Patent Publication Number: US-11383513-B2

Title: Liquid discharging apparatus, head control unit, and head unit

Description:
The present application is based on, and claims priority from JP Application Serial Number 2019-010398, filed Jan. 24, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a liquid discharging apparatus, a head control unit, and a head unit. 
     2. Related Art 
     In an ink jet printer as an example of a liquid discharging apparatus, a technique has been known which prints an image or a document on a medium by propagating a control signal, which is generated by a control circuit or the like provided on a main body of the ink jet printer, to a print head (printing head) which includes nozzles for discharging ink, and by controlling discharge timing of the ink based on the control signal. In the liquid discharging apparatus, the control signal supplied to the print head is propagated between a main body of the liquid discharging apparatus and the print head. 
     For example, JP-A-2008-183845 discloses a technique for controlling discharge timing of ink from nozzles included in a printing head mounted on a carriage by generating a printing data signal for controlling discharge of the ink in a printer control portion provided in a main body of a printer, converting the generated printing data signal into an optical signal, and propagating the optical signal from the printer control portion to the carriage. 
     The signal for controlling the discharge of the ink from the nozzles included in the print head is generated by performing various processes, such as a color conversion process, a half-tone process, an interlace process, and a nozzle complement process, with respect to image data input from an outside of the liquid discharging apparatus. In the various processes, the interlace process and the nozzle complement process are performed based on a signal corresponding to information of the print head. Therefore, as in JP-A-2008-183845, when the printing data signal generated in the main body of the liquid discharging apparatus is converted into the optical signal and the optical signal is propagated to the print head, it is necessary to convert the information of the print head into the optical signal and to propagate the optical signal from the print head to the main body of the liquid discharging apparatus. However, in optical communication in which the optical signal is propagated, conversion time is necessary for conversion from an electric signal into an optical signal and conversion from an optical signal to an electric signal. Therefore, there is a possibility that time is required for generation of the printing data, and thus there is room for improvement from a viewpoint of advancement of a control speed of the liquid discharging apparatus. 
     SUMMARY 
     According to an aspect of the present disclosure, there is provided a liquid discharging apparatus including a head unit that discharges a liquid from a nozzle, and a head control unit that controls an operation of the head unit, in which the head control unit includes a first conversion circuit that converts an image signal, which includes image data input from an outside, into a first electric signal, and a first photoelectric conversion circuit that converts the first electric signal into an optical signal, the head unit includes a second photoelectric conversion circuit that converts the optical signal into a second electric signal, a second conversion circuit that converts the second electric signal into a discharge control signal for controlling discharge of a liquid from the nozzle, and a liquid discharging head that includes a driving element, which is driven based on the discharge control signal, and that discharges a liquid from the nozzle in accordance with drive of the driving element, the first conversion circuit performs a first conversion process of converting the image signal into the first electric signal without depending on a discharge information of a liquid discharged from the liquid discharging head, and the second conversion circuit performs a second conversion process of converting the second electric signal into the discharge control signal by using the discharge information. 
     In the liquid discharging apparatus, the discharge information may include information which indicates whether or not to discharge a liquid from the nozzle. 
     In the liquid discharging apparatus, the first conversion process may include a color conversion process of converting color information corresponding to a hue of the image data included in the image signal into color information corresponding to a hue of a liquid discharged from the nozzle. 
     In the liquid discharging apparatus, the first conversion process may include a binarization process of converting the image signal into a signal which indicates whether or not a liquid corresponding to a pixel included in the image data is discharged. 
     In the liquid discharging apparatus, the first electric signal may be a signal acquired by performing the binarization process on a signal based on the image signal. 
     In the liquid discharging apparatus, the second conversion process may include a nozzle correspondence process of converting the second electric signal into a signal which indicates whether or not a liquid corresponding to the nozzle is discharged. 
     According to another aspect of the present disclosure, there is provided a head control unit, which controls an operation of a head unit that discharges a liquid from a nozzle, the head control unit including a first conversion circuit that converts an image signal, which includes image data input from an outside, into a first electric signal, and a first photoelectric conversion circuit that converts the first electric signal into an optical signal, in which the first conversion circuit performs a first conversion process of converting the image signal into the first electric signal without depending on a discharge information of a liquid discharged from the head unit. 
     According to another aspect of the present disclosure, there is provided a head unit, which discharges a liquid from a nozzle based on a signal input from a head control unit, the head unit including a second photoelectric conversion circuit that receives an optical signal input from the head control unit, and converts the optical signal into a second electric signal, a second conversion circuit that converts the second electric signal into a discharge control signal for controlling discharge of a liquid from the nozzle, and a liquid discharging head that includes a driving element, which is driven based on the discharge control signal, and that discharges a liquid from the nozzle in accordance with drive of the driving element, in which the second conversion circuit performs a second conversion process of converting the second electric signal into the discharge control signal by using a discharge information of a liquid discharged from the liquid discharging head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view illustrating a configuration of a liquid discharging apparatus. 
         FIG. 2  is a side view illustrating a peripheral configuration of a printing portion of the liquid discharging apparatus. 
         FIG. 3  is a front view illustrating the peripheral configuration of the printing portion of the liquid discharging apparatus. 
         FIG. 4  is a perspective view illustrating the peripheral configuration of the printing portion of the liquid discharging apparatus. 
         FIG. 5  is a block diagram illustrating an electrical configuration of the liquid discharging apparatus. 
         FIG. 6  is a diagram illustrating a configuration of an ink discharge surface. 
         FIG. 7  is a diagram illustrating a schematic configuration of one of a plurality of discharge portions. 
         FIG. 8  is a diagram illustrating examples of waveforms of driving signals COMA and COMB. 
         FIG. 9  is a diagram illustrating examples of waveforms of a driving signal VOUT. 
         FIG. 10  is a diagram illustrating a configuration of a driving signal selection circuit. 
         FIG. 11  is a table illustrating decoding content in a decoder. 
         FIG. 12  is a diagram illustrating a configuration of a selection circuit. 
         FIG. 13  is a diagram for illustrating an operation of the driving signal selection circuit. 
         FIG. 14  is a diagram illustrating configurations of a head control unit and a head unit. 
         FIG. 15  is a flowchart illustrating a conversion processing method for converting an image signal PDATA into a discharge control signal. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings. The used drawings are for convenience of description. The embodiments described below do not wrongfully limit the scope of the present disclosure as set forth in the claims. Further, not all of configurations described below are necessarily essential configuration requirements of the present disclosure. 
     1. Outline of Liquid Discharging Apparatus 
     A configuration of a liquid discharging apparatus  1  according to the present embodiment will be described with reference to  FIGS. 1 to 4 . 
       FIG. 1  is a side view illustrating a configuration of a liquid discharging apparatus  1 .  FIG. 2  is a side view illustrating a peripheral configuration of a printing portion  6  of the liquid discharging apparatus  1 .  FIG. 3  is a front view illustrating the peripheral configuration of the printing portion  6  of the liquid discharging apparatus  1 .  FIG. 4  is a perspective view illustrating the peripheral configuration of the printing portion  6  of the liquid discharging apparatus  1 . 
