Patent Publication Number: US-10308050-B2

Title: Printing apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a printing apparatus that includes a transport portion transporting a medium such as paper, a printing head performing printing on the transported medium, and a medium detection device capable of detecting a side edge of the medium in a width direction intersecting with a transport direction. 
     2. Related Art 
     In the related art, a printing apparatus that has a transport portion transporting a medium such as paper and a printing head performing printing on the medium has been widely known as one example of this type of printing apparatus (for example, JP-A-2001-287405 and the like). 
     For example, in JP-A-2001-287405, disclosed is a serial type printing apparatus (serial printer) that includes a paper width detection device (one example of a medium detection device) detecting the width of paper in a position upstream of a printing unit in the transport direction of paper (one example of a medium). This printing apparatus includes a carriage and two sensors (a right side detector and a left side detector). The carriage includes a printing head that moves by driving of a motor in a paper width direction (main-scanning direction) orthogonal with respect to the transport direction of the paper and performs printing. The two sensors are disposed in a position upstream of the printing head of the carriage in the transport direction. While the carriage moves in the paper width direction, the right side detector detects the right side edge of the paper, and the left side detector detects the left side edge of the paper. 
     The paper width detection device disclosed in JP-A-2001-287405 is disposed in the carriage that is movable in the main-scanning direction in order to perform printing in the serial printing type printing apparatus. Thus, printing apparatuses that do not include a printing carriage, for example, a line printing type printing apparatus (line printer), cannot employ a configuration in which a sensor is disposed in the printing carriage. In addition, in the case of separately attaching a device that can detect both side edges of a medium having the maximum width predetermined in the printing apparatus, a problem arises in that the size of the printing apparatus in the width direction may be increased. This type of problem is common to a serial printing type printing apparatuses as well in a case where a configuration in which a sensor is included in the printing carriage as in JP-A-2001-287405 is desired to be avoided in order to prevent a decrease in detection accuracy due to a sensor being stained with ink. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a printing apparatus that can detect both side edges of a medium in the width direction independently of printing types such as whether a printing head is movable or fixed, and that can have an apparatus size dimension in the width direction reduced to be comparatively small. 
     Hereinafter, means for solving the problems and operation effects thereof will be described. 
     According to an aspect of the invention, there is provided a printing apparatus including a transport portion that transports a medium, a printing head that performs printing on the medium, a medium detection device that is arranged upstream of the printing head in a transport direction of the medium and detects a side edge of the medium in a width direction intersecting with the transport direction, and a control unit that controls the transport portion, the printing head, and the medium detection device, in which the medium detection device includes a carriage that is movable in the width direction independently of the printing head in a position upstream of the printing head in the transport direction, two sensors that are disposed in different positions in the width direction in the carriage, and a source of motive power that moves the carriage, and the control unit renders the sensor to detect a side edge of the medium in the width direction by controlling the source of motive power to move the carriage. 
     According to this configuration, the two sensors are arranged in different positions in the width direction in the carriage of the medium detection device. Thus, using the two sensors for different uses when a side edge of the medium in the width direction is detected decreases the amount of movement of the carriage when a side edge is detected. Accordingly, both side edges of the medium in the width direction can be detected independently of printing types such as whether the printing head is movable or fixed, and the size dimension of the printing apparatus in the width direction can be reduced to be comparatively short even though the medium detection device is disposed. 
     In the printing apparatus, it is preferable that the carriage be disposed to be movable in the width direction in a position on the opposite side of a transport path of the medium transported by the transport portion from the printing head side, and the two sensors be optical sensors that irradiate the medium with light from a position on the opposite side of the transport path from the printing head side. 
     In this case, the carriage moves in the width direction in a position on the opposite side of the transport path of the medium from the printing head side, and thereby the two sensors configured of optical sensors irradiate the medium with light from a position on the opposite side of the transport path from the printing head side, and a side edge of the medium is detected. Since the two sensors move in a position on the opposite side of the medium from the printing head, ink from the printing head is unlikely to cling compared with a case where the two sensors are arranged on the same side of the medium as the printing head. Thus, a decrease in the accuracy of detection of the sensor due to staining with ink is easily avoided. 
     In the printing apparatus, it is preferable that the medium detection device include a casing that accommodates the carriage and the sensor, the casing include a medium support unit that supports the medium transported along the transport path, and the medium support unit include a window portion that is capable of transmitting light from the two sensors. 
     In this case, the medium support unit that includes the window portion capable of transmitting light from the sensor is disposed in the casing of the medium detection device between the medium transported along the transport path and a moving path of the sensor. Thus, the sensor can be protected from dust such as paper dust from the medium, ink mist from the printing head, and the like, and a side edge of the medium can be detected by the sensor through the window portion. In addition, since the window portion constitutes a part of the medium support unit, the distance between the sensor and the medium can be maintained approximately constantly. From this point as well, the accuracy of detection of the sensor can be highly maintained. 
     In the printing apparatus, it is preferable that the window portion be disposed in the medium support unit in plural numbers in the width direction. 
     In this case, the window portion is arranged in plural numbers in the width direction in a part of the medium support unit corresponding to a moving area of the sensor. Thus, comparatively high strength can be secured for the medium support unit compared with a configuration in which one long window portion is disposed across the moving area. 
     In the printing apparatus, it is preferable that a plurality of the window portions be disposed in a position where the two sensors are capable of detecting both side edges of the medium having the minimum width to the maximum width through the different window portions. 
     In this case, both side edges of the medium having the minimum width to the maximum width can be detected by the two sensors through different window portions. For example, both side edges of the medium having the minimum width to the maximum width can be continuously detected. 
     In the printing apparatus, it is preferable that a source of motive power of the medium detection device be a stepping motor. 
     In this case, since the source of motive power is a stepping motor, an encoder and the like required in the case of using a direct current motor (DC motor) is not required. Thus, the number of components of the medium detection device can be reduced to be small compared with the case of using a direct current motor. For example, it is easy to realize a small device size for the medium detection device. 
     In the printing apparatus, it is preferable that given that the width direction in which the carriage is movable is a left-right direction, in a case where the medium is a medium having the maximum width, when the carriage is in a left side end position in a movable range, the right side sensor of the two sensors sense the medium having the maximum width, and the left side sensor do not sense the medium having the maximum width, and meanwhile, when the carriage is in a right side end position in the movable range, the left side sensor of the two sensors sense the medium having the maximum width, and the right side sensor do not sense the medium having the maximum width. 
     In this case, when the medium has the maximum width, only the left side sensor is separated to the outside from the medium in the width direction when the carriage is in the left side end position, and only the right side sensor is separated to the outside from the medium in the width direction when the carriage is in the right side end position. That is, since the movable range of the carriage between the left and right end positions is relatively shorter than the width of the medium having the maximum width, the size dimension in the width direction of the printing apparatus in which the medium detection device is disposed is reduced to be comparatively short. In addition, in a case where the medium is a medium having the maximum width, both side edges of the medium having the maximum width can be detected by detecting the left side edge of the medium by using the left side sensor and detecting the right side edge of the medium by using the right side sensor. 
     In the printing apparatus, it is preferable that in a case where the width direction in which the carriage moves is a left-right direction, the control unit control the source of motive power to move the carriage in the width direction and thereby render the left side sensor to detect the left side edge of the medium and render the right side sensor to detect the right side edge of the medium. 
     In this case, the left side edge of the medium is detected by the left side sensor, and the right side edge of the medium is detected by the right side sensor. Accordingly, the distance of movement required for the carriage when both side edges of the medium in the width direction are detected is relatively short. Thus, the size dimension of the medium detection device in the width direction can be relatively small. For example, the size dimension of the printing apparatus in the width direction being relatively increased can be avoided even though the medium detection device is disposed, and the amount of time required for acquiring medium information related to the width direction of the medium can be reduced to be relatively small. 
     It is preferable that the printing apparatus further include a width information acquiring unit that acquires width information of the medium, in which the control unit, in a case where the width of the medium based on the width information is longer than a set width, controls the source of motive power in such a manner that the left side sensor detects the left side edge of the medium and that the right side sensor detects the right side edge of the medium. 
     In this case, in a case where the width based on the width information is longer than the set width, the left side edge of the medium is detected by the left side sensor, and the right side edge of the medium is detected by the right side sensor. Accordingly, the distance of movement required for the carriage when both side edges of the medium are detected is relatively short. Consequently, the size dimension of the medium detection device in the width direction can be small, and the amount of time required for acquiring the medium information can be reduced to be relatively small. 
     In the printing apparatus, it is preferable that the sensor be a light reflective type sensor, and a medium guide member that guides the medium along the transport path be arranged in a position facing a moving path of the sensor with the transport path of the medium transported by the transport portion interposed between the position and the moving path of the sensor, and a part of the medium guide member facing the moving path of the sensor be a light reflective surface. 
     In this case, a part of the medium guide member facing the moving path of the sensor is a light reflective surface. Thus, since a member dedicated for a light reflective surface is not required to be separately disposed, the medium guiding structure of the transport portion can be configured to be comparatively compact. 
     In the printing apparatus, it is preferable that the medium guide member be made of metal. 
     In this case, the light reflective surface can be comparatively simply formed by performing processing such as polishing or plating on a part of the medium guide member configured of metal that faces the moving path of the sensor. 
     In the printing apparatus, it is preferable that the control unit, when the transport portion does not transport the medium, regard the sensor as being normal if a detected signal of the sensor has a detected value when the medium is not present, and regard the sensor as failing if the detected signal has a detected value when the medium is present. 
     In this case, if, when the medium is not transported, the sensor does not receive reflective light from the light reflective surface and has a detected value when the medium is present, the sensor is regarded as failing. Accordingly, a problem that a wrong side edge of the medium based on a detected signal of the failing sensor is detected can be avoided to the greatest possible extent. 
     In the printing apparatus, it is preferable that the control unit, in a case where one of the two sensors fails, detect both side edges of the medium by using the other sensor. 
     In this case, since both side edges of the medium are detected by the other sensor in a case where one sensor fails, both side edge positions of the medium can be acquired even if one sensor fails. 
     It is preferable that the printing apparatus further include a width information acquiring unit that acquires the width information of the medium, in which the control unit, in a case where one sensor of the two sensors fails, detects both side edges of the medium by using the other sensor if the width of the medium based on the width information is less than or equal to the set width. 
     In this case, in a case where one sensor fails, both side edges of the medium are detected by the other sensor if the width of the medium based on the width information is less than or equal to the set width. Accordingly, even if one sensor fails, both side edge positions of a medium having a width less than or equal to the set width can be acquired. 
     In the printing apparatus, it is preferable that the control unit, in a case where one sensor of the two sensors fails, if the width of the medium based on the width information is greater than the set width, detect one side edge of both side edges of the medium by using the other sensor and estimate the position of the other side edge based on the result of detection of the one side edge and the width information. 
     In this case, in a case where one sensor fails, if the width of the medium based on the width information is greater than the set width, one side edge of both side edges of the medium is detected by the other sensor, and the position of the other side edge is estimated based on the result of detection of the one side edge and the width information. Accordingly, even for a medium having a width that is great to the extent that both side edges cannot be detected by the other sensor when one sensor fails, both side edge positions thereof can be acquired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective view illustrating a printing apparatus in one embodiment. 
         FIG. 2  is a schematic sectional view of the printing apparatus. 
         FIG. 3  is a perspective view of the printing apparatus with a cover opened. 
         FIG. 4  is a perspective view illustrating a main body side feed mechanism unit and a cover side feed mechanism unit with the cover opened. 
         FIG. 5  is a side sectional view illustrating a feed mechanism. 
         FIG. 6  is a side sectional view illustrating a part of the feed mechanism. 
         FIG. 7  is a side sectional view illustrating the main body side feed mechanism unit and the cover side feed mechanism unit with the cover opened. 
         FIG. 8  is a perspective view illustrating a medium detection device and a surrounding part thereof. 
         FIG. 9  is a perspective view illustrating the medium detection device. 
         FIG. 10  is an exploded perspective view illustrating the medium detection device. 
         FIG. 11  is a plan view illustrating the medium detection device with a cover thereof detached. 
         FIG. 12  is a perspective view of the medium detection device seen from a rear surface side thereof. 
         FIG. 13  is a plan view illustrating a carriage in a home position. 
         FIG. 14  is a sectional view taken along a line XIV-XIV in  FIG. 13  and illustrating a main portion of the medium detection device. 
         FIG. 15  is a sectional view taken along a line XV-XV in  FIG. 13  and illustrating a main portion of the medium detection device. 
         FIG. 16  is a block diagram illustrating an electrical configuration of the printing apparatus. 
         FIG. 17  is a schematic plan view illustrating a medium side edge detection process in the medium detection device. 
         FIG. 18  is a signal waveform diagram of each sensor illustrating a side edge detection process for a medium of a small size. 
         FIG. 19  is a signal waveform diagram of each sensor illustrating a side edge detection process for a medium of a large size. 
         FIG. 20  is a flowchart illustrating a medium side edge detection process routine. 
