Abstract:
In a serial communication control circuit, serial data transmitted from a transmission processing unit is switched to data generated in a mark ratio improvement data generation unit depending on a switch signal from the transmission processing unit, and is transmitted. Thereby, mark ratio improvement data is inserted in a transmission signal to improve a mark ratio during communication, thereby preventing reception signal&#39;s jitters from increasing.

Description:
RELATED APPLICATIONS 
     The present application claims priority to Japanese Application Number 2013-098457, filed May 8, 2013, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a serial communication control circuit. 
     2. Description of the Related Art 
     Serial communication may be employed for transferring various items of data between a piece of industrial equipment such as working machine or robot and a control device for controlling the same. At this time, an encoding scheme such as 4B5B or 8B10B is employed as an encoding scheme for generating a data reproduction clock from received data. 
     4B5B represents 4-bit data in 5-bit symbol data, and is configured such that at least one 1 is present in 5-bit symbol and the same values are not consecutive for more than 4 bits together with NRZI. 8B10B is such that 8-bit data is divided into upper 3 bits and lower 5 bits, 3B4B conversion for converting 3-bit data into 4-bit symbol data is performed on the upper 3 bits, 5B6B conversion for converting 5-bit data into 6-bit symbol data is performed on the lower 5 bits, and the respectively resultant items of 4-bit and 6-bit data are combined to be 10-bit symbol data. Thereby, the same bits are consecutive for 5 bits or less so that spread of a signal frequency is restricted and stability is improved. 
     The data subjected to the encoding is designed such that a ratio of the numbers of 0s and is contained in a series of transfer data set is totally kept equal. In the following, the ratio of time occupied by “1” in the series of transfer data set may be denoted as mark ratio (%). A state in which the numbers of is and 0s contained in the series of data set are equal may be called DC balance. 
     10-bit symbol data in 8B10B may contain five 1s and five 0s, may contain six 1s and four 0s, or may contain four is and six 0s. 8B10B has a running disparity function in which a code to be next transmitted is selected such that the DC balance is kept in consideration of the accumulative numbers of previously transmitted 1s and 0s. However, 4B5B does not have such a function, and if the same transmission data continues, the differences in the numbers of 1s and 0s are accumulated and the DC balance cannot be perfectly kept. 
     JP 2012-54870 A discloses that when a reception circuit receives data with a high or low mark ratio, pulse output is consequently distorted as a result of controlling a threshold to be higher or lower, and data with a high or low mark ratio needs to be transmitted and received for a sufficiently long time in order to acquire deterioration determination information when the distortion of pulse output is the largest. 
     Serial communication may utilize an AC coupling, or capacitive coupling termination form in which capacitors are inserted in series on a data signal line in order to secure specifications&#39; compatibility or to enhance a noise margin. In the AC coupling, if the DC balance cannot be kept, an average level of the signal cannot be kept at a half voltage of the amplitude, and signal jitters may increase on the reception side. 
     With the technique disclosed in JP 2012-54870 A, a deteriorated site can be determined by phase data determination, but a mark ratio of data is not adjusted and thus reception signal&#39;s jitters may increase. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a serial communication circuit for improving a mark ratio during communication and for preventing reception signal&#39;s jitters from increasing. 
     A serial communication control circuit according to the present invention is directed for transmitting and receiving encoded data, and comprises a transmission processing unit for generating serial data to be transmitted, and a mark ratio improvement data generation unit for generating mark ratio improvement data in addition to the generated transmission data. Further, the serial communication control circuit includes a transmission data switch unit for switching and transmitting serial data transmitted from the transmission processing unit to data generated in the mark ratio improvement data generation unit when a switch signal is output from the transmission processing unit. 
     Thereby, mark ratio improvement data is inserted in a transmission signal thereby to improve the mark ratio during communication, thereby preventing reception signal&#39;s jitters from increasing. 
