Patent Publication Number: US-7722026-B2

Title: Sheet conveyer device and image forming apparatus with error judging system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2008-100859 filed on Apr. 8, 2008, the entire subject matter of which is incorporated herein by reference. 
   BACKGROUND 
   1. Technical Field 
   Aspects of the present invention relate to a sheet conveyer device having a pair of discharge rollers, which nip a sheet therebetween and rotate to discharge the sheet out of a conveyer path, and an image forming apparatus having the sheet conveyer device. 
   2. Related Art 
   When an instruction for forming an image on a recording sheet is entered in an image forming apparatus (e.g., an inkjet printer), a feeder roller being pressed onto a surface of the recording sheet, which is stored in a sheet feed tray, rotates to direct the sheet into the sheet conveyer path. Along the sheet conveyer path in the image forming apparatus, several components for forming images are installed. Specifically, recording heads to record images on the recording sheet, a pair of conveyer rollers, and a pair of discharge rollers are provided. The conveyer rollers are arranged on an upstream side in the sheet conveyer path with respect to the recording heads, whilst the discharge rollers are arranged on a downstream side. The pair of conveyer rollers includes a driving roller, which is rotated by driving force provided by a drive source (e.g., a motor), and a subsidiary roller, which is pressed onto the driving motor and rotated according to the rotation of the driving roller. Similarly, the pair of discharge rollers includes a driving roller and a subsidiary roller. The driving rollers of the conveyer roller pair and the discharge roller pair are often connected to a single drive source and rotated in synchronization with each other. The recording sheet is carried in the sheet conveyer path by rotation of at least one of the conveyer roller pair and the discharge roller pair. The recording heads eject inks onto the surface of the recording sheet whilst the recording sheet is carried in the sheet conveyer path. 
   Japanese Patent Provisional Publication No. H09-58912 discloses a sheet discharging device for an image forming apparatus as above. The sheet discharging device specifically has a sheet sweeping rotary body, which is, according to the publication, a flexible and resilient piece (e.g., sponge) formed to have a shape of a gear. The sheet sweeping rotary body is attached onto a shaft of the driving roller of the discharge roller pair through an intervening one-way clutch. When the discharge rollers rotate in a direction to discharge the recording sheet into a discharge tray, the sheet sweeping rotary body rotates in the same discharging direction along with the driving roller of the discharge roller pair. When the sheet sweeping rotary body rotates, teeth in the gear can catch a rear end of the recording sheet so that the recording sheet is forwarded and swept out of the discharge rollers. When the discharge rollers rotate in a reverse direction which is opposite from the discharging direction, on the other hand, the sheet sweeping rotary body is not rotated due to an effect of the one-way clutch. Therefore, the recording sheet once discharged in the discharge tray is securely stopped by the discharge rollers from being reversely retracted once again between the discharge rollers, and reverse conveyance to convey the recording sheet in the reverse direction can be prevented. 
   Meanwhile, in the image forming apparatus, skew orientation of the recording sheet with respect to the conveyer path can be straightened by an operation so called a reverse registering operation. That is, when a feeder roller performs a feeding operation to pick up and feed the recording sheet into the sheet conveyer path, the conveyer rollers and the discharge rollers are rotated in the reverse directions so that the recording sheet conveyed to the conveyer roller pair is pushed back by the reverse rotation of the conveyer roller pair toward the upstream side of the sheet conveyer path. Thus, a front end of the recording sheet is halted at the conveyer roller pair and straightened with respect to the sheet conveyer path. 