         FIG. 21  is a flowchart illustrating a medium side edge detection process routine at the time of failure. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, one embodiment of a printing apparatus will be described with reference to the drawings. 
     As illustrated in  FIG. 1 , a printing apparatus  11  is configured of a multifunction peripheral that includes a printer unit  12  and a scanner unit  13  and, as a whole, has a shape of an approximately rectangular parallelepiped which is long in a vertical direction Z. The printer unit  12  has a printing function of performing printing on a medium P such as paper. The scanner unit  13  is arranged on the upper side of the printer unit  12 . An operation panel  14  is disposed in a position adjacent to (in  FIG. 1 , in front of) the scanner unit  13  in the upper surface portion of the printer unit  12 . The operation panel  14  includes a display unit  15  that has, for example, a touch panel function. In the present example, a touch panel constitutes one example of an operation unit  16 . The operation unit  16  may be an operation switch. 
     As illustrated in  FIG. 1 , the scanner unit  13  includes a scanner main body portion  17  and a document stand cover  18 . The scanner main body portion  17  has an upper surface configured as a document stand not illustrated. The document stand cover  18  is disposed to be openable and closable with respect to the upper surface (document stand glass surface) of the scanner main body portion  17 . An automatic document feed unit  19  (auto document feeder (ADF)) is mounted on the upper portion of the document stand cover  18 . 
     The scanner unit  13  includes a reading unit, not illustrated, that reads a document which is set on document stand glass, not illustrated, by opening the document stand cover  18 . The automatic document feed unit  19  feeds a plural sheets of documents set on a mount  19 A one sheet at a time in order, and documents after scanned by the reading unit are discharged onto a stack portion  19 B in order. The printing apparatus  11  configured as above includes a main body  20  and the document stand cover  18 . The main body  20  has a shape of an approximately rectangular parallelepiped and includes the above document stand and the operation panel  14  in the upper surface portion thereof. 
     As illustrated in  FIG. 1 , in the lower portion of the printer unit  12 , a cassette  21  that can accommodate a plural sheets of the medium P mounted in a stack is inserted in the main body  20  in plural numbers in the up-down direction to be insertable and withdrawable from the front. In addition, a feed tray  22  is disposed to be openable and closable about the lower end thereof in one side portion of the main body  20 . The feed tray  22  can be opened as illustrated by a double-dot chain line in  FIG. 1 , and on the feed tray  22 , the medium P on which printing is to be performed can be set. In addition, in one side portion of the main body  20 , a cover  23  for maintenance that includes the feed tray  22  and a cover  24  for maintenance that is arranged on the lower side of the cover  23  are disposed to be horizontally openable and closable about one side end thereof. In the present embodiment, each of the cassette  21  and the feed tray  22  constitutes one example of a medium mount unit. The feed tray  22  may be a manual feed tray in which only one sheet of the medium P can be set, or may be a configuration equipped with a hopper function in which a plurality of media can be set in a stack and that can automatically feed one sheet at a time. 
     A part of the printer unit  12  on the upper side of the cassette  21  is configured as a printing mechanism unit  25  (refer to  FIG. 2 ) that performs printing on the medium P fed from the cassette  21  or the feed tray  22 . A stacker unit  26  to which a medium after printing is discharged is disposed between the printer unit  12  and the scanner main body portion  17 . 
     Next, a detailed configuration of the printer unit  12  will be described with reference to  FIG. 2 . In  FIG. 2 , only the uppermost cassette  21  is illustrated, and the others are not illustrated. In addition, the direction in which the medium P is transported when printing is performed by a printing head  34  is set as a transport direction Y, and the direction that intersects with (particularly, orthogonal with respect to) the transport direction Y is set as a width direction X. 
     As illustrated in  FIG. 2 , the above printing mechanism unit  25  is disposed in the main body  20  of the printing apparatus  11  of the present embodiment. A transport unit  32  and a printing unit  33  are disposed in the printing mechanism unit  25 . The transport unit  32 , as one example of a transport portion, transports the medium P along a transport path  31 . The printing unit  33  includes the printing head  34  that performs printing on the medium P in transport. 
     The printing head  34  employs an ink jet type that discharges ink. The printing head  34  is configured of a line head of a long shape that extends slightly longer than the width of the medium P having the maximum width in the width direction X which is orthogonal with respect to the page of  FIG. 1 , and the printing head  34  is of a fixed type that is fixed to a predetermined position so that movement in the width direction X is not allowed. 
     The present embodiment employs a line printing type in which printing is performed in the shape of a line by discharging, by the printing head  34  configured of a fixed type line head, ink drops at the same time to the medium P in transport within an area extending in the width direction X. Ink that is discharged from the printing head  34  clings to the medium P, and thereby an image, a document, or the like is printed on the medium P. The present embodiment may also employ a serial printing type in which the printing unit  33  includes a printing carriage movable in the width direction X, the printing head  34  disposed in the printing carriage is of a moving type moving in the width direction X (main-scanning direction) along with the printing carriage, and a transport operation for the medium P and a printing operation by the printing head  34  are alternately performed. 
     As illustrated in  FIG. 2 , the transport unit  32  includes a feed mechanism unit  35 , a transport mechanism unit  37 , and a discharge mechanism unit  38 . The feed mechanism unit  35  feeds the medium P. The transport mechanism unit  37  transports the medium P along the transport path  36  when the printing unit  33  performs printing. The discharge mechanism unit  38  transports the medium P after printing along a discharge path  62  and discharges the medium P to the stacker unit  26 . 
     The feed mechanism unit  35  includes a first feed unit  41 , a second feed unit  42 , and a third feed unit  43 . The first feed unit  41  has the feed tray  22  as a feed source. The second feed unit  42  has the cassette  21  as a feed source. The third feed unit  43  feeds the medium P, after printing is performed on a single side thereof at the time of double-sided printing, again to the transport path  36 . The first feed unit  41  feeds, to the transport mechanism unit  37  along a first feed path  45  by rotation of a first feed roller pair  44 , the medium P that is set on the feed tray  22  and has the leading edge portion thereof inserted from an insertion port  20 A. The cover  23  (refer to  FIG. 1  and  FIG. 3 ) that includes the feed tray  22  is disposed to be openable and closable in a position upstream of the printing head  34  of the main body  20  in the transport direction Y. 
     The second feed unit  42  feeds the medium P from the cassette  21  to a second feed path  48 . The second feed unit  42  includes a pickup roller  49 , a separating roller pair  50 , a second feed roller pair  51 , and a passive roller  52 . The pickup roller  49  withdraws the uppermost medium P in the cassette  21 . The separating roller pair  50  separates one sheet from the withdrawn medium P. The second feed roller pair  51  and the passive roller  52  feed the separated one sheet of the medium P. 
     As illustrated in  FIG. 2 , the transport mechanism unit  37  includes a transport roller pair  46  and a belt transport mechanism  58 . The transport roller pair  46  is arranged in a position slightly downstream of the location where the first to third feed units  41  to  43  are joined in the transport direction Y. The belt transport mechanism  58  is arranged in a position facing the printing head  34 . The leading edge of the medium P abuts the stopped transport roller pair  46 , and thereby the medium P is skewed during feeding, and the medium P after skewed is transported to the transport path  36  by rotation of the transport roller pair  46 . 
     The belt transport mechanism  58  includes a pair of rollers  59  and  60  and a transport belt  61  that is wound on the pair of rollers  59  and  60 . In addition, a transport passive roller  47  that is driven in contact with the transport belt  61  is arranged in a position above the roller  59  of the belt transport mechanism  58 . 
     The belt transport mechanism  58  employs an electrostatic attraction type that attracts the medium P to the surface of the charged transport belt  61  by electrostatic force. The printing head  34  discharges ink toward the medium P that is transported at a constant speed by the belt transport mechanism  58  with a constant gap maintained with the printing head  34 , and thereby an image, a document, or the like is printed on the medium P. 
     The third feed unit  43 , at the time of double-sided printing, performs refeeding that turns over the medium P after printing is performed on one side (single side) thereof and guides the medium P again to the transport mechanism unit  37 . The medium P discharged from the transport mechanism unit  37  after printing is performed on one side thereof is guided to a branch transport path  54  by a branch mechanism  53  and is guided, by reverse rotation of the transport roller pair  55  after forward rotation thereof, to an inversion feed path  56  that is positioned above the printing unit  33  in  FIG. 2 . The medium P is fed along the inversion feed path  56  by rotation of a plurality of inversion transport roller pairs  57  and is inverted and joins the first feed path  45  and the second feed path  48 . Then, the medium P is guided again to the transport mechanism unit  37 , and the printing head  34  performs printing on the other side of the inverted medium P on which printing is not performed yet, and thereby double-sided printing is performed. 
     The discharge mechanism unit  38  discharges the medium P on which printing is ended from a medium discharge port  20 B onto the stacker unit  26  as illustrated by a double-dot chain line in  FIG. 2 , by rotation of a plurality of discharge roller pairs  63  arranged along the discharge path  62 . The discharged medium P after printing is stacked on the stacker unit  26 . A transport path  30  transports the medium P from the cassette  21  and the feed tray  22  along a path passing through a position in which the printing head  34  can perform printing. 
     As illustrated in  FIG. 2 , a medium detection device  80  is arranged slightly upstream of the transport roller pair  46  that is positioned upstream of the printing head  34  in the transport direction Y. The medium detection device  80  is positioned on the lower side of a position near a joint portion of the first feed path  45  and the second feed path  48  and near a joint portion of the transport path  30  and the inversion feed path  56 . The medium detection device  80  detects a side edge of the medium P fed by the first feed unit  41  and the second feed unit  42 , the side edge being in the width direction X that intersects with (particularly, orthogonal with respect to) the transport direction Y. Medium information that relates to a width and includes at least one of a side edge position of the medium P in the width direction X, a medium width (medium size), and a printing area in the width direction X is acquired from side edge information of the medium P detected by the medium detection device  80 . 
     Next, a configuration of a part of the feed mechanism unit  35  that includes surroundings of the medium detection device  80  will be described with reference to  FIG. 3  to  FIG. 7 .  FIG. 5  and  FIG. 6  illustrate the cover  23  that is closed, and  FIG. 7  illustrates the cover  23  that is opened. 
     As illustrated in  FIG. 3  and  FIG. 4 , when the cover  23  that is openable and closable with respect to the apparatus main body  20  of the printing apparatus  11  is opened, a part of each of the feed units  41  to  43  is exposed. Each of the feed units  41  to  43  is configured of a main body side feed mechanism unit  65  and a cover side feed mechanism unit  66 . The main body side feed mechanism unit  65  is attached to a frame, not illustrated, in the main body  20 . The cover side feed mechanism unit  66  is attached to the inner surface side of the cover  23 . The medium detection device  80  is attached on the main body  20  side. The medium detection device  80 , while supporting the medium P in feed or transport with the upper surface thereof, reads the medium P in the width direction X through a window portion  88  of the upper surface thereof and detects a side edge of the medium P. Thus, a medium guide unit  81 A and a medium support unit  81 B that constitute the upper surface part of the medium detection device  80  supporting the medium P also constitute a part of the main body side feed mechanism unit  65 . 
     As illustrated in  FIG. 5  and  FIG. 7 , the main body side feed mechanism unit  65  includes the separating roller pair  50  constituting the second feed unit  42 , a drive roller  51 A, a guide member  67 , a drive roller  57 A constituting the third feed unit  43 , guide members  68  and  69 , the transport roller pair  46 , and the like. In addition, the medium detection device  80  includes the medium guide unit  81 A of a sloping shape in the upstream part thereof in the transport direction Y in the upper surface portion of a casing  81 . The medium guide unit  81 A guides the medium P from the second feed path  48 . Furthermore, the medium detection device  80  includes the medium support unit  81 B in the upper surface part that is downstream of the medium guide unit  81 A in the transport direction Y in the casing  81 . The medium support unit  81 B supports the medium P from the first feed path  45  and the second feed path  48 . Thus, the medium guide unit  81 A and the medium support unit  81 B constitute a part of the main body side feed mechanism unit  65 . 
     As illustrated in  FIG. 5  and  FIG. 7 , the cover side feed mechanism unit  66  includes the feed roller pair  44  constituting the first feed unit  41 , guide members  70  and  71 , passive rollers  51 B and  52  constituting the second feed unit  42 , a guide member  72 , and a passive roller  57 B constituting the third feed unit  43 . The guide member  71  that is integrally attached to the cover  23  includes a medium guide surface  71 A constituting the first feed path  45  and a medium guide surface  71 B constituting the inversion feed path  56 . 
     The cover side feed mechanism unit  66  includes a feed mechanism that feeds the medium P from at least one of the feed tray  22  and the cassette  21  constituting one example of a plurality of medium mount units. In the present example, one example of a plurality of medium mount units is both of the feed tray  22  and the cassette  21 , and the entirety of the first feed unit  41  and a part of the second feed unit  42  are attached to the cover  23  as one example of at least a part of a feed unit. 