     As illustrated in  FIG. 1 , the liquid discharging apparatus  1  includes a delivery portion  3  that delivers a medium P, a support portion  4  that supports the medium P, a transport portion  5  that transports the medium P, the printing portion  6  that performs printing on the medium P, and a control portion  2  that controls these configurations. 
     In the following description, the width direction of the liquid discharging apparatus  1  is referred to as an X direction, the depth direction of the liquid discharging apparatus  1  is referred to as a Y direction, and the height direction of the liquid discharging apparatus  1  is referred to as a Z direction. Further, a direction in which the medium P is transported is referred to as a transport direction F. The X direction, the Y direction, and the Z direction are perpendicular to each other. Further, the transport direction F is a direction which intersects the X direction. 
     The control portion  2  is fixed to an inside of the liquid discharging apparatus  1  to generate various signals for controlling the liquid discharging apparatus  1  and to output the generated signals to corresponding various configurations. 
     The delivery portion  3  includes a holding member  31 . The holding member  31  rotatably holds a roll body  32  on which the medium P is wound and stacked. The holding member  31  holds different kinds of media P and roll bodies  32  having different dimensions in the X direction. Further, in the delivery portion  3 , as the roll body  32  is rotated in one direction, the medium P unwound from the roll body  32  is delivered to the support portion  4 . 
     The support portion  4  includes a first support portion  41 , a second support portion  42 , and a third support portion  43 , which constitute a transport path of the medium P from an upstream to a downstream in the transport direction F. The first support portion  41  guides the medium P delivered from the delivery portion  3  toward the second support portion  42 . The second support portion  42  supports the medium P on which printing is performed. Further, the third support portion  43  guides the printed medium P toward the downstream in the transport direction F. 
     The transport portion  5  includes a transport roller  52  that applies a transport force to the medium P, a driven roller  53  that presses the medium P against the transport roller  52 , and a rotary mechanism  51  that drives the transport roller  52 . 
     The transport roller  52  is disposed beneath the transport path of the medium P in the Z direction, and the driven roller  53  is disposed on the transport path of the medium P in the Z direction. The rotary mechanism  51  is configured with, for example, a motor and a reduction gear. Further, in the transport portion  5 , as the transport roller  52  rotates in a state in which the medium P is nipped by the transport roller  52  and the driven roller  53 , the medium P is transported in the transport direction F. 
     As illustrated in  FIGS. 2 to 4 , the printing portion  6  includes a carriage  71 , a guide member  62 , a movement mechanism  61 , and a heat dissipating case  81 . 
     The carriage  71  includes a carriage main body  72  and a carriage cover  73 , and is provided to reciprocate along the X direction in a state of facing the medium P. The carriage main body  72  forms an approximately L-shape when viewed from the X direction. The carriage cover  73  is detachably provided with respect to the carriage main body  72 . Further, an enclosed space is formed when the carriage cover  73  is attached to the carriage main body  72 . At a lower portion of the carriage main body  72 , five liquid discharging heads  400  are mounted at regular intervals in the X direction. Each of the liquid discharging heads  400  includes a lower end portion provided to protrude outward from a lower surface of the carriage main body  72 . A plurality of nozzles  651  for discharging the ink, as an example of a liquid, to the medium P are formed on a lower surface of the liquid discharging head  400 . 
     The guide member  62  extends along the X direction. Further, on the guide member  62 , the carriage  71  is supported to reciprocate along the X direction. Specifically, the guide member  62  includes a guide rail portion  63  extending from a lower portion of a front surface of the guide member  62  in the X direction. Further, the carriage  71  has a carriage support portion  64  at a lower portion of a rear surface of the carriage  71 . The carriage support portion  64  is supported to slide on the guide rail portion  63 . Therefore, the carriage  71  is coupled to reciprocate along the guide member  62 . 
     The movement mechanism  61  includes a motor and a reduction gear. Further, the movement mechanism  61  controls normal rotation and reverse rotation of the motor, and converts a rotational force of the motor into a moving force in the X direction of the carriage  71 . Therefore, the carriage  71  reciprocates along the X direction in a state in which the five liquid discharging heads  400 , five driving circuit boards  30 , and a discharge control circuit board  21  are mounted. Further, the movement mechanism  61  may adjust a position in the Z direction of the carriage  71  by controlling the motor and the reduction gear. Therefore, even when the medium P which has different thickness is used, it is possible to adjust a distance between the liquid discharging head  400  and the medium P, and thus it is possible to increase landing accuracy of the ink which lands on the medium P. 
     The heat dissipating case  81  has an approximately rectangular parallelepiped shape in which the discharge control circuit board  21  and the five driving circuit boards  30  are accommodated. A front end portion of the heat dissipating case  81  is fixed to an upper end portion of the rear portion of the carriage  71 . That is, the discharge control circuit board  21  and the five driving circuit boards  30  are mounted on the carriage  71  via the heat dissipating case  81 . 
     A connector  29  is provided on the discharge control circuit board  21 . A plurality of cables  82  for coupling the control portion  2  to the discharge control circuit board  21  are coupled to the connector  29 . That is, the cables  82  are provided between the discharge control circuit board  21 , which is mounted on the carriage  71  that reciprocates in the X direction, and the control portion  2 , which is fixed to the liquid discharging apparatus  1 , for communication, and the cables  82  propagate various signals. Further, the five driving circuit boards  30  are installed upward the discharge control circuit board  21  in the Z direction and are provided in parallel in the X direction. The discharge control circuit board  21  and each of the driving circuit boards  30  are connected through a connector  83  such as a Board to Board (B to B) connector. 
     Connectors  84  and  85  are provided at a front end portion of each of the five driving circuit boards  30 . Each of the connectors  84  and  85  is exposed from a front surface of the heat dissipating case  81 . One end of a cable  86  is coupled to the connector  84 , and one end of a cable  87  is coupled to the connector  85 . 
     Further, a connection board  74  is provided on an upper surface of each of the five liquid discharging heads  400 . The connection board  74  is electrically coupled to the liquid discharging head  400  via a connector  75  such as a B to B connector. Connectors  76  and  77  are provided on the connection board  74 . Another end of the cable  86  is coupled to the connector  76 , and another end of the cable  87  is coupled to the connector  77 . Therefore, the five driving circuit boards  30  and the five relevant liquid discharging heads  400  are electrically coupled to each other. 
     In the description with reference to  FIGS. 1 to 4 , description is performed such that the liquid discharging apparatus  1  includes the five driving circuit boards  30  and the five liquid discharging heads  400 . However, the number of driving circuit boards  30  and the number of liquid discharging heads  400  are not limited to five. 
     As above, in the liquid discharging apparatus  1 , the various signals, which are generated by the control portion  2  fixed to a main body of the liquid discharging apparatus  1 , are input to various configurations including the driving circuit board  30  and the liquid discharging head  400 , which are mounted on the carriage  71  provided to be reciprocated, via the cable  82 . Further, the carriage  71  reciprocates along the X direction, which is the scanning direction, under the control of the movement mechanism  61 , the medium P is transported along the transport direction F under the control of the rotary mechanism  51 , and the liquid discharging head  400  discharges the ink along the Z direction which is an ink discharge direction. Therefore, an image is formed on the medium P. 