   SUMMARY 
   Occasionally, the image forming apparatus may be obliged to have a small height difference between a top level of the discharge tray and a level of the discharge roller pair due to a volume limitation of the image forming apparatus, specifically when the entire image forming apparatus is configured to be small in height. Therefore, even when a small amount (e.g., several dozen) of discharged recording sheets are stacked in the discharge tray, a topmost sheet in the discharged sheet stack may be in a level barely lower than the level of the discharge roller pair. When a newly conveyed recording sheet is discharged on the stacked sheets, a front end of the recording sheet being newly discharged is subject to relatively large friction resistance caused by the topmost sheet in the sheet stack, and the recording sheet may halt, immediately upon completion of the discharging operation, with a rear end thereof approximated to the discharge rollers. When a next feeding operation starts, in synchronization with the reverse registering, with the rear end of the previously discharged recording sheet approximated to the discharge rollers, the previously discharged recording sheet may be retracted between the discharge rollers and reversely carried in the conveyer path. When the rear end of the previously discharged recording sheet is thus close to the discharge roller pair, preventing the reverse conveyance of the once discharged recording sheet is yet difficult, even with the sheet sweeping rotary body provided to the driving roller of the discharge roller pair. Specifically, it is to be noted that the reverse conveyance is likely to occur upon a sheet feeding error, for example, in which the feeder roller slips on the topmost recording sheet stacked in the sheet feed tray, and in which the recording sheet is jammed in the sheet conveyer path. When the feeding operation resumes after an error cause is removed, the recording sheet left in the sheet conveyer path may be reversed toward the upstream side further from the conveyer rollers and interfere a next feeding operation. Therefore, removal of the recording sheet remaining in the sheet conveyer path is required prior to the next feeding operation so that the remaining recording sheet should not interfere a succeeding recording sheet fed in the next feeding operation. 
   Additionally, when the reverse conveyance occurs in a scanner device having an ADF (auto document feeder), and a once discharged original document is withdrawn in a sheet conveyer path according to reverse registering of the original documents, a next original document to be scanned may be interfered with the reversed document in the sheet conveyer path, and an image on the next original document may not be scanned correctly. 
   In view of the above drawbacks, the present invention is advantageous in that a sheet conveyer device capable of preventing a reversely conveyed sheet from interfering a next feeding operation, which is resumed after a sheet feeding error, and an image forming apparatus having the sheet conveyer device are provided. 
   According to an aspect of the present invention, a sheet conveyer device to convey a sheet in a conveyer path is provided. The sheet conveyer device includes a first roller pair, which is arranged in an intermediate position in the conveyer path and conveys the sheet in the conveyer path, a feeder to feed the sheet in the conveyer path toward the first roller pair during a feeding operation, a discharge portion, on which the sheet carried through the conveyer path is discharged, a second roller pair, which is arranged in the conveyer path in a position closer to the discharge portion than the first roller pair and conveys the sheet in the conveyer path, a control unit to control the first roller pair and the second roller pair to rotate synchronously in one of a first direction, being a direction to convey the sheet toward the discharge portion, and a second direction, being a direction opposite from the first direction, and a judging system to judge as to whether a sheet feeding error has occurred in the feeder. The control unit controls the first roller pair and the second roller pair to rotate synchronously in the second direction during the feeding operation and to rotate synchronously in the first direction when the judging system judges that the sheet feeding error has occurred during the feeding operation. 
   According to the above configuration, when the judging system judges that a feeding error has occurred in the feeder during the feeding operation, the first roller pair and the second roller pair having been rotated synchronously in the second direction are switched to rotate in the first direction. Therefore, when the sheet having been conveyed in the second direction is present in the conveyer path, the sheet is discharged out of the conveyer path by the second roller pair. Accordingly, the feeding operation resumed after the feeding error can be performed without being interrupted by the sheet which was left in the conveyer path. 