     The serial communication control circuit comprises a mark ratio detection unit for finding a mark ratio of transmission data output from the transmission data switch unit, wherein the mark ratio improvement data generation unit receives a mark ratio output from the mark ratio detection unit, and determines a mark ratio of mark ratio improvement data to be generated. 
     Thereby, a mark ratio of transmission data is detected and a mark ratio of mark ratio improvement data is determined depending on the detected mark ratio, thereby more effectively improving a mark ratio during communication and preventing reception signal&#39;s jitters from increasing. 
     The present invention has the above structure thereby to provide a serial communication circuit capable of improving a mark ratio during communication and preventing reception signal&#39;s jitters from increasing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will be apparent from the following description of embodiments with reference to the accompanying drawings. 
         FIG. 1  is a block diagram of a serial communication control circuit according to a first embodiment; 
         FIG. 2  is a diagram illustrating exemplary mark ratio improvement data according to the first embodiment; 
         FIG. 3  is a diagram illustrating an example in which mark ratio improvement data is inserted between forward packets according to the first embodiment; 
         FIG. 4  is a diagram illustrating improvements in a mark ratio by inserted mark ratio improvement data according to the first embodiment; 
         FIGS. 5A to 5C  are diagrams illustrating a mark ratio improvement data insertion method according to a second embodiment; 
         FIG. 6  is a block diagram of a serial communication control circuit according to a third embodiment; 
         FIG. 7  is a diagram illustrating exemplary mark ratio improvement data according to the third embodiment; and 
         FIG. 8  is a diagram illustrating improvements in a mark ratio by inserted mark ratio improvement data according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
       FIG. 1  is a block diagram of a serial communication control circuit according to a first embodiment. Numeral  12  denotes a transmission processing unit, and the transmission processing unit generates encoded communication transmission data (i.e., forward packets)  32  and a switch signal  36  described later, and transmits them to a transmission data switch unit  16 . Numeral  14  denotes a mark ratio improvement data generation unit, and the mark ratio improvement data generation unit generates mark ratio improvement data  34  and transmits it to the transmission data switch unit  16 . The transmission data switch unit  16  selects either the communication transmission data (i.e., forward packets)  32  from the transmission processing unit  12  or the mark ratio improvement data  34  from the mark ratio improvement data generation unit  14  depending on the switch signal  36  transmitted from the transmission processing unit  12 , and transmits it to a transmission unit  18 , and the transmission unit  18  receiving either data sends a signal to the outside via serial communication. 
       FIG. 2  illustrates exemplary mark ratio improvement data  34  according to the first embodiment. In the present embodiment, as illustrated in  FIG. 2 , the mark ratio improvement data  34  uses data with a mark ratio of 50% in which a disparity or a difference in the numbers of 1s and 0s in a code is 0 or neutral. 
       FIG. 3  illustrates an example in which the mark ratio improvement data  34  is inserted between the random-interval forward packets  32 . Control information such as data length or error correction code in addition to data is added to the forward packet  32 . The forward packet  32  is generated in the transmission processing unit  12 , and thus the transmission processing unit  12  can grasp in which time zone the forward packet  32  is transmitted or in which time zone the forward packet  32  is not transmitted. Thus, the serial communication control circuit transmits the switch signal  36  from the transmission processing unit  12  to the transmission data switch unit  16  and switches the data to be transmitted to the transmission data switch unit  16  between the communication transmission data, that is, forward packet  32  from the transmission processing unit  12  and the mark ratio improvement data  34  from the mark ratio improvement data generation unit  14  at transmission start and transmission end of the forward packet  32 . 
       FIG. 4  illustrates improvements in a mark ratio by inserted mark ratio improvement data  34  according to the present embodiment. The forward packet  32  in the upper example is 10 bits of 1101010101 and its mark ratio is 60%, but when 16-bit mark ratio improvement data  34  of 0101010101010101 is inserted between the forward packets  32 , a total mark ratio is 14/26 or 53.8%, and the mark ratio is improved as compared with 60% before the insertion. 