     2. Electrical Configuration of Liquid Discharging Apparatus 
     Subsequently, an electrical configuration of the liquid discharging apparatus  1  will be described.  FIG. 5  is a block diagram illustrating the electrical configuration of the liquid discharging apparatus  1 . As illustrated in  FIG. 5 , the liquid discharging apparatus  1  includes a head control unit  10  and a head unit  20 . 
     The head control unit  10  includes a main control circuit  100  included in the above-described control portion  2 , and controls an operation of the head unit  20 . 
     The main control circuit  100  outputs, to the head unit  20 , a transmission signal Tx, which includes a signal acquired by performing various processes or the like on the image signal PDATA supplied from a not-shown host computer. Details of the processes performed on the image signal PDATA by the main control circuit  100  will be described later. 
     Further, the main control circuit  100  generates a control signal Ctrl-P for controlling transport of the medium P, and outputs the control signal Ctrl-P to the rotary mechanism  51 . The rotary mechanism  51  controls rotation of the above-described transport roller  52  by controlling the above-described motor and the reduction gear in accordance with the control signal Ctrl-P, and transports the medium P. Further, the main control circuit  100  generates a control signal Ctrl-C for controlling movement of the carriage  71 , and outputs the control signal Ctrl-C to the movement mechanism  61 . The movement mechanism  61  moves the carriage  71  by controlling the above-described motor, the reduction gear, and the like in accordance with the control signal Ctrl-C. 
     The head unit  20  causes the nozzles  651  to discharge the liquid. Specifically, the head unit  20  includes a discharge control circuit  200 , n driving signal output circuits  300  and n liquid discharging heads  400 . There are cases where the n driving signal output circuits  300  are respectively referred to as driving signal output circuits  300 - 1  to  300 - n  for discrimination, and the n liquid discharging heads  400  are respectively referred to as liquid discharging heads  400 - 1  to  400 - n  for discrimination. Further, description is performed such that a driving signal output circuit  300 - i  (i=any of 1 to n) is provided to correspond to a liquid discharging head  400 - i.    
     The discharge control circuit  200  is provided on the above-described discharge control circuit board  21 . Further, the discharge control circuit  200  generates printing data signals SI 1  to SIn, latch signals LAT 1  to LATn, change signals CH 1  to CHn, base driving signals dA 1  to dAn and dB 1  to dBn, and a clock signal SCK based on the transmission signal Tx, and outputs the generated signals to the relevant driving signal output circuits  300 - 1  to  300 - n . Further, the discharge control circuit  200  generates a reception signal Rx, which includes a signal indicative of reception of the transmission signal Tx input from the main control circuit  100 , and outputs the reception signal Rx to the main control circuit  100 . 
     Each of the driving signal output circuits  300 - 1  to  300   n  is provided on the above-described driving circuit board  30 . The driving signal output circuit  300 - 1  includes a first driving signal output circuit  310   a , a second driving signal output circuit  310   b , and a reference voltage signal output circuit  320 . The base driving signal dA 1 , which is a digital signal, is input to the first driving signal output circuit  310   a . The first driving signal output circuit  310   a  performs digital/analog signal conversion on the base driving signal dA 1 , generates a driving signal COMA 1  by performing class D amplification on the analog signal acquired through the digital/analog signal conversion, and outputs the driving signal COMA 1  to the liquid discharging head  400 - 1 . Further, the base driving signal dB 1 , which is the digital signal, is input to the second driving signal output circuit  310   b . The second driving signal output circuit  310   b  performs the digital/analog signal conversion on the base driving signal dB 1 , generates a driving signal COMB 1  by performing the class D amplification on the analog signal acquired through the digital/analog signal conversion, and outputs the driving signal COMB 1  to the liquid discharging head  400 - 1 . The first driving signal output circuit  310   a  and the second driving signal output circuit  310   b  may have the same configuration, and, for example, may include a class A amplification circuit, a class B amplification circuit, a class AB amplification circuit, or the like. 
     The reference voltage signal output circuit  320  generates a reference voltage signal VBS 1  indicative of a reference potential of the driving signals COMA 1  and COMB 1 , and outputs the reference voltage signal VBS 1  to the liquid discharging head  400 - 1 . For example, the reference voltage signal VBS 1  is a signal of a DC voltage having a voltage value of 6 V. 
     Further, the driving signal output circuit  300 - 1  propagates the printing data signal SI 1 , the latch signal LAT 1 , the change signal CH 1 , and the clock signal SCK, and outputs the printing data signal SI 1 , the latch signal LAT 1 , the change signal CH 1 , and the clock signal SCK to the liquid discharging head  400 - 1 . 
     The driving signal output circuits  300 - 1  to  300 - n  have the same configuration, and detailed description is not repeated. That is, the base driving signals dAi and dBi are input to the driving signal output circuit  300 - i . Further, the driving signal output circuit  300 - i  generates driving signals COMAi and COMBi and a reference voltage signal VBSi, and outputs the driving signals COMAi and COMBi and the reference voltage signal VBSi to the relevant liquid discharging head  400 - i . Further, the driving signal output circuit  300 - i  propagates a printing data signal SIi, a latch signal LATi, a change signal CHi, and the clock signal SCK, and outputs the printing data signal SIi, the latch signal LATi, the change signal CHi, and the clock signal SCK to the relevant liquid discharging head  400 - i.    
     The liquid discharging head  400 - 1  includes piezoelectric elements  60  which are examples of driving elements which are driven based on the driving signals COMA 1  and COMB 1 , and causes the nozzles  651  to discharge ink by driving the piezoelectric elements  60 . The liquid discharging head  400 - 1  includes a plurality of discharge modules  410 . Each of the plurality of discharge modules  410  includes a driving signal selection circuit  420  and a plurality of discharge portions  600 . 
     The driving signal selection circuit  420  includes, for example, an integrated circuit (IC) apparatus. The printing data signal SI 1 , the latch signal LAT 1 , the change signal CH 1 , the clock signal SCK, and the driving signals COMA 1  and COMB 1  are input to the driving signal selection circuit  420 . Further, the driving signal selection circuit  420  generates a driving signal VOUT by performing selection or non-selection in accordance with the printing data signal SI 1  on the input driving signals COMA 1  and COMB 1  at timing prescribed by using the latch signal LAT 1  and the change signal CH 1 . The driving signal VOUT generated by the driving signal selection circuit  420  is supplied to one end of the piezoelectric element  60  included in each of the plurality of discharge portions  600 . 
     Further, the reference voltage signal VBS 1  is supplied to another end of the piezoelectric element  60  included in each of the plurality of discharge portions  600  included in the liquid discharging head  400 - 1 . Further, the plurality of piezoelectric elements  60  are driven based on the driving signal VOUT and the reference voltage signal VBS 1 , and cause the amount of ink to be discharged in accordance with the drive of piezoelectric elements  60 . 