   According to another aspect of the present invention, an image forming apparatus is provided. The image forming apparatus includes a sheet conveyer device to convey a sheet in a conveyer path and an image forming unit to form an image on the sheet conveyed in the conveyer path. The sheet conveyer device includes a first roller pair, which is arranged in an intermediate position in the conveyer path and conveys the sheet in the conveyer path, a feeder to feed the sheet in the conveyer path toward the first roller pair during a feeding operation, a discharge portion, on which the sheet carried through the conveyer path is discharged, a second roller pair, which is arranged in the conveyer path in a position closer to the discharge portion than the first roller pair and conveys the sheet in the conveyer path, a control unit to control the first roller pair and the second roller pair to rotate synchronously in one of a first direction, being a direction to convey the sheet toward the discharge portion, and a second direction, being a direction opposite from the first direction, and a judging system to judge as to whether a sheet feeding error has occurred in the feeder. The control unit controls the first roller pair and the second roller pair to rotate synchronously in the second direction during the feeding operation and to rotate synchronously in the first direction when the judging system judges that the sheet feeding error has occurred during the feeding operation. The image forming unit is arranged along the conveyer path between the first roller pair and the second roller pair. 
   According to the above configuration, the feeding operation resumed after the feeding error can be performed without being interrupted by the sheet which was left in the conveyer path. Therefore, an image can be effectively formed on the sheet which is fed in the resumed feeding operation. 

   
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
       FIG. 1  illustrates an external and perspective view of an MFD (multi-function device)  10  according to an embodiment of the present invention. 
       FIG. 2  illustrates an internal configuration of a printer unit  11  of the MFD according to the embodiment of the present invention. 
       FIG. 3  illustrates a drive force transmission system  90  in the MFD  10  according to the embodiment of the present invention. 
       FIG. 4  is a block diagram to illustrate a control unit  100  in the MFD  10  according to the embodiment of the present invention. 
       FIG. 5  is a flowchart to illustrate a feeding operation to feed a recording sheet in the printer unit  11  in the MFD  10  according to the embodiment of the present invention. 
       FIG. 6  illustrates an internal configuration of the printer unit  11  according to a modified embodiment of the present invention. 
       FIG. 7  is a flowchart to illustrate a feeding operation to feed a recording sheet in the printer unit  11  in the MFD  10  according to the modified embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Hereinafter, embodiments according to the present invention will be described with reference to the accompanying drawings.  FIG. 1  illustrates an external and perspective view of the MFD  10  according to the embodiment of the present invention.  FIG. 2  illustrates an internal configuration of a printer unit  11  of the MFD  10  according to the embodiment of the present invention, although a part of a feed tray  20  and a part of a discharge tray  21  are omitted and not shown. First, an overall configuration of an MFD  10  according to the embodiment of the present invention will be described. 
   The MFD  10  according to the present embodiment is configured integrally with a printer unit  11  and a scanner unit  12  and provided with functionalities for printing, scanning, and facsimile transmission. However, the MFD  10  may not necessarily be equipped with the scanner unit  12 , but the MFD  10  may be replaced with a printer device having solely a printing function. 
   A body of the MFD  10  according to the present invention has an approximate shape of a box with a width and a depth being respectively greater than a height. In  FIG. 1 , directions of height, width, and depth of the MFD  10  are as indicated by arrows  121 ,  122 , and  123  respectively. 
   The printer unit  11  is provided in a bottom portion of the MFD  10 . The printer unit  11  is configured to form an image on a recording sheet  50  according to print data, which can be entered through the scanner unit  12  and from an external environment (not shown). 
   The scanner unit  12  being a flatbed scanner is provided at an upper portion of the MFD  10 , and a topmost portion includes a document cover  15 , which is openable/closable with respect to an original document holder  19  of the MFD  10 . The scanner unit  12  includes a contact glass (not shown) and an image sensor (not shown) at a topmost surface of the original document holder  19 . The contact class is a flat and transparent glass plate on which an original document to be scanned is placed. The image sensor is a line sensor, which extends in the direction of depth  123  and is installed underneath the contact glass. The image sensor is capable of being reciprocated in the direction of width  122  so that, when an original document with an image formed thereon is set on the contact glass and a scanning operation is started, the image is read by the image sensor. 