     As illustrated in  FIG. 5  and  FIG. 6 , the feed roller pair  44  includes a drive roller  44 A and a passive roller  44 B. The guide member  70  and the medium guide surface  71 A of the guide member  71  are arranged to face each other downstream of both of the rollers  44 A and  44 B in the feed direction and thereby form a part of the first feed path  45 . In addition, the guide members  67  and  72  facing each other and the medium guide unit  81 A of the medium detection device  80  form a part of the second feed path  48 . 
     As illustrated in  FIG. 5 , the separating roller pair  50  constituting the second feed unit  42  is configured of a drive roller  50 A and a passive roller  50 B. 
     In addition, as illustrated in  FIG. 5  and  FIG. 6 , the inversion transport roller pair  57  constituting the third feed unit  43  is configured of the drive roller  57 A and the passive roller  57 B. The downstream part of the inversion feed path  56  that inverts and feeds the medium P after printing is performed on the single side thereof is formed by the guide member  68  on the main body  20  side and the guide member  71  on the cover  23  side that are arranged to face each other. In addition, the surface of the medium detection device  80  facing the guide member  68  forms a part of the inversion feed path  56 . In addition, the transport roller pair  46  arranged in a position downstream of the medium detection device  80  in the transport direction Y is configured of a drive roller  46 A and a passive roller  46 B. 
     The drive roller  44 A illustrated in  FIG. 5  is driven by the motive power of a first feed motor  121  (refer to  FIG. 16 ). In addition, the drive rollers  50 A and  51 A are driven by the motive power of a second feed motor  122  (refer to  FIG. 16 ). In addition, the drive roller  46 A and the drive roller  57 A are driven by the motive power of a first transport motor  123  (refer to  FIG. 16 ). The belt transport mechanism  58  illustrated in  FIG. 2  is driven by the motive power of a belt motor  124  (refer to  FIG. 16 ). In addition, the discharge mechanism unit  38  illustrated in  FIG. 2  is driven by the motive power of a second transport motor  125  (refer to  FIG. 16 ). 
     As illustrated in  FIG. 6 , the first feed path  45  and the second feed path  48  are joined in a first joint portion  75 . Downstream of the first joint portion  75  in the feed direction (transport direction Y), the second feed path  48  as one example of a transport path and the inversion feed path  56  as one example of a double-sided printing path are joined in a second joint portion  76  as one example of a path joint portion. That is, a first common feed path  77  that is a common feed path for the medium P fed from the feed tray  22  and for the medium P fed from the cassette  21  is between the first joint portion  75  and the second joint portion  76 . A second common feed path  78  that is a common feed path for the medium P fed before printing in order to perform printing on one side of the medium P and for the medium P fed again after printing in order to perform printing on the other side for double-sided printing after printing is performed on one side is downstream of the second joint portion  76  in the feed direction (transport direction Y). The second joint portion  76  that is a joint portion of the first feed path  45 , the second feed path  48 , and the first common feed path  77  constituting one example of a single-sided printing path and the inversion feed path  56  exist upstream of the printing head  34  (refer to  FIG. 2 ) in the transport direction Y. 
     As illustrated in  FIG. 6 , a plurality of medium guide surfaces forming the second joint portion  76  is the surfaces of the downstream parts of the two medium guide surfaces  71 A and  71 B of the guide member  71 , the downstream part of a medium guide surface  68 A forming the inversion feed path  56 , and the slightly downstream part of the medium support unit  81 B of the medium detection device  80 . These surfaces form a part (joint region) in which the medium P is joined. The guide member  71  attached to the cover  23  (refer to  FIG. 5  and  FIG. 7 ) includes a part of the medium guide surfaces  71 A and  71 B forming the second joint portion  76 . Particularly, a location where the two medium guide surfaces  71 A and  71 B of the guide member  71  intersect with each other forms the second joint portion  76 . 
     As illustrated in  FIG. 5  and  FIG. 6 , the medium detection device  80  is arranged on the lower side of the first joint portion  75  and the second joint portion  76 . The upper surface portion of the casing  81  of the medium detection device  80  constitutes a part of the second feed path  48 , the first common feed path  77 , and the second common feed path  78 . In addition, a sensor  79  that can detect the edge portion of the medium P in the transport direction Y is disposed upstream of the transport roller pair  46  in the transport direction Y. The sensor  79  is used to determine the timing of a skew operation for the medium P performed by the transport roller pair  46 , by sensing the leading edge of the medium P in the transport direction Y. 
     As illustrated in  FIG. 6 , the medium detection device  80  includes the above casing  81 , a carriage  82 , a sensor  83 , and an electric motor  103  (refer to  FIG. 12  and  FIG. 16 ). The carriage  82  is accommodated in the casing  81  to be movable in the width direction X (the direction orthogonal with respect to the page of  FIG. 6 ). The sensor  83  is disposed on the side of the carriage  82  facing the transport path  30 . The electric motor  103  moves the carriage  82  as one example of a motive power source. 
     A pair of rail units  84  and  85  that guides the carriage  82  to be movable in the width direction X is disposed in the casing  81 . The carriage  82  is fixed to a part of a belt  87  of an endless shape that is wound to a plurality of pulleys  86  (only one of which is illustrated in  FIG. 6 ). The medium detection device  80  employs a belt drive type that renders the carriage  82  to reciprocate in the width direction X by rotation of the belt  87  by the motive power of the electric motor  103 . Since the medium detection device  80  is disposed separately from the printing head  34 , the carriage  82  and the sensor  83  can move in the width direction X independently of the printing head  34  (refer to  FIG. 2 ). 
     As illustrated in  FIG. 6 , the window portion  88  that is configured of a light transmissive member is disposed in a part of the casing  81  of the medium detection device  80 , the part facing the detection direction side of the sensor  83  when the carriage  82  moves. The light transmissive member constituting the window portion  88  is configured of a transparent member such as transparent glass or transparent plastic. The sensor  83  of the present example is an optical sensor and optically reads the medium P fed along the first common feed path  77  through the window portion  88  and thereby detects a side edge of the medium P in the width direction X. The carriage  82  can move in the width direction X in a position upstream of the printing head  34  in the transport direction Y. Thus, the sensor  83  can detect a side edge of the medium P in a position upstream of the printing head  34  in the transport direction Y. Accordingly, the side edge position of the medium P that is detected in advance by the medium detection device  80  can be used in control of the printing head  34  that performs printing in a position downstream of the side edge position. 
     As illustrated in  FIG. 6 , in a region that the sensor  83  faces through the window portion  88  in the detection direction thereof (light emission direction), that is, a facing region corresponding to a read position, a part of the medium guide surface  71 A of the guide member  71  horizontally extends parallel to the window portion  88  in the transport direction Y and parallel to the moving path of the sensor  83  in the width direction X. That is, a part of the medium guide surface  71 A is a horizontal surface from which the distance to the sensor  83  can be maintained equally in any position on the moving path of the sensor  83 . At least a region of the horizontal part of the medium guide surface  71 A facing the moving path of the sensor  83  is a light reflective surface  71 C that reflects light from the sensor  83 . The guide member  71  of the present example is made of metal, and the light reflective surface  71 C is formed by processing the corresponding part of the medium guide surface  71 A of the guide member  71  into a specular surface by, for example, polishing or plating. 
     The light reflective surface  71 C is formed to have a sufficiently higher light reflectance than the medium P. The sensor  83  of a light reflective type receives reflective light from the light reflective surface  71 C and outputs an L level in a case where a detected voltage value corresponding to the received light intensity exceeds a threshold, and receives reflective light from the medium P and outputs an H level in a case where the detected voltage value corresponding to the received light intensity is less than or equal to the threshold (refer to  FIG. 18  and  FIG. 19 ). That is, the sensor  83  outputs the L level when the medium P is not sensed, and outputs the H level in a case where the medium P is sensed. The sensor  83  is preferably capable of outputting a detected signal corresponding to the presence or absence of the medium P. The light reflective surface  71 C may be a surface having a sufficiently lower reflectance than the medium P by, for example, performing coating or surface roughening on the light reflective surface  71 C. 
     As illustrated in  FIG. 6 , a part of the second feed path  48  is formed by the medium guide unit  81 A of a sloping shape in the upper surface portion of the casing  81  of the medium detection device  80 . In addition, the window portion  88  and a medium guide unit  81 C, in the upper surface portion of the casing  81 , that is positioned downstream of the window portion  88  in the transport direction Y constitute the medium support unit  81 B by which both the medium P fed from the feed tray  22  and the medium P fed from the cassette  21  are supported. The medium detection device  80  includes an extending portion  81 D that extends from the upper portion of the casing  81  to the downstream side thereof in the transport direction Y. The extending portion  81 D is arranged in a position that faces the guide member  69  configured of a part of a support member supporting the passive roller  46 B, and forms a part of the second common feed path  78 . 
     As illustrated in  FIG. 6 , the carriage  82  is disposed to be movable in the width direction X in a position that is on the opposite side from the printing head  34  side of the transport path  30  of the medium P transported by the transport unit  32 . The sensor  83  irradiates the medium P with light from a position that is on the opposite side from the printing head  34  side with the transport path  30  interposed therebetween, and thereby can detect a side edge of the medium P. 
     As illustrated in  FIG. 6 , the medium detection device  80  is arranged in a position where the read position (a broken lined position in  FIG. 6 ) of the sensor  83  is upstream in the transport direction Y of the second joint portion  76  in which the first common feed path  77  and the inversion feed path  56  are joined. When the medium detection device  80  is arranged in, for example, a position where the medium P after printing is performed on one side thereof can be read, the second joint portion  76  is required to be arranged to be shifted upstream of the read position of the medium detection device  80  in the transport direction, and consequently, the third feed unit  43  including the inversion feed path  56  and the first feed unit  41  are required to be arranged to be shifted upstream in the transport direction Y. In this case, the feed mechanism unit  35  of the printing apparatus  11  is required to be arranged in a position moved back upstream in the transport direction Y, and the size dimension of the printing apparatus  11  in the transport direction Y is increased by the amount of moving back, and the size of the printing apparatus  11  is increased in the transport direction Y. 
     Regarding this point, the first feed unit  41  and the third feed unit  43  are not required to be arranged to be shifted upstream in the transport direction Y by arranging a second path joint portion J 2  in a position downstream of the read position with respect to the medium detection device  80  without reading a side edge of the medium P after printing is performed on one side thereof. Accordingly, the size dimension of the printing apparatus  11  in the transport direction Y is relatively short. 
     As illustrated in  FIG. 6 , a first feed path  91  that is a path of the medium P fed by rotation of the feed roller pair  44  and a second feed path  92  that is a path of the medium P fed along the second feed path  48  are joined in a first path joint portion J 1  that is positioned slightly downstream of the first joint portion  75 . Furthermore, a third feed path  93 , as one example of a double-sided printing path, that is a path of the medium P fed along the inversion feed path  56  is joined to the second feed path  92  in the second path joint portion J 2 , as one example of a joint portion, that is positioned slightly downstream of the second joint portion  76 . The read position of the medium detection device  80  is positioned upstream of the second path joint portion J 2  in the transport direction Y. In addition, the read position of the medium detection device  80  is positioned downstream of the first path joint portion J 1  in the transport direction Y. The position area of the read position is a part of a common transport path along which each medium P fed from the cassette  21  and the feed tray  22  is transported, the part being upstream of the second path joint portion J 2 . 
     Thus, regardless of the feed source (medium mount unit) from which the medium P comes, a side edge can be detected by reading the medium P with an approximately constant distance maintained between the medium P and the sensor  83 , and what is easily avoided is that ink from printing performed on one side of the medium P fed through the inversion feed path  56  clings to the window portion  88 . Accordingly, a decrease in the accuracy of detection of a side edge of the medium P or erroneous detection due to irregularity of the distance between the sensor  83  and the medium P in the read position, staining of the window portion  88  with ink, and the like are reduced. 
     In the present embodiment, the read position of the medium detection device  80  is preferably upstream of the second path joint portion J 2  in the transport direction Y. In addition, the read position is preferably upstream of the second joint portion  76  in the transport direction Y. Furthermore, the read position of the medium detection device  80  is preferably downstream of the first joint portion  75  in the transport direction Y. In addition, the read position is preferably downstream of the first path joint portion J 1  in the transport direction Y. 
     If the read position is positioned upstream of the second joint portion  76 , the guide member  71  that includes the two medium guide surfaces  71 A and  71 B forming the second joint portion  76  is positioned to face the moving path of the sensor  83 . In the present example, the light reflective surface  71 C that reflects light from the sensor  83  is formed in a region of the guide member  71  facing the sensor  83 , and the same member is intended to be used for guiding a medium and for the light reflective surface. 