     Here, the liquid discharging heads  400 - 1  to  400 - n  have the same configuration. Specifically, the printing data signal SIi, the latch signal LATi, the change signal CHi, the clock signal SCK, and the driving signals COMAi and COMBi are input to the liquid discharging head  400 - i , and the driving signal VOUT is generated. Further, the generated driving signal VOUT is supplied to one end of the piezoelectric element  60  included in each of the plurality of discharge portions  600  included in the liquid discharging head  400 - i . Further, the reference voltage signal VBSi is supplied to one end of the piezoelectric element  60  included in each of the plurality of discharge portions  600  included in the liquid discharging head  400 - i . Further, the plurality of piezoelectric elements  60  are driven based on the driving signal VOUT and the reference voltage signal VBSi, and cause the ink, the amount of which corresponds to the drive of piezoelectric elements  60 , to be discharged. 
     3. Configuration and Operation of Liquid Discharging Head 
     Subsequently, a configuration and an operation of the liquid discharging head  400  will be described. When the configuration of the liquid discharging head  400  is described, description is performed while the printing data signal SIi, the latch signal LATi, the change signal CHi, the clock signal SCK, the driving signals COMAi and COMBi, and the reference voltage signal VBSi, which are supplied to the liquid discharging head  400 , are respectively referred to as a printing data signal SI, a latch signal LAT, a change signal CH, a clock signal SCK, driving signals COMA and COMB, and a reference voltage signal VBS. 
       FIG. 6  is a diagram illustrating a configuration of an ink discharge surface  650 , on which the plurality of nozzles  651  are formed, in the liquid discharging head  400 .  FIG. 7  is a diagram illustrating a schematic configuration of one of the plurality of discharge portions  600  included in the discharge module  410 . As illustrated in  FIGS. 6 and 7 , the liquid discharging head  400  includes the plurality of nozzles  651  for discharging the ink and the piezoelectric elements  60  corresponding to the respective nozzles  651 . 
     As illustrated in  FIG. 6 , four discharge modules  410  are disposed in zigzag in the liquid discharging head  400 . In each of the discharge modules  410 , the nozzles  651 , which are provided in parallel in the Y direction, are formed in two lines in the X direction. Further, a not-shown ink channel, which communicates with the nozzles  651 , is provided in the discharge module  410 . The number of discharge modules  410  included in the liquid discharging head  400  is not limited to four. 
     Further, as illustrated in  FIG. 7 , the discharge module  410  includes the discharge portion  600  and a reservoir  641 . The ink is introduced from an ink supply port  661  into the reservoir  641 . 
     The discharge portion  600  includes the piezoelectric element  60 , a diaphragm  621 , a cavity  631 , and the nozzle  651 . The diaphragm  621  is deformed in accordance with drive of the piezoelectric element  60  provided on an upper surface in  FIG. 7 . The diaphragm  621  functions as a diaphragm that enlarges/reduces an internal volume of the cavity  631 . The ink is filled in the cavity  631 . Further, the cavity  631  functions as a compression chamber, the internal volume of which changes in accordance with the displacement of the diaphragm  621  due to the drive of the piezoelectric element  60 . The nozzle  651  is an opening portion which is formed in a nozzle plate  632  and which communicates with the cavity  631 . The ink stored inside the cavity  631  is discharged from the nozzle  651  in accordance with the change in the internal volume of the cavity  631 . 
     The piezoelectric element  60  has a structure in which a piezoelectric body  601  is interposed between a pair of electrodes  611  and  612 . In the piezoelectric body  601  having this structure, central portions of the electrodes  611  and  612  and the diaphragm  621  are bent in a vertical direction of  FIG. 7  with respect to both end portions in accordance with a potential difference between the electrode  611  and the electrode  612 . Specifically, the driving signal VOUT is supplied to the electrode  611  which is the one end of the piezoelectric element  60 , and the reference voltage signal VBS is supplied to the electrode  612  which is the other end of the piezoelectric element  60 . Further, when the voltage of the driving signal VOUT decreases, the piezoelectric element  60  is driven such that a central portion is bent upward, and when the voltage of the driving signal VOUT increases, the piezoelectric element  60  is driven such that the central portion is bent downward. When the piezoelectric element  60  is bent upward, the diaphragm  621  performs displacement upward, and internal volume of the cavity  631  is enlarged. Therefore, the ink is drawn from the reservoir  641 . Further, when the piezoelectric element  60  is bent downward, the diaphragm  621  performs displacement downward, and internal volume of the cavity  631  is reduced. Therefore, the ink, the amount of which corresponds a degree of the reduction of the internal volume of the cavity  631 , is discharged from the nozzle  651 . As above, the liquid discharging head  400  includes the piezoelectric element  60 , and discharges the ink to the medium by driving the piezoelectric element  60 . The piezoelectric element  60  is not limited to the illustrated structure, and may have any structure that can discharge the ink in accordance with the displacement of the piezoelectric element  60 . Further, the piezoelectric element  60  is not limited to bending vibration, and may be configured to use longitudinal vibration. 
     Here, examples of waveforms of the driving signals COMA and COMB, which are the basis of the driving signal VOUT supplied to the piezoelectric element  60 , and examples of waveforms of the driving signal VOUT will be described. 
       FIG. 8  a diagram illustrating examples of the waveforms of driving signals COMA and COMB. As illustrated in  FIG. 8 , the driving signal COMA has a waveform in which a trapezoidal waveform Adp 1  disposed in a period T 1  from rise of the latch signal LAT to rise of the change signal CH and a trapezoidal waveform Adp 2  disposed in a period T 2  from the rise of the change signal CH to the rise of the latch signal LAT. Further, when the trapezoidal waveform Adp 1  is supplied to the one end of the piezoelectric element  60 , a small amount of ink is discharged from the discharge portion  600  corresponding to the corresponding piezoelectric element  60 . When the trapezoidal waveform Adp 2  is supplied to the one end of the piezoelectric element  60 , a middle amount of the ink, which is larger than the small amount, is discharged from the discharge portion  600  corresponding to the corresponding piezoelectric element  60 . 
     Further, the driving signal COMB has a waveform in which a trapezoidal waveform Bdp 1  disposed in the period T 1  and a trapezoidal waveform Bdp 2  disposed in the period T 2  are continuous. Further, when the trapezoidal waveform Bdp 1  is supplied to the one end of the piezoelectric element  60 , the ink is not discharged from the discharge portion  600  corresponding to the relevant piezoelectric element  60 . The trapezoidal waveform Bdp 1  is a waveform for finely vibrating the ink near a nozzle opening portion of the discharge portion  600  to prevent an increase in the viscosity of the ink. Further, when the trapezoidal waveform Bdp 2  is supplied to the one end of the piezoelectric element  60 , the small amount of the ink is discharged from the discharge portion  600  corresponding to the corresponding piezoelectric element  60 , which is like a case where the trapezoidal waveform Adp 1  is supplied. 
