     Each user may be controlled and monitored by a programmable control device (not illustrated). Each user may have a data processing unit in a manner known as such. Such a data processing unit  11  is implemented in user  10 . This data processing unit is connected to transmitters  12  and  16  and to receivers  14  and  17 . 
     Transmitter  16  for user  10  transmits data DO 1 , synchronously with the spread spectrum clock signal SST 1 , to user  30  via data line  60 . Data DI 1  arriving from user  30  are received at receiver  17  for user  10  via data line  70 . The local spread spectrum clock signal SST 1  is transmitted to user  30  via clock line  50 . It is noted that data DI 1  have been coded by user  30  by means of the local spread spectrum clock signal SST 1  of user  10 , as explained in conjunction with the incoming interface for user  10 . 
     It is noted that the spread spectrum clock signal SST 1  does not have to be transmitted from user  10  to user  30  via clock line  50 . It is also possible for user  30 , the same as the other users  10  and  20 , to contain a clock recovery circuit (not illustrated) which recovers the spread spectrum clock signal SST 1  from data signal DO 1  which is received via data line  60  and modified with respect to its spectrum. 
     The operation of the field bus system is explained in greater detail below in conjunction with  FIGS. 1 and 2 . 
     It is assumed that user  10  intends to transmit data DO 1  to user  30 . It is further assumed that the spread spectrum clock  40  provides a spread spectrum clock signal SST 1  which is supplied to the transmitter  16  and to the receiver  17  via PLL circuit  15  and is transmitted to user  30  via clock line  50 . The data to be transmitted are supplied by the data processing unit  11 , for example, to the coder for the transmitter  16 . In response to the spread spectrum clock signal SST 1  the coder generates a correspondingly spectrally modified data signal which is transmitted to user  30  via data line  60 . 
       FIG. 2  shows an example of a modulation function f(clock O 1 ) which is used to modify the frequency of a clock signal in the spread spectrum clock  40  in order to generate the spread spectrum clock signal SST 1  (denoted by reference character O 1  in  FIG. 2 ). The variation of the modulation function over time is illustrated as a rising and falling straight line over three spread periods T, with the frequency deviation plotted on the ordinate axis. For low values of the modulation function f the frequency of the spread spectrum clock signal O 1  is low, and for high values of the modulation function f the clock frequency is higher. The relationships are depicted in an exaggerated manner in the drawing for purposes of illustration. The maximum frequency fmax or the minimum frequency fmin deviate only slightly from the mean frequency f 0 , for example by only approximately 0.2% greater or less than the mean frequency f 0 . This change in frequency results in different pulse lengths in the spread spectrum clock signal O 1 , and, correspondingly, different bit lengths in signals DO 1  and DI 1 . It is noted that data signal DO 1  corresponds to the output signal of transmitter  16 , whereas data signal DI 1  shown in  FIG. 2  corresponds to the input signal of receiver  17 . 
     To simplify the illustration, the spread spectrum clock signal O 1  matches data signal DO 1  in a 1:1 ratio. In practice, however, more rapid conversion is preferred. For example, transformation of a 50-MHz spread spectrum clock signal O 1  to a 200-MHz data signal DO 1  is achieved in practice. In addition, the period T of the modulation signal is illustrated in a greatly exaggerated manner. In one practical example the period T is 10 μs at a frequency of f 0 =100 MHz. 
     Once again the case is assumed for which user  10  receives data DO 2  from user  20  via data line  66 . At the same time, user  10  also receives the specific spread spectrum clock signal of user  20  via clock line  55  by means of which data signal DO 2  has been spectrally modified.  FIG. 2  also shows this spread spectrum clock signal O 2  as well as the associated modulation function f(clock O 2 ), likewise with rising and falling function values, but in a different phase position with respect to clock period T. The phase position of the specific spread spectrum clock signals O 1  and O 2  of users  10  and  20  may thus be differentiated. It is noted that the relative phase position of the local spread spectrum clock signals is random and may change; i.e., the clock signals are independently generated. As a result, data signal DO 2  is also spectrally modified in a different way than for data signal DO 1 . However, since user  10  has received the spread spectrum clock signal O 2  from user  20 , the received data signal DO 2  may be correctly decoded in the decoder of receiver  14  and sent to the data processing circuit  11 . It is noted that data signal DO 2  corresponds to the input signal of receiver  14 , whereas data signal DI 2  shown in  FIG. 2  corresponds to the output signal of transmitter  12 . 
     To enable transmission of data DI 2  to user  20  via data line  77 , user  10  uses the spread spectrum clock signal O 2  received from user  20  in the coder for transmitter  12 . Of course, user  20  knows its own specific spread spectrum clock signal and is therefore able to decode the received data signal DI 2 . 
     User  30  may transmit data to user  10  in a similar manner. For this purpose a spectrally modified data signal DI 1  is generated in a coder of user  30  in response to the specific spread spectrum clock signal of user  10 . The decoder for receiver  17  is then able to correctly decode the received, spectrally modified data signal DI 1 . 
     All variants have the advantage that interfering emissions are greatly reduced by use of a specific spread spectrum clock signal in each user, with economical generation of the spread spectrum clock signals.