     As illustrated in  FIG. 7 , when the cover  23  is opened, the cover side feed mechanism unit  66  that includes the guide member  71 , the guide member  72 , and the like moves in the receding direction (upstream in the transport direction Y) along with the cover  23 , and the medium detection device  80  is exposed. That is, since the read position of the medium detection device  80  is arranged to be positioned upstream of the second path joint portion J 2  in the transport direction Y, the medium detection device  80  is exposed as illustrated in  FIG. 7  and  FIG. 8  when the guide member  71  forming the second joint portion  76  recedes rearward. Particularly, since the read position is arranged to be positioned upstream of the second joint portion  76  in the transport direction Y, the window portion  88  is also exposed as illustrated in  FIG. 7  and  FIG. 8  when the guide member  71  forming the second joint portion  76  recedes rearward. For example, cleaning that removes stains on the surface of the window portion  88  can be performed with the medium detection device  80  attached to the main body  20 . 
     Next, a detailed configuration of the medium detection device  80  will be described with reference to  FIG. 8  to  FIG. 15 . 
     As illustrated in  FIG. 8 , the medium detection device  80  has a long shape that extends slightly longer in the width direction X than the maximum width of the medium P. Two window portions  88  are disposed in the width direction X (casing longitudinal direction). Center feeding in which the width center of the medium P passes through the position of the width center of a feed path independently of the size of the medium P is performed in the printing apparatus  11  of the present embodiment. The two window portions  88  and the medium guide unit  81 C form the medium support unit  81 B. In addition, the medium detection device  80  includes a connector  81 E in the lower portion of the casing  81 . A connector, not illustrated, that is connected to an interconnect from a control unit  120  is connected to the connector  81 E, and the detected signal of the sensor  83  is input into the control unit  120  through the connector  81 E and an interconnect not illustrated. 
     As illustrated in  FIG. 9 , the upper surface of the medium detection device  80  includes the medium guide unit  81 A, the two window portions  88 , a support unit  81 F interposed between the two window portions  88 , the medium guide unit  81 C, and the extending portion  81 D. The medium P fed from the cassette  21  is guided by the medium guide unit  81 A. In addition, the medium P fed from the cassette  21  and the medium P fed from the feed tray  22  are guided to a support surface that is configured of the window portion  88  and the upper surfaces of the support unit  81 F and the medium guide unit  81 C. The extending portion  81 D is extended in plural numbers in the shape of teeth of a comb, each of which is inserted in a gap between a plurality of the transport roller pairs  46  disposed in the width direction X. The medium P when being inserted into the transport roller pair  46  is supported by the upper surface of the plurality of extending portions  81 D. 
     As illustrated in  FIG. 10 , the casing  81  of the medium detection device  80  is configured of a base  100  and a cover  110 . The pair of rail units  84  and  85  is disposed to extend parallel to each other in the longitudinal direction on the upper surface of the base  100 . 
     The carriage  82  is attached to be movable in the longitudinal direction of the casing  81  along the pair of rail units  84  and  85 . In addition, a pair of the pulleys  86  is attached to the upper surface of the base  100  at a predetermined interval in the longitudinal direction in each position corresponding to both end portions of the base  100  in the longitudinal direction. The belt  87  of an endless shape is wound to the pair of pulleys  86 , and the carriage  82  is fixed to a part of the belt  87 . 
     The sensor  83  disposed in the carriage  82  is configured of a pair of sensors  83 A and  83 B that is arranged in a different position in the longitudinal direction of the casing  81  (width direction X). In addition, the central portions of the upper surfaces of the carriage  82  and the base  100  in the longitudinal direction are connected through a flexible flat cable  89 . In addition, a position sensor  90  that senses the carriage  82  being in a home position which is the end portion of the moving path thereof in the width direction X is disposed in the end portion of the upper surface of the base  100  in the longitudinal direction. In the following description, the sensor  83 A may be referred to as a first sensor  83 A, and the sensor  83 B may be referred to as a second sensor  83 B. 
     As illustrated in  FIG. 10 , a plurality of screw holes  101  is formed in the base  100  at appropriate intervals in the peripheral portion of the base  100  in the peripheral direction. In the peripheral portion of the cover  110 , a screw  111  that is inserted in a plurality of screw insertion holes, not illustrated, formed in a position corresponding to the screw hole  101  screws into the corresponding screw hole  101  on the base  100  side, and thereby the base  100  and the cover  110  are integrally attached to each other, and the casing  81  is formed. 
     As illustrated in  FIG. 11 , the carriage  82  can move between a home position HP illustrated by a solid line in  FIG. 11  and an anti-home position AP that is the opposite side end portion from the home position HP in the width direction X and is illustrated by a double-dot chain line in  FIG. 11 . One end portion of the flexible flat cable  89  is fixed to the central portion of the peripheral portion of the base  100  in the longitudinal direction. The part of the flexible flat cable  89  extending from the fixed location is wired along one rail unit  85  and is curved in an arc shape midway of the rail unit  85  and is then wired along the other rail unit  84 , and the other end portion of the flexible flat cable  89  is connected to the carriage  82 . The arc-shaped part of the flexible flat cable  89  moves in the width direction X along with movement of the carriage  82 , and thereby the flexible flat cable  89  maintains electrical connection with the carriage  82  in movement. 
     As illustrated in  FIG. 11 , the home position HP and the anti-home position AP of the carriage  82  are end positions on both sides when the carriage  82  moves in the width direction X. The position sensor  90  is ON when the carriage  82  is in the home position HP and is OFF when the carriage  82  is in a position separated from the home position HP. 
     As illustrated in  FIG. 12 , the electric motor  103  is attached to the rear surface of the base  100 . The drive axis of the electric motor  103  is connected to one pulley  86  (the pulley  86  on the left side in  FIG. 11 ). The electric motor  103  of the present example is configured of a stepping motor. A connector  104  is connected to the tip end of an interconnect extending from the electric motor  103 . A connector, not illustrated, that is connected to an interconnect extending from the control unit  120  is connected to the connector  104 , and electricity and a control signal are input from the control unit  120  into the electric motor  103  through the interconnect. The electric motor  103  is driven to rotate forward and reversely based on the control signal (step control signal) from the control unit  120 , and thereby the carriage  82  reciprocates in the width direction X by forward and reverse rotation of the belt  87 . 
     As illustrated in  FIG. 12 , an interconnect of the flexible flat cable  89  extends from the surface side (inner surface side) of the base  100  and is exposed to the rear surface side thereof on the rear surface of the base  100 . The interconnect is wired on the rear surface while being held along a predetermined path and is connected to the connector  81 E. In addition, in the connector  81 E, an interconnect  105  that extends from the position sensor  90  extends from the outer surface side (inner surface side) of the base  100  and is exposed to the rear surface side thereof, and the interconnect  105  is wired on the rear surface of the base  100  while being held along a predetermined path and is connected to the connector  81 E. 
     As illustrated in  FIG. 13 , the two sensors  83 A and  83 B include a light emitting unit  106  and a light receiving unit  107 . Light emitted from the light emitting unit  106  is reflected, and the light receiving unit  107  receives the reflective light, and thereby the sensors  83 A and  83 B outputs a detected signal at a voltage level corresponding to the received light intensity. An interconnect  89 A that extends from the first sensor  83 A is partially folded and convoluted midway and is connected with an interconnect  89 B extending from the second sensor  83 B, thereby constituting one flexible flat cable  89 . The flexible flat cable  89  is inserted into an interconnect hole formed in the carriage  82  and extends to the rear surface side of the carriage  82  and then is wired along the inside surface of the rail unit  84 . 
     As illustrated in  FIG. 13 , the position sensor  90  includes a light emitting unit  90 A and a light receiving unit  90 B. The carriage  82  includes a detected portion  82 A that protrudes outward in the width direction X from the side portion thereof on the home position HP side. When the carriage  82  is in the home position HP, the detected portion  82 A that is inserted in a recess  90 C between the light emitting unit  90 A and the light receiving unit  90 B blocks light projected from the light emitting unit  90 A to the light receiving unit  90 B, and thereby the position sensor  90  is placed into a sensing state. Meanwhile, in a state where the carriage  82  moves from the home position HP to the anti-home position AP side, the detected portion  82 A recedes from the recess  90 C, and the light receiving unit  90 B receives light that is projected from the light emitting unit  90 A to the light receiving unit  90 B, and thereby the position sensor  90  is placed into a non-sensing state. 
     As illustrated in  FIG. 14 , the rail unit  84  includes a vertically standing portion  84 A and a support unit  84 B. The vertically standing portion  84 A extends vertically with respect to the base  100  in the vertical direction Z. The support unit  84 B is curved from the upper end portion of the vertically standing portion  84 A and extends horizontally. A positioning mechanism  114  that is configured by pressing a pressing member  112  with pressing force of a compression spring  113  against the vertically standing portion  84 A of the rail unit  84  is disposed in the carriage  82 . The pressing member  112  is pressed to the vertically standing portion  84 A of the rail unit  84  with predetermined pressing force in the horizontal direction, and thereby the carriage  82  is positioned in the transport direction Y. Thus, rattling of the carriage  82  in the transport direction Y is prevented. 
     As illustrated in  FIG. 15 , the carriage  82  includes a positioning mechanism  118  that is configured by pressing a pressing member  116  with pressing force of a compression spring  117  against the support unit  84 B of the rail unit  84 . The pressing member  116  is pressed to the support unit  84 B of the rail unit  84  with predetermined pressing force, and thereby the carriage  82  is positioned in the vertical direction Z. Thus, rattling of the carriage  82  in the vertical direction is prevented. In addition, the carriage  82  is fixed to the belt  87  with the belt  87  grasped by a grasping unit  82 B that is disposed to protrude to the upstream side portion of the carriage  82  in the transport direction Y. In addition, a guide unit  82 C of a plate shape that extends downstream in the transport direction Y from the carriage  82  is guided to the upper surface of the rail unit  85 . The carriage  82 , with the attitude thereof held in the transport direction Y and the vertical direction Z through the two types of positioning mechanisms  114  and  118 , can move in the width direction X along the rail units  84  and  85  by driving of the belt  87 . 
     The transport roller pair  46  illustrated in  FIG. 2  and  FIG. 6  is resist rollers and determines the timing of starting transport of the medium P downstream thereof. The leading edge portion of the medium P fed abuts the stopped transport roller pair  46 , and thereby an unskewing operation of removing or reducing skewing (slanting movement) of the medium P is performed. When the medium P is abutting the transport roller pair  46 , the medium P stops and rotates in plane at an angle corresponding to the slanting movement, and thereby skewing thereof is removed. After the unskewing operation, a feed speed at which the trailing edge portion side of the medium P is withdrawn by the feed units  41  and  42  and a transport speed at which the leading edge portion side of the medium P is withdrawn by rotation of the transport roller pair  46  are set to be equal, and thereby the medium P is brought onto the transport belt  61  of the belt transport mechanism  58  at a constant transport speed. 
     As illustrated in  FIG. 6 , the read position of the sensor  83  is set to a position upstream of a nip location in the transport direction Y. The nip location is where the transport roller pair  46  nips (pinches) the medium P. The medium detection device  80  detects a side edge of the medium P in the width direction X after unskewing. The medium detection device  80  can detect a side edge of the medium P that is stopped after unskewing or the medium P that starts to be transported at a low speed after unskewing. Thus, in the case of using a configuration in which, for example, the medium detection device detects a side edge of the medium P in a position downstream of the transport roller pair  46  in the transport direction Y, a side edge of the medium P after unskewing can be detected, but the target of detection is the medium P that reaches a certain transport speed and is comparatively fast. Thus, in the case of, for example, detecting both side edges of the medium P, one side edge and the other side edge are detected in significantly different positions in the transport direction Y of the medium P. In this case, when skewing of the medium P remains slightly, the accuracy of detecting the width and a side edge of the medium P is slightly decreased. Regarding this point, according to the present embodiment in which the read position of the sensor  83  is set to be upstream in the transport direction Y of the nip location of the transport roller pair  46  used in the unskewing operation, the side edge position and the width of the medium P can be more accurately acquired than in the case of setting the read position of the sensor  83  to be downstream of the nip location of the transport roller pair  46  in the transport direction Y. A side edge detection process for the medium P may also be performed during transport of the medium P at a timing other than after unskewing, and side edge detection may be performed at a plurality of different locations in the transport direction Y of one sheet of the medium P. 
     Next, an electrical configuration of the printing apparatus  11  will be described with reference to  FIG. 16 . As illustrated in  FIG. 16 , the printing apparatus  11  includes the control unit  120  generally controlling the printing apparatus  11 , the medium detection device  80 , the above operation panel  14 , the transport unit  32  transporting the medium P, and the printing head  34  performing printing on the medium P in transport. The transport unit  32  includes the first feed motor  121  and the second feed motor  122 . The first feed motor  121  is the source of motive power of the first feed unit  41  that feeds the medium P set in the feed tray  22 . The second feed motor  122  is the source of motive power of the second feed unit  42  that feeds the medium P accommodated in the cassette  21 . In addition, the transport unit  32  includes the first transport motor  123 , the belt motor  124 , and the second transport motor  125 . The first transport motor  123  is the source of motive power of the transport roller pair  46 , the discharge mechanism unit  38 , and the like transporting the fed medium P. The belt motor  124  is the source of motive power of the belt transport mechanism  58 . The second transport motor  125  is the source of motive power of the transport roller pair  55  and the inversion transport roller pair  57  that transport the medium P after printing is performed on one side thereof. 