     Here, all voltages at start timings and termination timings of the trapezoidal waveforms Adp 1 , Adp 2 , Bdp 1 , and Bdp 2  are commonly a voltage Vc. That is, each of the trapezoidal waveforms Adp 1 , Adp 2 , Bdp 1 , and Bdp 2  is a waveform that starts at the voltage Vc and ends at the voltage Vc. Further, a period Ta including the period T 1  and the period T 2  corresponds to a printing period during which dots are formed on the medium P. 
     Although  FIG. 8  illustrates that the trapezoidal waveform Adp 1  and the trapezoidal waveform Bdp 2  have the same waveform, the trapezoidal waveform Adp 1  and the trapezoidal waveform Bdp 2  may have different waveforms. Further, in the following description, it is described that the small amount of the ink is discharged both when the trapezoidal waveform Adp 1  is supplied to the piezoelectric element  60  and when the trapezoidal waveform Bdp 2  is supplied to the piezoelectric element  60 . However, the present disclosure is not limited thereto. That is, the waveforms of the driving signals COMA and COMB are not limited to the waveforms illustrated in  FIG. 8 , and signals of combinations of various waveforms may be used in accordance with a moving speed of the carriage  71  on which the liquid discharging head  400  is mounted, properties of the discharged ink, and materials of the medium P. Further, the waveforms of the driving signals COMA and COMB supplied to each of the plurality of liquid discharging heads  400  may be different from each other. 
       FIG. 9  is a diagram illustrating examples of waveforms of the driving signal VOUT, corresponding to a “large dot”, a “middle dot”, and a “small dot” formed on the medium P and “non-recording”, respectively. 
     As illustrated in  FIG. 9 , the driving signal VOUT corresponding to the “large dot” has a waveform in which, in the period Ta, the trapezoidal waveform Adp 1  disposed in the period T 1  and the trapezoidal waveform Adp 2  disposed in the period T 2  are continuous. When the driving signal VOUT is supplied to the one end of the piezoelectric element  60 , in the period Ta, the small amount of the ink and the middle amount of the ink are discharged from the discharge portion  600  corresponding to the corresponding piezoelectric element  60 . Thus, the ink lands and is coalesced, so that the large dot is formed on the medium P. 
     The driving signal VOUT corresponding to the “middle dot” has a waveform in which the trapezoidal waveform Adp 1  disposed in the period T 1  and the trapezoidal waveform Bdp 2  disposed in the period T 2  are continuous in the period Ta. When the driving signal VOUT is supplied to the one end of the piezoelectric element  60 , the small amount of the ink is discharged twice from the discharge portion  600  corresponding to the corresponding piezoelectric element  60  in the period Ta. Thus, the ink lands and is coalesced, so that the middle dot is formed on the medium P. 
     The driving signal VOUT corresponding to the “small dot” has a waveform in which the trapezoidal waveform Adp 1  disposed in the period T 1  and a waveform that is disposed in the period T 2  and is constant at the voltage Vc are continuous in the period Ta. When the driving signal VOUT is supplied to the one end of the piezoelectric element  60 , in the period Ta, the small amount of the ink is discharged from the discharge portion  600  corresponding to the corresponding piezoelectric element  60 . Thus, the ink lands, so that the small dot is formed on the medium P. 
     The driving signal VOUT corresponding to the “non-recording” has a waveform in which the trapezoidal waveform Bdp 1  disposed in the period T 1  and a waveform that is disposed in the period T 2  and is constant at the voltage Vc are continuous in the period Ta. When the driving signal VOUT is supplied to the one end of the piezoelectric element  60 , in the period Ta, the ink near the nozzle opening portion of the discharge portion  600  corresponding to the corresponding piezoelectric element  60  slightly vibrates, so that the ink is not discharged. Thus, as the ink does not land, no dot is formed on the medium P. 
     Here, the waveform that is constant at the voltage Vc is a waveform in which when none of the trapezoidal waveforms Adp 1 , Adp 2 , Bdp 1 , and Bdp 2  is selected as the driving signal VOUT, the immediately preceding voltage Vc is maintained by a capacitive component of the piezoelectric element  60 . When none of the trapezoidal waveforms Adp 1 , Adp 2 , Bdp 1 , and Bdp 2  is selected as the driving signal VOUT, the voltage Vc as the driving signal VOUT is supplied to the piezoelectric element  60 . 
     Next, a configuration and an operation of the driving signal selection circuit  420  that selects the waveforms of the driving signals COMA and COMB and generates the driving signal VOUT will be described.  FIG. 10  is a diagram illustrating a configuration of the driving signal selection circuit  420 . As illustrated in  FIG. 10 , the driving signal selection circuit  420  includes the selection control circuit  430  and a plurality of selection circuits  440 . 
     The printing data signal SI, the latch signal LAT, the change signal CH, and the clock signal SCK are input to the selection control circuit  430 . Further, in the selection control circuit  430 , a set of a shift register (S/R)  432 , a latch circuit  434 , and a decoder  436  is provided to correspond to each of the plurality of discharge portions  600 . That is, the driving signal selection circuit  420  includes the sets of the shift registers  432 , the latch circuits  434 , and the decoders  436 , the number of which is the same as the total number m of the corresponding discharge portions  600 . 
     In detail, the printing data signal SI is a signal synchronized with the clock signal SCK, and is a signal having 2 m bits totally including 2-bit printing data [SIH, SIL] for selecting any one of the “large dot”, the “middle dot”, the “small dot”, and the “non-recording” with respect to each of the m discharge portions  600 . The printing data signal SI is held in the shift register  432  for each 2-bit printing data [SIH, SIL] included in the printing data signal SI, corresponding to the discharge portion  600 . In detail, the m stages of the shift registers  432  corresponding to the discharge portions  600  are cascade-coupled to each other, and the serially input printing data signal SI is sequentially transferred to the subsequent stage in accordance with the clock signal SCK.  FIG. 10 , in order to distinguish the shift registers  432 , the shift registers  432  are sequentially represented by a first stage, a second stage, . . . , an m-th stage from an upstream where the printing data signal SI is input. 
     The m latch circuits  434  latch the 2-bit printing data [SIH, SIL] held by the m shift registers  432  at rising of the latch signal LAT, respectively. 
       FIG. 11  is a diagram illustrating decoding contents in the decoder  436 . The decoder  436  outputs selection signals S 1  and S 2  in accordance with the latched 2-bit printing data [SIH, SIL]. For example, when the 2-bit printing data [SIH, SIL] is [1, 0], the decoder  436  outputs a logic level of the selection signal S 1  as levels H and L in the periods T 1  and T 2 , and outputs a logic level of the selection signal S 2  as levels L and H in the periods T 1  and T 2  to the selection circuit  440 . 
     The selection circuits  440  are provided to correspond to the respective discharge portions  600 . That is, the number of selection circuits  440  included in the driving signal selection circuit  420  is the same as the total number m of the relevant discharge portions  600 .  FIG. 12  is a diagram illustrating a configuration of the selection circuit  440  corresponding to one discharge portion  600 . As illustrated in  FIG. 12 , the selection circuit  440  includes inverters  442   a  and  442   b  which are NOT circuits, and transfer gates  444   a  and  444   b.    