   The document cover  15  makes a top level of the MFD  10  and includes an ADF  29 . The ADF  29  transports original documents stacked in the original document tray  30  to a document discharge tray  31  through a conveyer path (not shown). Whilst being transported by the ADF  29 , each of the original documents passes over the contact glass so that the image sensor can read an image formed on the original document through the contact glass. The original documents passed over the contact glass are held by the document discharge tray  31  at opposing sides thereof so that the discharged original documents can be held separately from the unread original documents on the original document tray  30 . 
   The MFD  10  is provided with an operation panel  14  in an upper front portion thereof. (In the present embodiment, a near left side in  FIG. 1  corresponds to the front of the MFD  10 .) The operation panel  14  includes a display, which presents various information concerning operations, and keys, through which a user enters information and instructions. The MFD  10  operates according to the information and instructions entered through the operation panel  14  and transmitted from external devices. 
   Next, the printer unit  11  will be described. The printer unit  11  includes a sheet cassette  33 , which is configured to be inserted into the printer unit  11  through an opening  13 . The sheet cassette  33  includes a detachable feed tray  20  and a discharge tray  21 , and the discharge tray  21  is disposed above the feed tray  20  in the sheet cassette  33 . The feed tray  20  is a substantially box-shaped container to store a stack of recording sheets  50  (see  FIG. 2 ) therein. The feed tray  20  accepts various sizes of the recording sheets  50 , e.g., letter, legal, and postcard, to store therein. Meanwhile, the discharge tray  21  is a substantially flat plate having a depthwise length in the direction of depth  123  smaller than a depthwise length of the feed tray  20 . The sheet cassette  20  with the feed tray  20  and the discharge tray  21  is arranged on the front side (i.e., a left-hand side in  FIG. 2 ) of the MFD  10 , and the feed tray  20  is partially open-ended at a side closer to the rear (i.e., a right-hand side in  FIG. 2 ) of the MFD  10 . The recording sheets  50  stored in the feed tray  20  are fed and carried in the paths  23 ,  27  to the printer unit  11 , in which images are formed thereon, and discharged out of the path  27 . The discharged recording sheets  50  are settled and piled up on the discharge tray  21 . Various materials of the recording sheets  50  may be accepted in the sheet cassette  33 , for example, standard paper, gloss paper, and inkjet-printer paper. 
   The printer unit  11  according to the present embodiment includes a feeder unit  32 , a conveyer path including paths  23 ,  27 , a conveyer roller pair  59 , a discharge roller pair  64 , an image forming unit  24 , in addition to the feed tray  20  and the discharge tray  21 . 
   The paths  23 ,  27  are courses in which the recording sheets  50  fed by the feed tray  20  are conveyed. The path  23  starts in the vicinity of a slope board  22  and includes a curve arched upward to orient the front of the MFD  10 . The path  27  is continuous from the path  23  and extends linearly along the image forming unit  24  toward the discharge tray  21 . 
   The slope  22  is arranged at a rear end (i.e., right-hand end) of the feed tray  20  in an inclined orientation to lean toward the rear of the MFD  10 . When the feed tray  20  is installed in the printer unit  11 , the slope board  22  comes in a position below a point where the path  23  starts. The recording sheets  50  stored in the feed tray  20  are fed along the slope bode  22  into the path  23 . 
   The feeder unit  32  is provided substantially above the feed tray  22 . The feeder unit  32  feeds the recording sheets  50  in the feed tray  20  into the path  23 , through the conveyer roller pair  59 , and into the feed tray  27 . The feeder unit  32  includes a feed roller  25 , an arm  26 , and a shaft  28 . The feed roller  25  is configured to come in contact with a topmost surface of the recording sheets  50  stored in the feed tray  20  and picks up at least the topmost recording sheet  50  to feed. The arm  26  is supported by a frame (not shown) of the printer unit  11  and rotatable about the shaft  28 . The arm  26  is pivoted to orient in a direction to droop downward by weights of the arm  26  itself and the feed roller  25  and/or expanding force of a resilient member (not shown) such as a spring. Thus, the feed roller  25  is pressed onto the topmost recording sheet  50  in the feed tray  20  with effectual pressure. With the feed roller  25  pressed onto the recording sheet  50 , driving force generated in a motor  84  (see  FIG. 4 ) is transmitted to the feed roller  25  through the shaft  28  and a transmitting system (not shown) in the arm  26  so that the feed roller  25  rotates. The recording sheet  50  in contact with the feed roller  25  is therefore picked up according to rotation of the feed roller  25  and fed along the inclination of the slope board  22  into the path  23 . The slope board  22  has a separator piece (not shown), which separates the topmost recording sheet from succeeding recording sheets  50 . 