     The forward packet  32  in the lower example is 10 bits of 0101010100 and its mark ratio is 40%, but when 16-bit mark ratio improvement data  34  of 0101010101010101 is inserted between the forward packets  32 , a total mark ratio is 12/26 or 46.2%, and the mark ratio is improved as compared with 40% before the insertion. In this way, the mark ratio improvement data is transmitted depending on traffic in the serial transmission path, thereby preventing jitters from occurring on the reception side. 
     The examples illustrated in  FIG. 4  are merely exemplary insertion of the mark ratio improvement data  34 , and the forward packet  32  and the mark ratio improvement data  34  are not necessarily 10 bits and 16 bits, respectively. Actually, the data length of the forward packet  32  variably changes, and the length between the forward packets  32  is not constant. As many items of mark ratio improvement data  34  as possible are inserted between the forward packets  32 , thereby keeping even more the DC balance. 
     (Second Embodiment) 
     A second embodiment is similar to the first embodiment in the structure of the serial control communication control circuit and in that the mark ratio improvement data  34  to be inserted has a mark ratio of 50%, but is different therefrom in a method for inserting mark ratio improvement data between the forward packets  32 . 
       FIGS. 5A to 5C  illustrate a mark ratio improvement data insertion method according to the second embodiment, and illustrate an example in which mark ratio improvement data is inserted in an unnecessary part in a packet during periodical packet transfer. 
     In a communication protocol ensuring a real-time property, all the units have to be completely communicated within a period determined by a specification. For example, there is assumed a protocol that packets are periodically transferred to go round all the units in the cycle. In this case, when there is a unit which does not need to be transferred in a shortest cycle among the units or when the number of connected slaves is small relative to the maximum number of slaves, a vacancy occurs in any field in many cases. Mark ratio improvement data is inserted in the empty field while a protocol format is met. 
     As illustrated in  FIG. 5A , the forward packets  32  are periodically transmitted according to the present embodiment. As illustrated in  FIG. 5B , an individual forward packet  32  consists of a start sequence  42 , an address field  44 , a control field  46 , an information field  48 , a check sequence  50  and an end sequence  52 . 
       FIG. 5C  illustrates an exemplary internal structure of the information field  48 . As illustrated in  FIG. 5C , the information field  48  contains as many fields as corresponding to up to 10 slaves, but when the number of actually connected slaves is 5, only the shaded part in the information field is used and the rest is empty. There is described herein an example in which less slaves than the maximum number of slaves are connected in the information field  48 , but also when there is a unit which does not need to be transferred in a shortest cycle among the units, a vacancy occurs in the field. A vacancy occurs in not only the information field  48  but also the control field  46  or the check sequence  50  for the same reason, and thus the mark ratio improvement data  34  may be inserted in the empty part. 
     The forward packet  32  is generated in the transmission processing unit  12  which grasps empty fields, and thus the transmission processing unit  12  can propagate a switch signal to the transmission data switch unit while grasping empty fields. 
     In this way, it is possible to transmit mark ratio improvement data and to prevent jitters from occurring on the reception side depending on a vacancy of a field in a packet. 
     (Third Embodiment) 
       FIG. 6  is a block diagram of the serial communication control circuit according to a third embodiment. Numeral  12  denotes a transmission processing unit, and the transmission processing unit generates encoded communication transmission data (i.e., forward packets)  32  and a switch signal  36  described later, and transmits them to the transmission data switch unit  16 . Numeral  14  denotes a mark ratio improvement data generation unit, and the mark ratio improvement data generation unit receives a mark ratio data transmitted from a mark ratio detection unit  22  described later, and generates and transmits mark ratio improvement data  34  depending on the mark ratio to the transmission data switch unit  16 . The transmission data switch unit  16  selects either the communication transmission data (i.e., forward packets)  32  from the transmission processing unit  12  or the mark ratio improvement data  34  from the mark ratio improvement data generation unit  14  depending on the switch signal  36  transmitted from the transmission processing unit  12 , and transmits the data to the transmission unit  18 , and the transmission unit  18  receiving either data transmits the data to the outside via serial communication. 