     The plurality of motors  121  to  125  is electrically connected to the control unit  120  through motor drive circuits  126  to  130  of the same number as the number of motors in a transport system. The control unit  120  controls each of the motors  121  to  125  through the motor drive circuits  126  to  130  and thereby performs inversion and discharge at the time of feeding and transporting of the medium P and double-sided printing. The second transport motor  125  may be removed by disposing an electromagnetic clutch that can switch the transport roller pair  55  to rotate forward or reversely, and by setting the source of motive power of the transport roller pair  55  and the inversion transport roller pair  57  to the first transport motor  123  which is the same source of motive power as the transport roller pair  46 . 
     The printing head  34  is electrically connected to the control unit  120 . The control unit  120  controls the printing head  34  based on graphic print data in the print job data PD received from, for example, a host apparatus (not illustrated) and thereby discharges ink drops from a nozzle of the printing head  34  to the part of the medium P in transport on the transport belt  61  and prints an image or the like based on the graphic print data on the medium P. In addition, the operation unit  16  and the display unit  15  constituting the operation panel  14  are electrically connected to the control unit  120 . The control unit  120 , based on an operation signal input from the operation unit  16 , receives various types of setting information corresponding to items selected from a menu displayed on the display unit  15  or instruction information indicating performing of printing, scanning, copying, and the like. In addition, the control unit  120  displays the above menu, a message notifying a user when failure or malfunctioning occurs, and the like on the display unit  15 . 
     The control unit  120  performs the unskewing operation for the medium P as follows. The control unit  120  performs the unskewing operation of removing or reducing skewing (slanting movement) of the medium P by, with driving of the first transport motor  123  stopped, driving the feed motor  121  or  122  to render the leading edge portion of the fed medium P to abut the stopped transport roller pair  46 . 
     The control unit  120  temporarily stops driving of the feed motor  121  or  122  when the feed motor  121  or  122  finishes rotating the set amount of rotation required for the unskewing operation. 
     After the unskewing operation, the feed motor  121  or  122  and the first transport motor  123  are driven with the rotation speeds thereof synchronized, and thereby the medium P is transported onto the transport belt  61  at a constant transport speed. The control unit  120  starts driving the belt motor  124  before the medium P is transported to the transport belt  61 , and the medium P is brought at a constant transport speed onto the transport belt  61  that is driven at a constant transport speed. 
     The electric motor  103  that is the source of motive power of the carriage  82  of the medium detection device  80 , the position sensor  90 , and the first sensor  83 A and the second sensor  83 B on the carriage  82  are electrically connected to the control unit  120  illustrated in  FIG. 16 . The control unit  120  drives and controls the electric motor  103  through the motor drive circuit  131  and thereby performs movement control that renders the carriage  82  to reciprocate in the width direction X of the medium P and performs position control that renders the carriage  82  to stop in a target stop position. 
     In addition, the control unit  120  illustrated in  FIG. 16  recognizes whether or not the carriage  82  is in the home position HP based on a detected signal SH (refer to  FIG. 18  and  FIG. 19 ) that is input from the position sensor  90 . The control unit  120  recognizes the carriage  82  as being in the home position HP if the detected signal SH input from the position sensor  90  is at a signal level (for example, the H level) indicating that the carriage  82  is in the home position HP, and recognizes the carriage  82  as not being in the home position HP if the detected signal SH is at a signal level (for example, the L level) indicating that the carriage  82  is not in the home position HP. 
     Furthermore, a detected signal SA from the first sensor  83 A and a detected signal SB from the second sensor  83 B (for each, refer to  FIG. 18  and  FIG. 19 ) are input into the control unit  120  illustrated in  FIG. 16  during movement of the carriage  82 . The control unit  120  detects both side edges of the detection target medium P in the width direction X based on each of the detected signals SA and SB input from the first sensor  83 A and the second sensor  83 B. The control unit  120 , for example, from the result of detection of the side edges of the medium P in the width direction X, acquires medium information that is related to width such as side edge positions PE 1  and PE 2  (refer to FIG.  17 ), a printing area in the width direction X when the printing head  34  performs printing, the width of the medium P (for example, a paper width), and a medium size (for example, a paper size) defined from the width of the medium P. 
     The control unit  120  illustrated in  FIG. 16  includes, for example, a computer and a memory, not illustrated, and includes a plurality of functional units that functions by the computer executing a program stored on the memory and is required for a medium detection process. The control unit  120 , as the plurality of functional units, includes a width information acquiring unit  141 , a carriage control unit  142 , a failure detection unit  143 , and a detection processing unit  144 . The width information acquiring unit  141  acquires width information of the medium P. The carriage control unit  142  drives and controls the electric motor  103 . The failure detection unit  143  can detect failure of the sensors  83 A and  83 B. The detection processing unit  144  can detect the side edge positions PE 1  and PE 2  of the medium P based on the detected signals SA and SB of the sensors  83 A and  83 B. The width information acquiring unit  141  acquires the width information of the medium P from printing setting information that is included in the print job data PD received by the control unit  120 . The width information is, for example, information as to the medium size. The memory stores, for example, reference data that indicates a correspondence between the medium paper size and the width information, and the width information acquiring unit  141  acquires the width information by referencing the reference data based on the medium size acquired. 
     The carriage control unit  142  illustrated in  FIG. 16  controls the electric motor  103  through the motor drive circuit  131  and thereby performs control that renders the carriage  82  to move in the width direction X and performs position control that renders the carriage  82  to stop in a target position. The failure detection unit  143 , when the medium P is not transported, determines whether or not each of the detected signals SA and SB from the first sensor  83 A and the second sensor  83 B has a value at the time of failure and, if there is a sensor having a value at the time of failure, detects the sensor as failing. In the present example, the H level is a detected value when the medium P is present, and if there is a sensor, of the sensors  83 A and  83 B, having the H level even though the medium P is not present, the sensor is detected as failing. In addition, if the sensors  83 A and  83 B receive reflective light from the light reflective surface  71 C and have the L level that is a detected value when the medium P is not present, the sensors  83 A and  83 B are regarded as being normal. 
     The detection processing unit  144  detects a side edge of the medium P in the width direction X based on the detected signals SA and SB of the first sensor  83 A and the second sensor  83 B. The detection processing unit  144  includes a counter  145  and a calculating unit  146 . The counter  145  counts the positions of the carriage  82  in the width direction X with the home position HP as the origin. The calculating unit  146  calculates the other side edge position of the medium P based on one side edge position and the width information of the medium P detected by the other of the first and second sensors  83 A and  83 B when one fails. 
       FIG. 17  is a schematic diagram illustrating a method for detecting a side edge of a medium with two sensors and is a diagram seen from the rear surface side of the medium. As illustrated in  FIG. 17 , the first sensor  83 A and the second sensor  83 B are mounted on the upper portion of the carriage  82  at an inter-center distance L 1  therebetween in the width direction X. When the carriage  82  is positioned in the home position HP (a solid line position in  FIG. 17 ), in the case of a medium SP of a small size having a small width dimension, both of the sensors  83 A and  83 B are positioned outside of the medium SP in the width direction and do not sense the medium SP. Meanwhile, in the case of a medium LP of a large size having a large width dimension, when the carriage  82  is positioned in the home position HP, one sensor (the second sensor  83 B in the example of  FIG. 17 ) on the home position HP side is positioned outside of the medium LP in the width direction X and does not sense the medium LP. Regarding this point, the other sensor (the first sensor  83 A in the example of  FIG. 17 ) on the anti-home position AP side is in a position facing the medium LP and senses the medium LP. 
     In the present example, in a case where the medium P having the maximum width is a detection target, when the carriage  82  is in a left side end position E 1  (for example, the home position HP) in a movable range thereof, the right side sensor  83 A of the two sensors  83 A and  83 B senses the medium P having the maximum width, and the left side sensor  83 B does not sense the medium P having the maximum width. Meanwhile, when the carriage  82  is in a right side end position E 2  in the movable range thereof, the left side sensor  83 B of the two sensors  83 A and  83 B senses the medium P having the maximum width, and the right side sensor  83 A does not sense the medium P having the maximum width. The movable range of the carriage  82  is relatively narrower than the width of the medium P having the maximum width. Thus, the size dimension of the medium detection device  80  in the width direction X is reduced to be relatively smaller than the width of the medium P having the maximum width, and even if the medium detection device  80  is disposed in the apparatus main body  20  in a direction in which the longitudinal direction of the medium detection device  80  matches the width direction X, the size dimension of the printing apparatus  11  in the width direction X is reduced to be relatively small. 
     The control unit  120  illustrated in  FIG. 16  switches the number of sensors, of the two sensors  83 A and  83 B, used in the side edge detection process of detecting both side edge positions PE 1  and PE 2  of the medium P according to the size (width dimension) of the medium P. That is, in the case of the medium SP of a small size of which the medium width based on the width information is less than or equal to a set width, the control unit  120  uses only one sensor (for example, the first sensor  83 A) to detect the side edge positions PE 1  and PE 2  of the medium SP. In addition, in the case of the medium LP of a large size of which the medium width exceeds the set width, the control unit  120  uses both of the first and second sensors  83 A and  83 B to detect the side edge positions PE 1  and PE 2  of the medium LP. With the carriage  82  being in the home position HP illustrated by a solid line, the set width is set to a value greater than or equal to the maximum medium width of medium widths in which both of the two sensors  83 A and  83 B cannot sense the medium P (for example, the medium SP) and less than the minimum medium width of medium widths in which the first sensor  83 A can sense the medium P (for example, the medium LP), as illustrated in  FIG. 17 . In the case of regarding the width direction X in which the carriage  82  can move as the left-right direction in  FIG. 17 , the first sensor  83 A corresponds to one example of a right side sensor, and the second sensor  83 B corresponds to one example of a left side sensor. In addition, the first side edge PE 1  of the medium P corresponds to one example of a left side edge, and the second side edge PE 2  corresponds to one example of a right side edge. 
       FIG. 18  and  FIG. 19  illustrate detected signals output from each of the sensors  83 A,  83 B, and  90  when both side edges of the medium P in the width direction X are detected.  FIG. 18  is a signal waveform of each detected signal when a side edge of the medium SP of a small size of which the medium width is less than or equal to the set width is detected, and  FIG. 19  is a signal waveform of each detected signal when a side edge of the medium LP of a large size of which the medium width exceeds the set width is detected. In  FIG. 18  and  FIG. 19 , the horizontal direction indicates a time t, and the vertical direction indicates the voltage level of each of the detected signals SA, SB, and SH. The carriage  82  moves at a constant speed V 1 . Thus, the time t in the horizontal direction in  FIG. 18  and  FIG. 19  corresponds to the number of steps of the electric motor  103 , that is, the distance of movement of the carriage  82 . 
     As illustrated in  FIG. 18 , when the carriage  82  is positioned in the home position HP (the solid line position in  FIG. 17 ) and the detected signal SH is at the H level in a case where the medium SP of a small size is a detection target, both of the sensors  83 A and  83 B do not sense the medium SP, and both of the detected signal SA and SB are at the L level. When the carriage  82  starts to move from the home position HP, first, the right side first sensor  83 A senses the left side first side edge PE 1 , and the detected signal SA rises from the L level to the H level. 
     Furthermore, when the carriage  82  moves in a time t 1  (=L 1 /V 1 ) corresponding to the inter-center distance L 1  between the two sensors  83 A and  83 B, the left side second sensor  83 B senses the right side second side edge PE 2 , and the detected signal SB rises from the L level to the H level. Next, when the first sensor  83 A moves in a time t 2  corresponding to the medium width from the time point when the first side edge PE 1  is detected, the first sensor  83 A senses the second side edge PE 2 , and the detected signal SA falls from the H level to the L level. The number of steps of the electric motor  103  output during the period of the time t 2  corresponds to the distance of movement of the carriage  82  during the period, and the distance of movement of the carriage  82  corresponds to the width of the medium SP. 
     In addition, as illustrated in  FIG. 19 , when the carriage  82  is positioned in the home position HP (the solid line position in  FIG. 17 ) and the detected signal SH is at the H level in a case where the medium LP of a large size is a detection target, the right side first sensor  83 A senses the medium LP and is at the H level, and the left side second sensor  83 B does not sense the medium LP and is at the L level. When the carriage  82  starts to move from the home position, first, the left side second sensor  83 B senses the left side first side edge PE 1 , and the detected signal SB rises from the L level to the H level. Furthermore, when the carriage  82  moves in a time t 3  corresponding to the medium width, the right side first sensor  83 A senses the right side second side edge PE 2 , and the detected signal SA falls from the H level to the L level. The number of steps of the electric motor  103  output during the period of the time t 3  corresponds to the distance of movement of the carriage  82  during the period, and the distance of movement of the carriage  82  corresponds to the width of the medium LP. 