   In the printer unit  11 , a platen  42  to hold the recording sheet  50  substantially flat in the path  27  is provided in a position below the path  27  to oppose a lower surface of the image forming unit  24 . Thus, the recording sheet is maintained substantially flat on the platen  24  to have the image recorded thereon by the image forming unit  24  with a predetermined clearance from the lower surface of the image forming unit  24 . 
   The pair of conveyer rollers  59  is provided in an intermediate position in the path  27  on the upstream side with respect to a conveying direction  17  than the image forming unit  24 . The conveyer roller pair  59  includes a conveyer roller  60  and a pinch roller  61 , which convey the recording sheet  50  in the path  27 . In the present embodiment, the conveyer roller  60  is arranged on an upper side of the path  27 , whilst the pinch roller  61  is arranged below the path  27  to oppose the conveyer roller  60 . The conveyer roller  60  is rotated by driving force of a motor  85  (see  FIG. 4 ) through a drive force transmission system  90  (see  FIG. 3 ). The pinch roller  61  is biased toward and pressed onto the conveyer roller  60 , therefore rotatable along with the rotation of the conveyer roller  60 . When the recording sheet  50  is forwarded in the path  27  between the conveyer roller  60  and the pinch roller  61 , the pinch roller  61  is shifted away from the conveyer roller  60  for a thickness amount of the recording sheet  50 . Still the pinch roller  61  is biased toward the conveyer roller  60  so that the rotation of the conveyer roller  60  is transmitted to the recording sheet  50 . The recording sheet  50  is thus nipped with the conveyer roller  60  and the pinch roller  61  and carried to the position above the platen  42 . 
   The pair of discharge rollers  64  is arranged in the path  27  on the downstream side with respect to the conveying direction  17  than the image forming unit  24  in the vicinity of the discharge tray  21 . That is, the discharge roller pair  64  is arranged in the path  27  in a position closer to the discharge tray  21  than the conveyer roller pair  59 . The discharge roller pair  64  includes a discharge roller  62  and a spur  63 , which carry the recording sheet  50  in the path  27 . In the present embodiment, the discharge roller  62  is arranged on the lower side of the path  27 , whilst the spur  63  is arranged on the upper side of the path  27  to oppose the discharge roller  62 . The discharge roller pair  64  is separated from the conveyer roller pair  59  to have a length L therebetween in the conveying direction  17  so that the image forming unit  24  can be arranged within the length L. Further, according to the present embodiment, the discharge roller  62  is formed to have a circumference equivalent to a circumference of the conveyer roller  60 . The discharge roller  62  is rotated by driving force of the motor  85  through the drive force transmission system  90 . The spur  63  is biased toward and pressed onto the discharge roller  62 , therefore rotatable along with the rotation of the discharge roller  62 . When the recording sheet  50  is forwarded in the path  27  between the discharge roller  62  and the spur  63 , the spur  63  is shifted away from the discharge roller  62  for the thickness amount of the recording sheet  50 . Still the discharge roller  62  is biased toward the discharge roller  62  so that the rotation of the discharge roller  62  is transmitted to the recording sheet  50 . The recording sheet  50  is thus nipped with the discharge roller  62  and the spur  63  and forwarded to the discharge tray  21 . 