     The selection signal S 1  is input to a positive control end not marked by a circle in the transfer gate  444   a , is logically inverted by the inverter  442   a , and is input to a negative control end marked by a circle in the transfer gate  444   a . Further, the driving signal COMA is supplied to an input end of the transfer gate  444   a . The selection signal S 2  is input to a positive control end not marked by a circle in the transfer gate  444   b , is logically inverted by the inverter  442   b , and is input to a negative control end marked by a circle in the transfer gate  444   b . Further, the driving signal COMB is supplied to an input end of the transfer gate  444   b . Further, output ends of the transfer gates  444   a  and  444   b  are commonly coupled to each other, and the driving signal VOUT is output. 
     Specifically, the transfer gate  444   a  conducts an input end and an output end when the selection signal S 1  is at the level H, and does not conduct the input end and the output end when the selection signal S 1  is at the level L. Further, the transfer gate  444   b  conducts the input end and an output end when the selection signal S 2  is at the level H, and does not conduct the input end and the output end when the selection signal S 2  is at the level L. As above, the selection circuit  440  selects the waveforms of the driving signals COMA and COMB based on the selection signals S 1  and S 2 , and outputs the driving signal VOUT. 
     Here, an operation of the driving signal selection circuit  420  will be described with reference to  FIG. 13 .  FIG. 13  is a diagram for illustrating the operation of the driving signal selection circuit  420 . The printing data signal SI is serially input in synchronization with the clock signal SCK, and is sequentially transferred in the shift registers  432  corresponding to the discharge portions  600 . Further, when the input of the clock signal SCK is stopped, the shift registers  432  hold the 2-bit printing data [SIH, SIL] corresponding to the discharge portions  600 , respectively. The printing data signal SI is input in an order corresponding to the discharge portions  600  of the m-th stage, . . . , the second stage, and the first stage of the shift registers  432 . 
     Further, when the latch signal LAT rises, the latch circuits  434  latch the 2-bit printing data [SIH, SIL] held in the shift registers  432  all at once, respectively. In  FIG. 13 , LT 1 , LT 2 , . . . , LTm indicate the 2-bit printing data [SIH, SIL] latched by the latch circuits  434  corresponding to the shift registers  432  of the first stage, the second stage, . . . , the m-th stage. 
     The decoder  436  outputs the logic levels of the selection signals S 1  and S 2  in the periods T 1  and T 2 , using contents illustrated in  FIG. 11 , in accordance with the size of a dot prescribed by the latched 2-bit printing data [SIH, SIL]. 
     Specifically, when the printing data [SIH, SIL] is [1, 1], the decoder  436  sets the selection signal S 1  to levels H and H in the periods T 1  and T 2 , and sets the selection signal S 2  to levels L and L in the periods T 1  and T 2 . In this case, the selection circuit  440  selects the trapezoidal waveform Adp 1  in the period T 1 , and selects the trapezoidal waveform Adp 2  in the period T 2 . As a result, the driving signal VOUT corresponding to the “large dot” illustrated in  FIG. 9  is generated. 
     Further, when the printing data [SIH, SIL] is [1, 0], the decoder  436  sets the selection signal S 1  to levels H and L in the periods T 1  and T 2 , and sets the selection signal S 2  to levels L and H in the periods T 1  and T 2 . In this case, the selection circuit  440  selects the trapezoidal waveform Adp 1  in the period T 1 , and selects the trapezoidal waveform Bdp 2  in the period T 2 . As a result, the driving signal VOUT corresponding to the “middle dot” illustrated in  FIG. 9  is generated. 
     Further, when the printing data [SIH, SIL] is [0, 1], the decoder  436  sets the selection signal S 1  to levels H and L in the periods T 1  and T 2 , and sets the selection signal S 2  to levels L and L in the periods T 1  and T 2 . In this case, the selection circuit  440  selects the trapezoidal waveform Adp 1  in the period T 1 , and selects neither the trapezoidal waveform Adp 2  nor the trapezoidal waveform Bdp 2  in the period T 2 . As a result, the driving signal VOUT corresponding to the “small dot” illustrated in  FIG. 9  is generated. 
     Further, when the printing data [SIH, SIL] is [0, 0], the decoder  436  sets the selection signal S 1  to levels L and L in the periods T 1  and T 2 , and sets the selection signal S 2  to levels H and L in the periods T 1  and T 2 . In this case, the selection circuit  440  selects the trapezoidal waveform Bdp 1  in the period T 1 , and selects neither the trapezoidal waveform Adp 2  nor the trapezoidal waveform Bdp 2  in the period T 2 . As a result, the driving signal VOUT corresponding to “non-recording” illustrated in  FIG. 9  is generated. 
     As above, the driving signal selection circuit  420  selects waveforms of the driving signals COMA and COMB based on the printing data signal SI, the latch signal LAT, the change signal CH, and the clock signal SCK, and outputs the driving signal VOUT. In other words, the driving signal selection circuit  420  controls supply of the driving signals COMA and COMB to the piezoelectric element  60 . 
     4. Details of Electrical Connection of Main Control Circuit and Discharge Control Circuit 
     Here, details of configurations of the head control unit  10  and the head unit  20  and details of signals propagated between the head control unit  10  and the head unit  20  will be described. 
       FIG. 14  is a diagram illustrating configurations of the head control unit  10  and the head unit  20 . As illustrated in  FIG. 14 , the head control unit  10  includes the main control circuit  100  which includes a conversion circuit  110  and a photoelectric conversion circuit  130 . Further, the head control unit  10  and the head unit  20  are connected through optical cables  170   a  and  170   b  in the cable  82  for communication. The optical cables  170   a  and  170   b  may be, for example, an optical fiber cable. 
     The conversion circuit  110  converts the image signal PDATA, which is supplied from a not-shown host computer or the like, into an image signal ePDATA 1  which is an electric signal. Further, the conversion circuit  110  outputs the image signal ePDATA 1  to the photoelectric conversion circuit  130 . Further, a response signal eREP 2  is input to the conversion circuit  110 . The response signal eREP 2  is a signal which indicates that the image signal ePDATA 1  output by the conversion circuit  110  is normally propagated to the relevant head unit  20 . Here, an image signal oPDATA corresponds to the transmission signal Tx illustrated in  FIG. 1 , and a response signal oREP corresponds to the reception signal Rx illustrated in  FIG. 1 . 
     The photoelectric conversion circuit  130  includes an E/O circuit  131  and an O/E circuit  132 . The E/O circuit  131  includes a light emitting element or the like, and converts the input electric signal into the optical signal. Specifically, the image signal ePDATA 1 , which is the electric signal, is input to the E/O circuit  131 . Further, the E/O circuit  131  converts the image signal ePDATA 1  into the image signal oPDATA which is the optical signal. Further, the O/E circuit  132  includes a light receiving element or the like, and converts the input optical signal into the electric signal. Specifically, the response signal oREP, which is the optical signal, is input to the O/E circuit  132 . Further, the O/E circuit  132  converts the response signal oREP into the response signal eREP 2  which is the electric signal. 