     Numeral  22  denotes a mark ratio detection unit, and the mark ratio detection unit detects a mark ratio of the transmission data transmitted from the transmission data switch unit  16 . The mark ratio detection method includes counting 0s and 1s in a digital manner, or detecting them in an analog manner. A mark ratio detected by the mark ratio detection unit  22  is transmitted to the mark ratio improvement data generation unit  14 . Further, data transmitted from the outside via serial communication is received in a reception unit  20  and is transmitted to a reception processing unit  24 . 
     The serial communication control circuit according to the present embodiment is different from the first embodiment in that the mark ratio detection unit  22  for finding a mark ratio of encoded transmission data, and the mark ratio improvement data generation unit  14  for receiving a mark ratio output from the mark ratio detection unit  22 , determining a mark ratio of mark ratio improvement data  34  to be inserted, and outputting the mark ratio improvement data  34  are provided therein. 
     Like the aforementioned 4B5B code, also for a code non considering a running disparity, a running disparity can be calculated in the mark ratio improvement data generation unit and mark ratio improvement data for keeping the DC balance can be generated and transmitted, and thus a jitter suppression effect on the reception side can be expected. 
     The mark ratio improvement data  34  according to the present embodiment will be described with reference to  FIG. 7  and  FIG. 8 .  FIG. 7  illustrates exemplary mark ratio improvement data  34  in 8B10B. As illustrated in  FIG. 7 , three items of mark ratio improvement data  34  are employed in the present embodiment, which include data with a disparity of 0 or neutral and a mark ratio of 50%, data with a disparity of −2 and a mark ratio of 40%, and data with a disparity of +2 and a mark ratio of 60%. 
     The mark ratio improvement data generation unit  14  determines which mark ratio improvement data  34  is to be used depending on a value of the mark ratio transmitted from the mark ratio detection unit  22 . Specifically, the mark ratio improvement data generation unit  14  uses mark ratio improvement data  34  with a mark ratio of 40% when the mark ratio is 55% or more, or mark ratio improvement data  34  with a mark ratio of 60% when the mark ratio is 45% or less, or mark ratio improvement data  34  with a mark ratio of 50% when the mark ratio is between 45% and 55% thereby to select mark ratio improvement data  34  so as to eliminate a varied mark ratio. 
     For insertion of mark ratio improvement data  34  according to the present embodiment, the mark ratio improvement data  34  may be inserted between the random-interval transfer packets as in the first embodiment, or the mark ratio improvement data may be inserted in an unnecessary part in a packet during periodical packet transfer as in the second embodiment. 
       FIG. 8  illustrates improvements in a mark ratio by inserted mark ratio improvement data  34  according to the present embodiment. In the upper example of  FIG. 8 , the forward packet  32  is 10 bits of 1101010101 and its mark ratio is 60%, but when mark ratio improvement data  34  with 10 bits of 0101010100 and a mark ratio of 40% is inserted between the forward packets  32 , a total mark ratio is 10/20 or 50%. In the lower example of  FIG. 8 , the forward packet  32  is 10 bits of 0101010100 and its mark ratio is 40%, but when mark ratio improvement data  34  with 10 bits of 1101010101 and a mark ratio of 60% is inserted between the forward packets  32 , a total mark ratio is 10/20 or 50%. In this way, mark ratio improvement data is transmitted depending on traffic of the serial transmission path, thereby preventing jitters from occurring on the reception side. 
     The examples illustrated in  FIG. 8  are merely exemplary insertion of mark ratio improvement data  34 , and the forward packet  32  and the mark ratio improvement data  34  are not necessarily 10 bits, respectively. Actually, the data length of the forward packet  32  variably changes and the length between the forward packets  32  is not constant. As many items of mark ratio improvement data  34  as possible are inserted between the forward packets  32 , thereby keeping even more the DC balance.