   The drive force transmission system  90  transmits the driving force generated in the motor  85  to the conveyer roller  60  and the discharge roller  62 .  FIG. 3  illustrates a drive force transmission system  90  in the MFD  10  according to the embodiment of the present invention. The drive force transmission system  90  includes a motor gear  91 , a gear  92 , a connection gear  95 , a pulley  94 , a belt  98 , and a pulley  93 . In  FIG. 3 , the conveyer roller  60  and the discharge roller  62  are omitted. Further, the motor gear  91 , the gear  92 , the connection gear  95 , the pulley  94 , the belt  98 , and the pulley  93  are represented in circles, and teeth formed on those are omitted. 
   In the drive force transmission system  90 , the motor gear  91  is coupled to the motor  85 . The gear  92  is fixed to a shaft  76  (see  FIG. 2 ) of the conveyer roller  60  and rotates about the shaft  76  along with the conveyer roller  60 . The gear  92  is engaged with the motor gear  91 . The gear  92  is also engaged with the connection gear  95 , which is arranged in the vicinity of the gear  92  and rotatable about a shaft  77 . The pulley  93  is fixed to a shaft  78  (see  FIG. 2 ) of the discharge roller  62  and rotatable about the shaft  78  along with the discharge roller  62 . The belt  98 , which is an endless belt with teeth formed inside, is extended between the pulley  93  and the pulley  94 . The pulley  94  is fixed to the shaft  77  of the connection gear  95 . 
   When the driving force generated in the motor  85  is transmitted to the gear  92  through the motor gear  91 . Accordingly, the conveyer roller  60  is rotated. Further, the driving force transmitted to the gear  92  is conveyed to the pulley  93  through the connection gear  95 , the pulley  94 , and the belt  98 . Accordingly, the discharge roller  62  is rotated. Thus, the conveyer roller  60  and the discharge roller  62  are rotated synchronously by a same drive source, i.e., the motor  85 . Moreover, the discharge roller  62  and the spur  63  are rotated in synchronization with the conveyer roller  60  and the pinch roller  61 . The recording sheet  50  forwarded in the path  27  is therefore conveyed to the position above the platen  43  by the conveyer roller  60  and the pinch roller  61 . Thereafter, the recording sheet  50  is forwarded to the discharge roller pair  64  between the discharge roller  62  and the spur  63  and further forwarded to the position above the discharge tray  21 . When the discharge roller pair  64  releases the rear end of the recording sheet  50 , the recording sheet  50  is discharged in the discharge tray  21 . 
   A height difference between the upper level of the discharge tray  21  and the nipped position of the discharge roller  62  with the spur  63  is determined based on an allowable number of recording sheets to be stacked in the discharge tray  21 . In the present embodiment, a height to allow the discharge tray  21  to hold several tens of recording sheets is set between the upper level of the discharge tray  21  and the nipped position of the discharge roller  62  with the spur  63 . 
   In  FIG. 3 , directions of rotation of the motor gear  91  and the shafts  76 ,  77 ,  78  to convey the recording sheet  50  in the conveying direction  17  are indicated by arrows. As shown in  FIG. 3 , when the driving force of the motor  85  is transmitted to the shaft  76 ,  78  through the drive force transmission system  90 , the shaft  76  of the conveyer roller  60  and the shaft  78  of the discharge roller  62  rotate in the directions opposite from each other. However, in the present embodiment, the discharge roller  62  is arranged on the side opposite from the conveyer roller  60  with respect to the path  27 ; therefore, the direction of the conveyer roller pair  59  to forward the recording sheet  50  coincides with the direction of the discharge roller pair  64  to forward the recording sheet  50 . In the present embodiment, when polarity of the electricity to be supplied to the motor  85  is inversed, the motor gear  91  and the shafts  76 ,  77 ,  78  rotate in directions opposite from the directions indicated by the respective arrows in  FIG. 3 . 