     Here, the conversion circuit  110 , which converts the image signal PDATA including image data input from the host computer provided on the outside of the liquid discharging apparatus  1  into the image signal ePDATA 1  that is the electric signal, is an example of a first conversion circuit, the image signal ePDATA 1  is an example of a first electric signal, and a process of converting the image signal PDATA into the image signal ePDATA 1  by the conversion circuit  110  is an example of a first conversion process. Further, the photoelectric conversion circuit  130 , which converts the image signal ePDATA 1  that is the electric signal into the image signal oPDATA that is the optical signal, is an example of a first photoelectric conversion circuit. 
     The head unit  20  includes a discharge control circuit  200  including a conversion circuit  210  and a photoelectric conversion circuit  230 . 
     The photoelectric conversion circuit  230  includes an O/E circuit  231  and an E/O circuit  232 . The O/E circuit  231  includes the light receiving element or the like, and converts the input optical signal into the electric signal. Specifically, the image signal oPDATA, which is the optical signal, is input to the O/E circuit  231 . Further, the O/E circuit  231  converts the image signal oPDATA into an image signal ePDATA 2  which is the electric signal. Further, the E/O circuit  232  includes a light emitting element or the like, and converts the input electric signal into the optical signal. Specifically, a response signal eREP 1 , which is the electric signal, is input to the E/O circuit  232 . Further, the E/O circuit  232  converts the response signal eREP 1  into the response signal oREP which is the optical signal. 
     Discharge information DI of the liquid discharging head  400  is input to the conversion circuit  210 . Further, the conversion circuit  210  converts the image signal ePDATA 2  into the printing data signal SI, the latch signal LAT, the change signal CH, the clock signal SCK, and the base driving signals dA and dB based on the discharge information DI. Further, the conversion circuit  210  generates the response signal eREP 1 , which indicates that the image signal ePDATA 2  is normally received, and outputs the response signal eREP 1  to the E/O circuit  232 . 
     Here, the photoelectric conversion circuit  230 , which converts the optical signal that is input from the head control unit  10  into the image signal ePDATA 2 , is an example of a second photoelectric conversion circuit, and the image signal ePDATA 2  is an example of a second electric signal. Further, the conversion circuit  210 , which converts the image signal ePDATA 2  that is the electric signal into the printing data signal SI, the latch signal LAT, the change signal CH, and the clock signal SCK, is an example of a second conversion circuit, at least one of the printing data signal SI, the latch signal LAT, the change signal CH, and the clock signal SCK is an example of a discharge control signal for controlling discharge of a liquid from the nozzles  651 , and a process for converting the image signal ePDATA 2  into the discharge control signal is an example of the second conversion process. 
     The image signal ePDATA 1  acquired before being converted into the optical signal in the main control circuit  100  may be the same signal as the image signal ePDATA 2  which is converted from the optical signal in the discharge control circuit  200 . Further, the response signal eREP 1  acquired before being converted into the optical signal in the discharge control circuit  200  may be the same signal as the response signal eREP 2  which is converted from the optical signal in the main control circuit  100 . 
     5. Generation of Discharge Control Signal 
     As described above, in the liquid discharging apparatus  1  of the exemplary embodiment, the image signal PDATA, which includes the image data input from the host computer, is converted into the discharge control signal corresponding to each of the nozzles  651  in a process of being propagated in the conversion circuit  110 , the photoelectric conversion circuit  130 , the photoelectric conversion circuit  230 , and the conversion circuit  210 , and is output from the discharge control circuit  200 . 
     Here, a conversion process of converting the image signal PDATA, which includes the image data input from the host computer, into the discharge control signal, which corresponds to each of the nozzles  651 , will be described with reference to  FIGS. 14 and 15 .  FIG. 15  is a flowchart illustrating a conversion processing method for converting the image signal PDATA into the discharge control signal. 
     In the liquid discharging apparatus  1  according to the exemplary embodiment, the conversion circuit  110  converts the image signal PDATA into the image signal ePDATA 1  without depending on the discharge information DI of the ink discharged from the liquid discharging head  400 , and the conversion circuit  210  converts the image signal ePDATA 2  into the discharge control signal by using the discharge information DI. 
     Specifically, as illustrated in  FIG. 15 , the image signal PDATA, which includes the image data, is input from the host computer to the main control circuit  100  of the liquid discharging apparatus  1  (step S 100 ). 
     Further, the image signal PDATA, which is input to the main control circuit  100 , is input to the conversion circuit  110 . Further, the conversion circuit  110  performs a color conversion process on the image signal PDATA (step S 110 ). The color conversion process is a process of converting color information corresponding to a hue of the image data included in the image signal PDATA into the color information corresponding to a hue of the liquid which is discharged from the nozzles  651 . For example, when the image data included in the image signal PDATA is realized by combining grayscale values of red, green, and blue, the color conversion process is a process of converting the image data into image data which is realized by combining grayscales of cyan, magenta, yellow, and black of a color of the ink used in the liquid discharging apparatus  1 . It is possible to rapidly perform the color conversion process by referring to a 3-dimensional numerical table called a color conversion table. The ink used in the liquid discharging apparatus  1  is not limited to the above described ink, and, for example, light cyan or light magenta may be included. 
     Next, the conversion circuit  110  performs a half-tone process with respect to the image signal PDATA on which the color conversion process is performed (step S 120 ). The half-tone process is a binarization process of converting the image signal PDATA into a signal which indicates whether or not the ink corresponding to a pixel included in the image data is discharged, and is a process of determining a position of the medium to which the ink is discharged in order to reproduce grayscale information of the input image signal PDATA. The binarization process may include a process of determining the amount of ink to be discharged to the medium, in addition to a process of determining whether or not the ink is discharged with respect to the-above described medium. Further, the half-tone process may be performed through, for example, dithering using a dither mask. 
     Further, the image signal PDATA, on which the color conversion process and the half-tone process are performed, is output as the image signal ePDATA 1  from the conversion circuit  110 . That is, the conversion circuit  110  outputs a signal, which is acquired by performing the binarization process on the image signal PDATA, as the image signal ePDATA 1 . Further, the image signal ePDATA 1  is supplied to the head unit  20  after being converted into the image signal oPDATA, which is the optical signal, in the photoelectric conversion circuit  130 . 
     Here, the above-described color conversion process and the half-tone process are performed with respect to the image signal PDATA in accordance with a predetermined arithmetic operation. That is, the color conversion process and the half-tone process are processes which do not rely on the discharge information DI of the ink discharged from the liquid discharging head  400 , and the conversion circuit  110  performs the process which does not rely on the discharge information DI. Further, the conversion circuit  110  may perform only the color conversion process and may output a signal, which is acquired by performing the color conversion process on the image signal PDATA, as the image signal ePDATA. However, as illustrated in the exemplary embodiment, it is preferable that the conversion circuit  110  performs the process up to the half-tone process, and outputs a signal, which is acquired by performing the color conversion process and the half-tone process on the image signal PDATA, as the image signal ePDATA 1 . As will be described later, the head unit  20  converts the image signal ePDATA 2  into the discharge control signal corresponding to the plurality of nozzles  651  included in the liquid discharging head  400 . Therefore, a load of the process performed in the head unit  20  becomes large, compared to the process performed by the head control unit  10 . As illustrated in the exemplary embodiment, when a larger number of processes, which are possible without using the discharge information DI, are performed in the head control unit  10 , it is possible to reduce the load of the process in the head unit  20 . 