     Next, operation of the printing apparatus  11  will be described with reference to  FIG. 18  to  FIG. 21 . The control unit  120  performs a medium side edge detection process illustrated in  FIG. 20  when a power supply of the printing apparatus  11  is turned ON. In addition, the control unit  120 , in the case of detecting failure of one of the sensors  83 A and  83 B in the medium side edge detection process illustrated in  FIG. 20 , performs a medium side edge detection process at the time of failure illustrated in  FIG. 21 . 
     First, in Step S 11 , a sensor failure detection process is performed. The sensor failure detection process is performed when the transport unit  32  does not transport the medium P. Examples of the time when the medium P is not transported include when the power supply of the printing apparatus  11  is turned ON, when the printing apparatus  11  is in a printing standby state of waiting for reception of a print job, and when the printing apparatus  11  returns from a pause mode (sleep mode). In the present example, the light reflective surface  71 C is disposed in a region facing the moving paths of the sensors  83 A and  83 B. Thus, the sensors  83 A and  83 B face the light reflective surface  71 C in any position on the moving paths thereof. For example, with the carriage  82  being in the home position HP, the sensors  83 A and  83 B emit light and receive reflective light that is formed by reflection of the emitted light by the light reflective surface  71 C. The control unit  120  determines failure in a case where the received light intensity from reception of the reflective light formed by reflection of the light from the sensors  83 A and  83 B by the light reflective surface  71 C does not exceed a threshold and where the detected signal of the received light is at the H level indicating the presence of a medium. The failure detection process is performed for each of the two sensors  83 A and  83 B. 
     In a case where the light reflective surface  71 C exists in only a region facing a part of the moving paths of the sensor  83 A and  83 B and where the two sensors  83 A and  83 B are not in a position facing the light reflective surface  71 C, the control unit  120  drives and controls the electric motor  103  and arranges the two sensors  83 A and  83 B to a position facing the light reflective surface  71 C. The control unit  120  renders the two sensors  83 A and  83 B arranged in a position facing the light reflective surface  71 C to emit light and thereby performs the failure detection process based on each detected signal. 
     In Step S 12 , a determination as to whether or not a sensor fails is performed. In the failure determination, failure is determined if any of the two sensors  83 A and  83 B fails. If any sensor fails, the process proceeds to Step S 13 . If no sensor fails, the process proceeds to Step S 14 . 
     In Step S 13 , failure notification is performed. That is, the control unit  120 , for example, displays a message indicating failure on the display unit  15  and thereby notifies a user of failure. The failure notification may be performed as notification with light emission or blinking of a light emitting unit such as a light emitting diode, notification with a buzzer or audio, or notification by combining these plural types of notification methods. 
     In Step S 14 , a determination as to whether or not a print job including the width information is received is performed. In the printing apparatus  11  of the present example, a print job includes the printing setting information, and one item in the printing setting information includes the width information of a medium (for example, the paper size). That is, the process of Step S 14  has the same meaning as a determination as to whether or not a print job is received. In the case of receiving a printing instruction by a method other than a print job, for example, in the case of connecting a memory card not illustrated to the printing apparatus  11  and printing an image selected from the memory card by operating the operation unit  16 , the control unit  120  acquires the width information (for example, the paper size) of the medium P selected by the operation unit  16 . 
     In Step S 15 , a determination as to whether or not all sensors are normal is performed. Specifically, the failure detection unit  143  of the control unit  120  performs the failure detection process of detecting failure of the first and second sensors  83 A and  83 B and determines whether or not all of the sensors  83 A and  83 B are normal based on the result of the failure detection process. As the failure detection process, with no medium present before the medium P is fed, if the values of the detected signals SA and SB from the sensors  83 A and  83 B that are supposed to receive reflective light reflected by the light reflective surface  71 C are not equal to a value when there is no medium (for example, the L level) and are equal to a value when a medium is sensed (for example, the H level), those sensors are determined as failing. 
     If all of the sensors  83 A and  83 B are normal, the process proceeds to Step S 16 . The process proceeds to Step S 19  in a case where any of the sensors  83 A and  83 B fails. 
     In Step S 16 , a determination as to whether or not the width of a medium is less than or equal to the set width is performed. If the width of the medium P acquired from the width information is less than or equal to the set width, that is, if the medium P is the medium SP of a small size, the control unit  120  proceeds to Step S 17 . Meanwhile, if the width of the medium P acquired from the width information exceeds the set width, that is, if the medium P is the medium LP of a large size, the process proceeds to Step S 18 . 
     In Step S 17 , a first side edge detection process of detecting both side edges of a medium by using one sensor is performed. The carriage control unit  142  of the control unit  120  drives and controls the electric motor  103  through the motor drive circuit  131  and renders the carriage  82  to move in the width direction X. In the present example in which the electric motor  103  is a stepping motor, the carriage control unit  142  controls the electric motor  103  by specifying the number of steps. The counter  145  is reset when the carriage  82  is in the home position HP and the position sensor  90  senses the detected portion  82 A, and performs a counting process of adding or subtracting the number of steps used in control in the advancing direction of the carriage  82 . Accordingly, the counter  145  stores the count value corresponding to the positions of the carriage  82  in the width direction X. In the case of the medium SP of a small size such as A4 of which the width of the medium P based on the width information is less than or equal to the set width, the carriage control unit  142  renders the carriage  82  to move from the home position HP to a position A 1  illustrated by a double-dot chain line in  FIG. 17 . During movement of the carriage  82 , the detection processing unit  144  detects both side edge positions PE 1  and PE 2  of the medium SP of a small size by using only one first sensor  83 A. 
     At this point, as illustrated in  FIG. 18 , the detected signal SH of the position sensor  90 , the detected signal SA of the first sensor  83 A, and the detected signal SB of the second sensor  83 B are input into the control unit  120 . The detection processing unit  144  of the control unit  120  monitors the detected signal SA of the first sensor  83 A. When the detected signal SA rises from the L level to the H level, the detection processing unit  144  acquires the count value of the counter  145  at that time and calculates the first side edge position PE 1  by using the count value and a known first distance (for example, L 1 / 2 ) in the width direction X from the center position of the carriage width to the first sensor  83 A. The calculated first side edge position PE 1  is stored on the memory. When the detected signal SA of the first sensor  83 A falls from the H level to the L level during movement of the carriage  82 , the count value of the counter  145  at that time is acquired. The second side edge position PE 2  is calculated by using the count value and the above known first distance, and the calculated second side edge position PE 2  is stored on the memory. If both of the two sensors  83 A and  83 B do not sense the medium SP when the carriage  82  is in the home position HP, using only the first sensor  83 A that is positioned on the front side in the advancing direction during movement of the carriage  82  at the time of medium detection allows both side edge positions PE 1  and PE 2  of the medium P to be detected in a relatively short distance of movement of the carriage  82 . 
     In Step S 18  in  FIG. 20 , a second side edge detection process of detecting both side edges of a medium by using two sensors is performed. The carriage control unit  142  of the control unit  120  drives and controls the electric motor  103  through the motor drive circuit  131  by specifying the number of steps and renders the carriage  82  to move in the width direction X. At this point, the counter  145  counts the positions of the carriage  82  in the width direction X with the origin set to the position when the carriage  82  is in the home position HP. In the case of the medium LP of a large size such as A3 of which the width of the medium P based on the width information exceeds the set width, the carriage control unit  142  renders the carriage  82  to move in the width direction X from the home position HP to, for example, a position A 2  illustrated by a double-dot chain line in  FIG. 17 . During movement of the carriage  82 , the detection processing unit  144  detects both side edge positions PE 1  and PE 2  of the medium LP of a large size in the width direction X by using both of the first and second sensors  83 A and  83 B. 
     Specifically, as illustrated in  FIG. 19 , when the carriage  82  is in the home position HP and the detected signal SH is at the H level, the first sensor  83 A senses the medium LP, and the detected signal SA thereof is at the H level. Meanwhile, the detected signal SB of the second sensor  83 B that does not sense the medium LP is at the L level. When the carriage  82  starts to move from the home position HP, the detection processing unit  144  first monitors the detected signal SB of the second sensor  83 B. When the detected signal SB of the second sensor  83 B rises from the L level to the H level, the detection processing unit  144  acquires the count value of the counter  145  at that time and calculates the first side edge position PE 1  by using the count value and a known second distance (for example, L 1 / 2 ) in the width direction X from the center position of the carriage width to the second sensor  83 B. The calculated first side edge position PE 1  is stored on the memory. When the detected signal SA of the first sensor  83 A falls from the H level to the L level during subsequent movement of the carriage  82 , the count value of the counter  145  at that time is acquired. The second side edge position PE 2  is calculated by using the count value and the above known first distance, and the calculated second side edge position PE 2  is stored on the memory. If one of the two sensors  83 A and  83 B does not sense the medium LP when the carriage  82  is in the home position HP, the first side edge position PE 1  is detected by the second sensor  83 B that is positioned on the rear side in the advancing direction of the carriage  82  at the time of medium detection, and the second side edge position PE 2  is detected by the first sensor  83 A that is positioned on the front side in the advancing direction. Thus, both side edge positions PE 1  and PE 2  of the medium P can be detected in a relatively short distance of movement of the carriage  82 . 
     In Step S 19  in  FIG. 20 , the medium side edge detection process at the time of failure is performed. The medium side edge detection process at the time of failure is performed by the computer of the control unit  120  executing a medium side edge detection process routine at the time of failure illustrated in  FIG. 21 . 
     Hereinafter, the medium side edge detection process at the time of failure performed by the control unit  120  will be described with reference to  FIG. 21 . 
     First, in Step S 21 , a determination as to whether or not only one sensor fails is performed. That is, the control unit  120  determines whether or not only one sensor fails by using the detection result of the sensor failure detection process in Step S 11  of  FIG. 20 . If only one sensor fails, the process proceeds to Step S 22 . If not only one sensor fails, that is, if both of two sensors fail, the process proceeds to Step S 26 . 
     In Step S 22 , a determination as to whether or not the width of a medium is less than or equal to the set width is performed. This determination process is the same as the determination process of Step S 16  in  FIG. 20  and is performed based on the width information. If the width of the medium P is less than or equal to the set width, the process proceeds to Step S 23 . Meanwhile, if the width of the medium P is not less than or equal to the set width, that is, if the width of the medium P exceeds the set width, the process proceeds to Step S 24 . 
     In Step S 23 , a third side edge detection process of detecting both side edges of a medium by using the other normal sensor is performed. For example, in a case where the first sensor  83 A is normal and the second sensor  83 B fails, both side edge positions PE 1  and PE 2  of the medium P are detected by using the normal first sensor  83 A, and the first side edge detection process which is the same as Step S 17  in  FIG. 19  is performed as the third side edge detection process. Meanwhile, in a case where the first sensor  83 A fails and the second sensor  83 B is normal, the third side edge detection process of detecting both side edge positions PE 1  and PE 2  of the medium P by using the normal second sensor  83 B is performed. In the case of the latter, the count value of the counter  145  when the detected signal SB from the second sensor  83 B rises from the L level to the H level is acquired, the first side edge position PE 1  is calculated by using the count value and the above known second distance, and the calculated first side edge position PE 1  is stored on the memory. In addition, the count value of the counter  145  when the detected signal SB falls from the H level to the L level is acquired, the second side edge position PE 2  is calculated by using the count value and the above known second distance, and the calculated second side edge position PE 2  is stored on the memory. 
     In Step S 24 , a determination as to whether or not a fourth side edge detection process accompanying estimation may be performed is performed. In the present example, the control unit  120  displays, on the display unit  15 , an inquiry message of whether or not the fourth side edge detection process accompanying estimation may be performed, and determines whether or not to perform the fourth side edge detection process based on an operation signal generated by a user operating the operation unit  16  as a response to the inquiry. Alternatively, a user operates the operation unit  16  of the printing apparatus  11  to read preregistered registered information from the memory and determines whether or not to perform the fourth side edge detection process based on the registered information. The control unit  120  proceeds to Step S 25  if the fourth side edge detection process is permitted and proceeds to Step S 27  if the fourth side edge detection process is not permitted. 
     In Step S 25 , the fourth side edge detection process of detecting one side edge by using the other normal sensor and estimating the other side edge by using the detection result for one side edge and the width information is performed. As illustrated in  FIG. 17 , the movable range in which the carriage  82  can move from the end position E 1  on the home position HP side to the end position E 2  on the anti-home position AP side is relatively shortened in the printing apparatus  11  of the present example. Thus, the size dimension of the medium detection device  80  in the width direction X and, furthermore, the size dimension of the printing apparatus  11  in the width direction X are small, and this contributes to rendering the printing apparatus  11  compact. However, since the movable range of the carriage  82  is relatively short, both side edges of the medium LP cannot be detected by only the other normal sensor in a case where the medium LP of a large size is a detection target. 