   Next, the image forming unit  24  will be described. As shown in  FIG. 2 , the image forming unit  24  is disposed in along the path  27  between the conveyer roller pair  59  and the discharge roller pair  64 . The image forming unit  24  includes recording heads  39  being inkjet heads and a carriage  38  on which the recording heads  39  are mounted. The carriage  38  is driven to reciprocate in the widthwise direction  122  of the MFD  10  during a printing operation. The recording heads  39  are provided with nozzles, which are directed downward and exposed through a bottom level of the carriage  38 . The recording heads  39  are supplied with inks by ink cartridges (not shown), which are arranged inside the printer unit  11 . The inks in ink drops are ejected from the nozzles of the recording heads  39  downward onto the recording sheets  50  held by the platen  42  whilst the carriage  38  is reciprocated. Thus, an image is formed on the upper surface of the recording sheet  50 . 
   Next, mechanism to detect presence of the recording sheet  50  in the paths  23 ,  27  will be described. As shown in  FIG. 2 , the shaft  76  of the conveyer roller  60  is provided with a disk-shaped encoder disk  35 . Further, an optical sensor  73 , which is arranged to have a circumferential edge of the encoder disk  35  in-between, is provided. The encoder disk  35  is a transparent disk to rotate coaxially along with the conveyer roller  60  and have radiated marks which are arranged in a predetermined pitch. The optical sensor  73  is arranged in the vicinity of the conveyer roller  60 . The optical sensor  73  has a light-emitting element (not shown) and a light-receiving element (not shown) and is arranged to have the light-emitting element and the light-receiving element on either side of the circumferential edge of the encoder disk  35 . When the light emitted from the light-emitting element is received by the light-receiving element, a sensor signal depending on a level of brightness being received is generated. When one of the marks is present in between the light-emitting element and the light-receiving element, a high-leveled sensor signal is generated. On the other hand, when no mark is present in between, a low-leveled sensor signal is generated. The sensor signal generated in the optical sensor  73  is transmitted to a control unit  100  (see  FIG. 4 ). 
   In the printer unit  11 , a register sensor  71  is provided in a position P 1  in the conveyer path including the paths  23 ,  27 . The position P 1  is a position opposite from the discharge roller pair  64  with respect to the conveyer roller pair  59 . In other words, the position P 1  is on the upstream side of the paths  23 ,  27 , closer to the feed tray  20  in the conveying direction  17  than the conveyer roller pair  59 . The register sensor  71  is to detect presence of the recording sheet  50  in the conveyer path. In the present embodiment, the register sensor  71  is a so-called mechanical sensor, which includes a photo-interrupter and a rotatably-supported feeler. The photo-interrupter has a light emitter to emit light toward a light receiver and the light receiver to receive the emitted light. When the recording sheet  50  carried in the path  23  reaches the position P 1 , the recording sheet  50  becomes in contact with the feeler, and the feeler is rotated. Thus, the levels of the sensor signals generated in the register sensor  71  are varied. The generated sensor signals are transmitted to the control unit  100 , which detects presence of the recording sheet  50  in the position P 1 . 
   Next, the control unit  100  to control overall behaviors of the MFD  10  will be described.  FIG. 4  is a block diagram to illustrate the control unit  100  in the MFD  10  according to the embodiment of the present invention. The control unit  100  is a microcomputer which includes a CPU  101 , a ROM  102 , a RAM  103 , an EEPROM  104 , and an ASIC (Application Specific Integrated Circuit)  109 . The control unit  100  controls behaviors of the motors  84 ,  85 , and the printer unit  11 , and the scanner unit  12 . 
   The ROM  102  stores therein various programs to be run to control the behaviors of the motors  84 ,  85 , and the entire MFD  10 . The RAM  103  is a storage area being a memory area, in which various data to be used in the running programs is stored, and a work area for various data processing. In the beginning of a printing operation in the printer unit  11 , the control unit  100  starts counting for an elapsed time period t when the recording sheet  50  starts being fed from the feed tray  20  into the path  23 . In this regard, the elapsed time t is temporarily stored in the RAM  103 . Meanwhile, the EEPROM  104  stores data such as settings and flags concerning the MFD  10  which is to be maintained even after the control unit  100  is powered off. A predetermined set time T is stored in the EEPROM  104 . Behaviors of the MFD  10  based on the elapsed time t and the set time T will be described later in detail. 