     The image signal oPDATA supplied to the head unit  20  is input to the conversion circuit  210  after being converted into the image signal ePDATA 2 , which is the electric signal, in the photoelectric conversion circuit  230 . Further, the conversion circuit  210  performs a nozzle complement process with respect to the image signal ePDATA 2  (step S 130 ). When a nozzle  651  which is not capable of normally discharging the ink exists in the plurality of nozzles  651  included in the liquid discharging head  400 , the nozzle complement process is a process of complementing the ink to be originally discharged from the nozzle  651 , from which the ink is not discharged, by adjusting the discharge amount of the ink which is discharged from another nozzle  651  provided in the vicinity of the relevant nozzle  651 . Specifically, information, which indicates whether or not the liquid is discharged from the nozzles  651 , is input as the discharge information DI to the conversion circuit  210 . In other words, the discharge information DI includes information, which indicates whether or not the liquid is discharged from the nozzles  651 . Further, based on the discharge information DI, a process of determining whether or not the nozzle  651 , which is not capable of normally discharging the ink, exists is performed. Further, when the nozzle  651  from which the ink is not discharged exists, the process of adjusting the discharge amount of the ink discharged from another nozzle  651  provided in the vicinity of the relevant nozzle  651  is performed. 
     Here, as the discharge information DI which indicates whether or not the nozzle  651  which is not capable of normally discharging the ink exists, there are provided, for example, a method for detecting a waveform, which is generated after discharging the ink, of a residual vibration of the piezoelectric elements  60 , a method for causing ink drops discharged from the nozzles  651  to be irradiated with laser light and detecting a change in the amount of light of the laser light, and the like. 
     Further, the conversion circuit  210  performs an interlace process with respect to the image signal ePDATA 2  (step S 140 ). The interlace process is a process of performing conversion on pixel information included in the input image signal ePDATA 2  in order corresponding to the nozzles  651  included in the liquid discharging head  400 . For example, when the image data included in the input image signal ePDATA 2  is the pixel information arranged in a direction intersecting with respect to a transport direction of the medium P and columns of the nozzles included in the liquid discharging head  400  are provided to be parallel to the transport direction of the medium P, the interlace process includes a vertical conversion process of rearranging the pixel information included in the image signal ePDATA 2  in order corresponding to the nozzles  651 . Further, in the liquid discharging head  400  in which one pseudo nozzle column is formed by a plurality of nozzle columns, when the nozzles  651  provided in different nozzle columns are provided in positions, which overlap in a movement direction of the carriage  71 , the interlace process includes a process of correcting the discharge amount of the ink in each of the overlapping nozzles  651 . 
     Further, the conversion circuit  210  performs a pixel shift process with respect to the image signal ePDATA 2  (step S 150 ). The pixel shift process is a process of correcting discharge timing of the ink which is discharged from the nozzles  651  based on a difference between an ideal landing position of the medium P, which is desired to make the ink discharged from the nozzles  651  land, and a real landing position on which the ink actually lands. Specifically, the liquid discharging head  400  acquires information of the real landing position using image recognition using a camera or the like, hue recognition based on a color of an image formed on the medium, or the like. Further, the discharge information DI, which includes the difference between the real landing position and the ideal landing position, is input to the conversion circuit  210 . The conversion circuit  210  performs a process of correcting the discharge timing of the ink which is discharged from the nozzles  651  based on the input discharge information DI. 
     As above, the conversion circuit  210  converts the image signal ePDATA 2  from a signal corresponding to the pixel included in the image data into the signal corresponding to each of the nozzles  651 . In other words, the conversion circuit  210  includes a nozzle correspondence process of performing conversion into a signal which indicates whether or not the ink is discharged from the nozzles  651  in correspondence to the nozzles  651 . Steps S 130 , S 140 , and S 150  illustrated in  FIG. 15  may be performed in any order. 
     After the nozzle complement process, the interlace process, and the pixel shift process, which are described above, are performed, the conversion circuit  210  generates and outputs the printing data signal SI, the latch signal LAT, the change signal CH, and the clock signal SCK, as the discharge control signal (step S 160 ). 
     6. Effects 
     The liquid discharging apparatus  1  of the above-described exemplary embodiment includes: the head control unit  10  including the conversion circuit  110  that converts the image signal PDATA including the image data input from the outside into the image signal ePDATA, and the photoelectric conversion circuit  130  that converts the image signal ePDATA into the image signal oPDATA which is the optical signal; and the head unit  20  including the photoelectric conversion circuit  230  that converts the image signal oPDATA, which is the optical signal, into the image signal ePDATA 2 , the conversion circuit  210  that converts the image signal ePDATA into the discharge control signal for controlling the discharge of the liquid from the nozzles  651 , and the liquid discharging head  400  that discharges the liquid from the nozzles  651  by driving the piezoelectric elements  60  based on the discharge control signal. That is, in the liquid discharging apparatus  1 , the discharge control signal for controlling the discharge of the ink from the liquid discharging head  400  is propagated as the optical signal between the head control unit  10  and the head unit  20 . 
     Further, the conversion circuit  110  performs the process of converting the image signal PDATA into the image signal ePDATA 1  without depending on the discharge information DI of the liquid discharged from the liquid discharging head  400 , and the conversion circuit  210  performs the process of converting the image signal ePDATA 2  into the discharge control signal using the discharge information DI. That is, the conversion circuit  210 , which is included in the liquid head unit  20  that includes the discharging head  400 , performs a conversion process using the discharge information DI of the ink discharged from the liquid discharging head  400 , and the conversion circuit  110 , which is included in the head control unit  10  that does not include the liquid discharging head  400 , performs a conversion process which does not rely on the discharge information DI of the ink discharged from the liquid discharging head  400 . Therefore, it is not necessary to propagate the discharge information DI with respect to the conversion circuit  110  which performs the process that does not rely on the discharge information DI of the ink. Accordingly, it is not necessary to convert the discharge information DI of the ink into the optical signal in order to generate the discharge control signal for controlling drive of the piezoelectric elements  60 . Therefore, it is possible to reduce time which is required to generate the discharge control signal, and thus it is possible to improve a control speed of the liquid discharging apparatus  1 . 
     Although the exemplary embodiments and the modification examples have been described above, the present disclosure is not limited to these exemplary embodiments, and may be carried out in various modes without departing from the gist thereto. For example, the above-described embodiments can be combined appropriately. 
     The present disclosure includes substantially the same configuration (for example, a configuration having the same function, method, and result or a configuration of the same purpose and effect) as the configuration described in the exemplary embodiment. Further, the present disclosure includes configurations in which nonessential parts of the configurations described in the embodiments are replaced. Further, the present disclosure also includes configurations that have the same effects as those of the embodiments or configurations that can achieve the same objects as those of the embodiments. Further, the present disclosure includes a configuration obtained by adding a Known technique to the configurations described in the embodiments.