     Thus, one side edge position is detected by a normal sensor, and the other side edge position that cannot be detected due to failure of the other sensor is estimated by calculation using the side edge position detected by the normal sensor and the width information. Specifically, the control unit  120  detects one side edge by using the other sensor of the first and second sensors  83 A and  83 B that does not fail. For example, in a case where the first sensor  83 A is normal and the second sensor  83 B fails, one side edge position PE 2  is detected by the first sensor  83 A. In addition, in a case where the first sensor  83 A fails and the second sensor  83 B is normal, one side edge position PE 1  is detected by the second sensor  83 B. The calculating unit  146  of the detection processing unit  144  calculates the other side edge position by using the detected one side edge position and the width information of the medium P. Given that one side edge position detected by the other sensor is x 1  and the medium width is W 1 , an other side edge position x 2  of the medium P is calculated by x 2 =x 1 −W 1  in a case where the first sensor  83 A is normal and by x 1 +W 1  in a case where the second sensor  83 B is normal. Both side edge positions PE 1  and PE 2  of the medium P are acquired by the fourth side edge detection process. 
     The control unit  120  controls the printing area of the printing head  34  in the width direction X based on both side edge positions PE 1  and PE 2  of the medium P acquired by the medium side edge detection process. Consequently, printing is performed in an appropriate position range in the width direction X of the medium P. The control unit  120  acquires the width of the medium P from the side edge positions PE 1  and PE 2  and, in a case where the width is different from the medium width acquired from the width information and exceeds an allowable range, renders the display unit  15  to display a message indicating medium size error. 
     In Step S 26 , a determination as to whether or not printing may be performed without side edge detection is performed. In the present example, for example, the control unit  120  renders the display unit  15  to display an inquiry message and inquires a user as to whether or not printing may be performed without detecting a side edge of the medium P. Based on an operation signal from the operation unit  16 , when the control unit  120  receives a response indicating that printing is performed without side edge detection (positive determination in S 26 ), the control unit  120  proceeds to Step S 27 . Meanwhile, when the control unit  120  receives a response indicating that printing is not performed without side edge detection (negative determination in S 26 ), the control unit  120  ends the routine. In the case of the latter, printing is stopped. 
     In Step S 27 , the side edge detection process is stopped. The control unit  120  includes a flag for managing whether or not to perform the side edge detection process and writes a value indicating stopping into the flag. In this case, the control unit  120  performs printing based on the print job data PD without performing the side edge detection process. 
     At this point, the control unit  120  controls the printing area of the printing head  34  based on the medium width acquired from the width information. 
     In  FIG. 20 , the processes of Step S 24  and Step S 25  may be removed, and the fourth side edge detection process accompanying estimation may not be performed. In addition, when a negative determination is made in Step S 24 , the process may proceed to the determination process of Step S 26 . Furthermore, the process of Step S 26  may be removed, and printing may be stopped in a case where all of the sensors  83 A and  83 B fail. 
     According to the first embodiment described in detail heretofore, the following effect can be achieved. 
     (1) The medium detection device  80  that detects a side edge of the medium P in the width direction X intersecting with the transport direction Y is arranged upstream of the printing head  34  in the transport direction Y of the medium P. The medium detection device  80  includes the carriage  82  capable of moving in the width direction X independently of the printing head  34  in a position upstream of the printing head  34  in the transport direction Y, the two sensors  83 A and  83 B disposed in different positions in the width direction X in the carriage  82 , and the electric motor  103  as one example of a motive power source rendering the carriage  82  to move. The control unit  120  renders the carriage  82  to move by controlling the electric motor  103  and thereby renders the sensors  83 A and  83 B to detect a side edge of the medium P in the width direction X. Since the two sensors  83 A and  83 B are arranged in different positions in the width direction X in the carriage  82 , using the two sensors  83 A and  83 B for different uses decreases the amount of movement of the carriage  82  when a side edge of the medium P in the width direction X is detected. Accordingly, both side edges of the medium P in the width direction X can be detected without depending on a printing type like whether the printing head  34  moves or not (for example, a line printing type or a serial printing type). In addition, the size dimension of the printing apparatus  11  in the width direction can be reduced to be comparatively small even though the medium detection device  80  is disposed. 
     (2) The carriage  82  is disposed to be movable in the width direction X in a position on the opposite side of the transport path  30  of the medium P transported by the transport unit  32  from the printing head  34  side. Thus, the two sensors  83 A and  83 B configured of optical sensors irradiate the medium P with light from a position on the opposite side of the transport path  30  from the printing head  34  side. Accordingly, ink mist from the printing head  34  is unlikely to cling compared with a case where the two sensors  83 A and  83 B are arranged on the same side of the medium P as the printing head  34 . Thus, a decrease in the accuracy of detection of the sensors  83 A and  83 B due to staining with ink is easily avoided. 
     (3) The casing  81  of the medium detection device  80  includes the medium support unit  81 B that supports the medium P transported along the transport path  30 , and the window portion  88  that can transmit light from the two sensors  83 A and  83 B is disposed in the medium support unit  81 B. Accordingly, since the sensors  83 A and  83 B are protected by the window portion  88 , the sensors  83 A and  83 B are not directly rendered dirty by dust such as paper dust from the medium P, ink mist from the printing head  34 , and the like. Thus, the accuracy of detection of the sensors  83 A and  83 B can be comparatively highly maintained. In addition, the window portion  88  constitutes a part of the medium support unit  81 B, and the distance between the medium P sliding on the upper surface of the window portion  88  and the sensors  83 A and  83 B can be comparatively short. From this point as well, the accuracy of detection of the sensors  83 A and  83 B can be highly maintained. 
     (4) The two window portions  88  are arranged in the width direction X in the medium support unit  81 B. Accordingly, compared with a configuration in which one long window portion extending in the area of movement of the sensors  83 A and  83 B is disposed, a comparatively high strength of the medium support unit  81 B can be secured, and the cost of components of the window portion  88  can be reduced to be relatively inexpensive. 
     (5) A plurality of the window portions  88  is arranged in a position where the two sensors  83 A and  83 B can detect both side edges of the medium P having the minimum width to the maximum width in the width direction X through different window portions  88 . For example, both side edges of the medium P having the minimum width to the maximum width can be continuously detected. For example, even if the medium P of an undefined shape other than the medium P of the defined shape is transported, a side edge of the medium P of the undefined shape can be detected. 
     (6) Since the electric motor  103  that is one example of the source of motive power of the medium detection device  80  is configured of a stepping motor, an encoder or the like that is required in the case of using a direct current motor (DC motor) for acquiring the position of the carriage  82  is not required. Accordingly, the number of components of the medium detection device  80  can be reduced to be small compared with the case of using a direct current motor. For example, it is easy to realize a small device size for the medium detection device  80 . 
     (7) When the carriage  82  is in the left side end position E 1  in the movable range thereof in the case of the medium P having the maximum width, the right side sensor  83 A of the two sensors  83 A and  83 B senses the medium P having the maximum width, and the left side sensor  83 B does not sense the medium P having the maximum width. Meanwhile, when the carriage  82  is in a right side end position E 2  in the movable range thereof, the left side sensor  83 B of the two sensors  83 A and  83 B senses the medium P having the maximum width, and the right side sensor  83 A does not sense the medium P having the maximum width. That is, when the medium P has the maximum width, only the left side sensor  83 B is separated to the outside from the medium P in the width direction when the carriage  82  is in the left side end position E 1 , and only the right side sensor  83 A is separated to the outside from the medium P in the width direction X when the carriage  82  is in the right side end position. Thus, since the movable range of the carriage  82  is relatively narrower than the width of the medium P having the maximum width, the size dimension in the width direction X of the printing apparatus  11  in which the medium detection device  80  is disposed is reduced to be comparatively small. In addition, in the case of the medium P having the maximum width, the left side edge PE 1  of the medium P is detected by the left side sensor  83 B, and the right side edge PE 2  of the medium P is detected by the right side sensor  83 A, and thereby both side edges PE 1  and PE 2  of the medium P having the maximum width can be detected. 
     (8) The left side edge (first side edge PE 1 ) of the medium P is detected by the left side second sensor  83 B, and the right side edge (second side edge PE 2 ) of the medium P is detected by the right side first sensor  83 A. Accordingly, the distance of movement required for the carriage  82  when both side edges of the medium P in the width direction X are detected is relatively short. Thus, the size dimension of the medium detection device  80  in the width direction X can be short. 
     For example, the size of the printing apparatus  11  in the width direction being increased due to disposing of the medium detection device  80  can be avoided, and the amount of time required for acquiring the medium information related to the width direction X of the medium P can be reduced to be relatively small. 
     (9) In a case where the width of the medium P based on the width information acquired by the width information acquiring unit  141  is longer than the set width, the control unit  120  controls the electric motor  103 , renders the left side edge (first side edge PE 1 ) of the medium P to be detected by the left side second sensor  83 B, and renders the right side edge (second side edge PE 2 ) of the medium P to be detected by the right side first sensor  83 A. Accordingly, the distance of movement required for the carriage  82  when both side edges PE 1  and PE 2  of the medium P are detected can be relatively short. Accordingly, the size dimension of the medium detection device  80  in the width direction can be relatively small, and the amount of time required for acquiring medium information can be reduced to be relatively small. 
     (10) The guide member  71  as one example of a medium guide member is arranged in a position where the sensors  83 A and  83 B configured of light reflective type sensors face the window portion  88  transmitting light at the time of detection with the transport path  30  interposed therebetween, and the part of the guide member  71  facing the moving paths of the sensors  83 A and  83 B is configured as a light reflective surface. Accordingly, since a member dedicated for a light reflective surface is not required to be separately disposed, the medium guiding structure of the transport unit  32  that is upstream of the printing head  34  in the transport direction Y can be configured to be comparatively compact. 
     (11) Since the guide member  71  is made of metal, the light reflective surface  71 C can be comparatively simply formed if the part of the guide member  71  facing the moving paths of the sensors  83 A and  83 B is, for example, polished. 
     (12) When the transport unit  32  does not transport the medium P, the control unit  120  monitors the detected signals SA and SB of the sensors  83 A and  83 B. If a sensor receives reflective light from the light reflective surface  71 C, the control unit  120  regards the sensor as being normal. If a sensor does not receive reflective light from the light reflective surface  71 C, the control unit  120  regards the sensor as failing. Accordingly, a problem that wrong medium information is acquired, such as erroneous detection of a side edge position based on the detected signal of a failing sensor, can be avoided to the greatest possible extent. 
     (13) The control unit  120 , in a case where one of the two sensors  83 A and  83 B fails, detects both side edges PE 1  and PE 2  of the medium P by using the other sensor. Accordingly, even if one sensor fails, the medium information required can be acquired by detecting both side edges PE 1  and PE 2  of the medium P. 
     Particularly, even in a case where a comparatively short movable range in which both side edges PE 1  and PE 2  cannot be detected by only one sensor when the medium P has a large size is set for the carriage  82 , the medium information required can be acquired for the medium SP of a small size having a width less than or equal to the set width by detecting both side edges PE 1  and PE 2  of the medium SP by using the other sensor that does not fail. 
     (14) The control unit  120 , if the width of the medium P is greater than the set width in a case where one of the two sensors  83 A and  83 B fails, detects one side edge of both side edges PE 1  and PE 2  of the medium P by using the other sensor, and the calculating unit  146  estimates the other side edge position by calculation based on the result of detection of one side edge and the width information. Accordingly, even for the medium LP having a width that is great to the extent that both side edges cannot be detected by the other sensor when one sensor fails, the medium information including both side edge positions PE 1  and PE 2  of the medium LP can be acquired. 
     (15) The printing apparatus  11  includes the transport path  30  transporting the medium P from the cassette  21  and the feed tray  22  as one example of a medium mount unit along a path passing through a position where the printing head  34  can perform printing, and includes the third feed path  93  (one example of a double-sided printing path) that inverts the medium P after performing printing on one side thereof through the transport path  30  by the printing head  34  and returns the medium P to a position midway of the transport path  30 . 
     The medium detection device  80  that can detect both side edges PE 1  and PE 2  of the medium P by reading the medium P in the width direction X is arranged in a position upstream of the printing head  34  in the transport direction Y of the medium P so that the read position of the medium detection device  80  is positioned upstream in the transport direction Y of the second path joint portion J 2  (one example of a joint portion) in which the transport path  30  and the third feed path  93  are joined. Accordingly, the size dimension of the printing apparatus  11  in the transport direction Y can be comparatively small even though the medium detection device  80  is disposed. For example, when a configuration in which the medium P that is refed through the inversion feed path  56  after printing is performed on one side thereof is read by the medium detection device is employed, the joint portion is required to be arranged to be shifted upstream of the read position of the medium detection device in the transport direction Y. Accordingly, the part of the transport path  30  and the inversion feed path  56  that is positioned upstream of the joint portion  76  in the transport direction is required to be shifted upstream in the transport direction Y. In this case, the size dimension of the printing apparatus  11  in the transport direction Y is relatively increased. Regarding this point, the medium P after printing is performed on one side thereof is not read, and the joint portion  76  is arranged downstream of the read position in the transport direction Y. Thus, the part of the transport path  30  and the inversion feed path  56  that is positioned upstream (rear side) of the joint portion  76  in the transport direction Y can be arranged downstream to the greatest possible extent. Consequently, both side edges PE 1  and PE 2  of the medium P in the width direction X can be detected without depending on a printing type like whether the printing head  34  is a moving type or a fixed type, and the size dimension of the printing apparatus  11  in the transport direction Y can be reduced to be comparatively small. 
     (16) The medium detection device  80  has a long shape in the width direction X in which the carriage  82  can move with the electric motor  103  as the source of motive power within a moving range in which the sensors  83 A and  83 B can detect both side edges of the medium P having the maximum width in the width direction X. That is, even if the medium detection device  80  that is slightly longer than the width of the medium P having the maximum width is disposed in the printing apparatus  11  in a direction in which the longitudinal direction of the medium detection device  80  matches the width direction X, the size dimension of the printing apparatus  11  in the transport direction Y can be reduced to be comparatively small by designing a space for arrangement of the medium detection device  80  in such a manner that the second path joint portion J 2  is positioned downstream of the read position in the transport direction Y. 
     (17) The printing apparatus  11  can detect the side edges PE 1  and PE 2  of the medium P fed from both of the cassette  21  and the feed tray  22  by using the medium detection device  80 , and the medium detection device  80  can be disposed while the size of the printing apparatus  11  in the transport direction Y is rendered small. 
     (18) The read position of the medium detection device  80  is positioned downstream of the first joint portion  75  in which the two feed paths  45  and  48  feeding the medium P from the cassette  21  and the feed tray  22  in the transport direction Y are joined, and upstream of the second path joint portion J 2 . Accordingly, the medium P that is fed from any of the cassette  21  and the feed tray  22  is read in the read position of the medium detection device  80 , and both side edges PE 1  and PE 2  thereof can be detected. Thus, printing can be performed in an appropriate position in the width direction X on the medium P that is fed from any of the cassette  21  and the feed tray  22 . 
     (19) Particularly, the read position is positioned downstream of the first path joint portion J 1  and upstream of the second path joint portion J 2  in the transport direction Y. According to this configuration, the distance between the sensors  83 A and  83 B and the medium P can be maintained approximately constantly, and thus a higher side edge detection accuracy can be secured. 
     Furthermore, the read position is positioned downstream of the first path joint portion J 1  and upstream of the second joint portion  76  in the transport direction Y. According to this configuration, the sensing area of the sensors  83 A and  83 B with respect to the medium P that is refed through the inversion feed path  56  after printing is performed on one side thereof is protected by the guide member  71 , and thus a decrease in the side edge detection accuracy due to staining with ink can be prevented. For example, since the ink of printing performed on one side of the refed medium P is unlikely to cling to the window portion  88 , the side edge detection accuracy for the medium P can be maintained highly even in a configuration in which the sensors  83 A and  83 B reads the medium P through the window portion  88 . 
     (20) The cover  23  to which the first feed unit  41  as one example of a feed mechanism feeding the medium P mounted in the feed tray  22  and the guide member  71  including a part of the medium guide surfaces  71 A and  71 B forming the second joint portion  76  are attached is disposed to be openable and closable with respect to the apparatus main body  20  in the printing apparatus  11 . Thus, when the cover  23  is opened, the first feed unit  41  and the guide member  71  are separated from the apparatus main body  20  along with the cover  23 , and a part of the medium detection device  80  is exposed. Consequently, maintenance and the like are easily performed for the medium detection device  80 , and the medium detection device  80  can be comparatively easily detached in a case where the medium detection device  80  is required to be detached for maintenance or replacement. 
     The above embodiment can be modified in the following forms.
         The first side edge and the second side edge may be detected by different sensors even in the case of a medium of a small size of which the width based on the width information is less than or equal to the set width. In addition, even in the case of a medium of a large size of which the width based on the width information is longer than the set width, the first side edge and the second side edge may be detected by the same sensor by slightly increasing the distance of movement of the carriage.   In a case where one of two sensors fails and in the case of a medium of which the medium width is longer than the set width, a message may be displayed on the display unit to inquire a user as to whether or not an estimation process of detecting one side edge of the first side edge and the second side edge by the other non-failing sensor and estimating the other side edge by calculation may be performed. For this inquiry, the inquiry may be performed by audio instead of or in addition to display of a message.   While the medium detection device  80  is arranged on the lower side of the transport path  30 , the medium detection device  80  may be arranged on the upper side of the transport path  30 . Even in this configuration, a side edge of the medium P can be detected by reading the medium P downwards by the two sensors  83 , and arranging the medium detection device  80  in a position in which the read position thereof is upstream of the second path joint portion J 2  in the transport direction Y can relatively decrease the size dimension of the printing apparatus  11  in the transport direction Y and render the size thereof small.   The medium information acquired by the sensor  83  detecting a side edge of the medium is preferably at least one of, one side edge position PE 1  or PE 2  of the medium P in the width direction, both of the side edge positions PE 1  and PE 2 , the medium width (includes the medium size defined from the medium width), and the printing area in the width direction X. For example, the medium information may be only one of, one side edge position, both side edge positions, the medium width, and the printing area. In addition, the content of the medium information acquired from the result of side edge detection may be different according to a printing mode, and there may exist a printing mode in which the side edge detection process is not performed. In the case of detecting only one side edge, the carriage  82  is required to be capable of moving in a moving range in which both side edges of a medium having the maximum width can be detected, provided that one side edge of the medium is detected in a certain printing mode and the other side edge of the medium is detected in another printing mode.   The sensors  83 A and  83 B may sense at least one of the leading edge and the trailing edge of the medium P in the transport direction Y in addition to a side edge of the medium P. For example, before the medium P is fed to the read position, the carriage  82  is rendered to move from the home position HP, and the sensor  83  is rendered to wait in a position where the leading edge of the medium P in the transport direction Y can be detected. When the medium P is fed, the leading edge thereof is sensed.       

     After the leading edge is sensed, the carriage is rendered to move to one side (for example, the right side) in the width direction X to sense one side edge, and next the carriage is rendered to move to the other side (for example, the left side) in the width direction X to sense the other side edge. For example, the control unit  120  may control the timing of starting a predetermined operation such as the unskewing operation for the medium based on leading edge sensing information acquired by sensing the leading edge of the medium P. In this case, for example, the sensor  79  can be removed. In addition, the control unit  120  may recognize the position of the medium P in the transport direction Y (transport position) based on the leading edge sensing information and control the timing of starting printing by the printing head  34 .
         In a case where the sensor  83  is configured of an optical sensor, the optical sensor is not limited to a light reflective type and may be a light transmissive type. For example, an illuminant that can move along with the sensor  83  or an illuminant of a line shape that can be turned on in the range across the moving range of the sensor may be arranged in a position facing the medium detection device  80  with the transport path interposed therebetween, and a side edge of the medium may be detected by switching between a light reception state where light from the illuminant is received and a light non-reception state where light is blocked by the medium and cannot be received.   The sensor  83  may be a contact type sensor instead of an optical sensor. Even if the sensor  83  is a contact type sensor, a side edge of the medium can be detected.   The source of motive power of the medium detection device  80  may be configured of a direct current motor (DC motor) instead of a stepping motor. In the case of a DC motor, for example, a linear encoder or a rotary encoder that can output pulse signals of which the number is proportional to the distance of movement of the carriage  82 , and a counter that can count a value indicating the position of the carriage  82  by counting pulse edges of the pulse signals output by the encoder may be disposed. The control unit may acquire a side edge position that is detected by the sensors  83 A and  83 B based on the count value of the counter.   One window portion may be arranged in a region corresponding to the moving path of a sensor instead of arranging the window portion  88  in plural numbers in the width direction X. In addition, the number of the window portions  88  arranged in the width direction X is not limited to two and may be three or more such as three or four. In this case, a plurality of window portions is preferably arranged in a position where both side edges of a plurality of types of the media P having different widths can be detected. For example, a plurality of window portions is preferably arranged in a position in which the position and the width center (center line) of the medium P transported are symmetric about the width direction X.   The inversion feed path  56  (or the third feed path  93 ) may be an inversion path that joins a joint portion via the opposite side (lower side) of the transport path  30  from the printing head  34  side, instead of being an inversion path that joins a joint portion via the printing head  34  side (upper side) of the transport path  30 .   The movable distance of the carriage  82  may be extended by a length in which both side edges PE 1  and PE 2  of the medium P having the maximum width can be detected by only one sensor of two sensors, and the number of sensors used in accordance with the width of the medium P may not be switched. In this case, a process of comparing the width based on the width information with the set width can be removed. In addition, both side edges of a medium can be detected by only one sensor, and at the time of failure, both side edges of a medium can be detected by only the other sensor.   The medium detection device  80  may be arranged in a position where the read position thereof is upstream of the second path joint portion J 2  in the transport direction Y and downstream of the second joint portion  76  in the transport direction Y. In this configuration as well, the medium detection device  80  in the read position is separated from the third feed path  93  along which the medium P is refed after printing is performed on one side thereof. Thus, ink of printing performed on one side is unlikely to cling to the window portion  88 . Accordingly, a decrease in the side edge detection accuracy due to this type of ink staining is easily prevented.   The medium detection device  80  may be arranged in a position where the read position thereof is upstream of the first path joint portion J 1  in the transport direction Y and downstream of the first joint portion  75  in the transport direction Y. In this configuration as well, the medium P that is fed through any of the two feed paths  45  and  48  is read by one common medium detection device  80 , and a side edge thereof can be detected by the medium detection device  80 .       

     In this case, the different feed paths  91  and  92  of a medium comparatively closely pass through the joint location in the first joint portion  75 . Thus, if the sensor  83  that has a comparatively long detectable distance is selected, required side edge detection accuracy can be secured.
         The medium detection device  80  may be arranged in a position where the read position thereof is upstream of the first joint portion  75  in the transport direction Y. In this case, if the medium detection device  80  is arranged for each of the plurality of feed paths  91  and  92 , a side edge of each medium P passing through the different feed paths  91  and  92  can be detected.       

     In addition, the medium detection device  80  may detect a side edge of only a medium that is fed along one feed path of the plurality of feed paths  91  and  92 .
         The medium detection device  80  may be arranged in a position where the read position thereof is downstream of the second path joint portion J 2  in the transport direction Y. For example, the medium detection device  80  may be arranged downstream of the transport roller pair  46  in the transport direction Y and upstream of the most upstream nozzle of the printing head  34  in the transport direction Y. Furthermore, the medium detection device  80  may be arranged in a position where the medium P after printing can be read. In addition, a plurality of the medium detection device  80  may be disposed in such a manner that a side edge of the medium P can be detected at a plurality of locations on the transport path thereof.   Only a part of the first feed unit  41  may be disposed in the cover  23 , only a part of the second feed unit  42  may be disposed in the cover  23 , or a part of the first feed unit  41  and a part of the second feed unit may be disposed in the cover  23 . In addition, the cover may not include the feed tray  22  as one example of a medium mount unit.   The medium mount unit may be only one of the cassette  21  and the feed tray  22 . In addition, the cassette  21  is not necessarily in plural numbers, and only one cassette  21  may be disposed. Furthermore, the number of feed trays is not necessarily one, and the feed tray may be disposed in plural numbers.   The printing apparatus is not limited to a line printing type printing apparatus (line printer) or a serial printing type printing apparatus (serial printer) and may be a lateral printing type printing apparatus (lateral printer) in which the carriage can move in two directions of a main-scanning direction and a sub-scanning direction.       

     If the invention is applied to this type of serial printer or lateral printer, the size dimensions of the printing apparatus in the width direction X and the transport direction Y can be comparatively decreased in the case of disposing the medium detection device.
         Each functional unit built in the control unit is not necessarily realized by software by the computer executing a program and may be realized by hardware by an electronic circuit such as a field-programmable gate array (FPGA) or an application-specific IC (ASIC) or may be realized by cooperation of software and hardware.   The medium is not limited to paper and may be configured of a film or sheet made of resin, a composite film of resin and metal (laminated film), fabric, non-woven fabric, metal foil, a ceramic sheet, or the like.   The printing apparatus is not limited to a multifunction peripheral and may be a printer that includes a printer unit and does not include a scanner unit.   The printing apparatus is not limited to an ink jet type printer and may be a dot impact type printer, a thermal transfer type printer, or an electrophotographic printer.   The printing apparatus is not limited to a printing apparatus performing printing on a medium such as paper and may be a 3D printer that discharges liquid resin drops to a medium configured of a base and the like of a component to form a three-dimensional object. In this type of printing apparatus as well, the size dimension of the apparatus main body can be comparatively decreased, and a high accuracy three-dimensional object can be formed on a medium such as a base.       

     The entire discovery of Japanese Patent Application No. 2016-026142, filed Feb. 15, 2016 is expressly incorporated by reference herein.