   The ASIC  109  is connected with drive circuits  80 ,  81 , the register sensor  71 , and a rotary encoder  83 . Further, although not shown in  FIG. 4 , the ASIC  109  is connected with a head control circuit to control the recording heads  39 , the operation panel  14 , and the scanner unit  12 . Description of these devices controlled by the ASIC  109  is herein omitted. 
   The register sensor  71  generates sensor signals based on presence/absence and a position of the recording sheet  50  in the paths  23 ,  27  so that the control unit  100  judges presence or absence of the recording sheet  50  in the paths  23 ,  27 , as to whether the front end of the recording sheet  50  reached the position P 1 , and as to whether the rear end of the recording sheet  50  has passed the position P 1 . 
   The rotary encoder  83  counts the marks provided on the encoder disk  35  (see  FIG. 2 ) according to the detected result obtained by the optical sensor  73  (see  FIGS. 2 and 4 ) so that a rotation amount of the encoder disk  35  is obtained. Each time when the optical sensor  73  detects the mark on the encoder disk  35 , a high-leveled sensor signal is generated. In the present embodiment, the encoder disk  35  rotates coaxially along with the conveyer roller  60 ; therefore, a rotation amount of the conveyer roller  60  can be obtained based on the rotation amount of the encoder disk  35 . The obtained rotation amount of the conveyer roller  60  is passed to the control unit  100 . Thus, the control unit  100  obtains a rotation speed of the conveyer roller  60 , and the discharge roller  62  as well, based on the rotation amount within a unit of time achieved from the rotary encoder  83 . 
   The drive circuit  80  drives the motor  84 , which is coupled to the feed roller  25  through the shaft  28  and the arm  26 . The drive circuit  80  drives the motor  84  according to signals output from the ASIC  109 . Thus, the recording sheets  50  in the feed tray  20  are fed into the path  23  in a feeding operation. 
   The drive circuit  81  drives the motor  85 , which is coupled to the shaft  76  of the conveyer roller  60  and the shaft  78  of the discharge roller  62  through the drive force transmission system  90 . The drive circuit  81  drives the motor  85  according to signals output from the ASIC  109 . The drive force generated in the motor  85  is thus transmitted to the shafts  76 ,  78  respectively through the drive force transmission system  90 . Therefore, the conveyer roller  60  and the discharge roller  62  are rotated in synchronization with each other. 
   According to the present embodiment, the conveyer roller  60  and the discharge roller are capable of rotating in first and normal directions  111 ,  112  (see  FIG. 2 ) and in second and reverse directions  113 ,  114  respectively under control of the control unit  100 . When the recording sheet  50  is picked up from the sheet tray  20  into the path  23  in the feeding operation, the conveyer roller pair  59  and the discharge roller pair  64  are at the same time rotated in reverse directions in a reverse rotating operation so that the recording sheet  50  reached the conveyer roller pair  59  is pushed back in an opposite direction from the conveying direction  17 , and the orientation of the recording sheet  50  is straightened with respect to the conveyer path. Therefore, in the reverse registering operation, the control unit  100  controls the drive circuit  81  to drive the motor  85  so that the conveyer roller  60  and the discharge roller  62  are rotated in the second and reverse directions  113 ,  114  respectively and synchronously. Thus, the front end of the recording sheet  50  carried in the paths  23 ,  27  is pushed back by the conveyer roller pair  59  and stops. 
   When the reverse registering operation is completed, the control unit  100  controls the conveyer roller  60  and the discharge roller  62  in the normal directions  111 ,  112 , which are opposite from the second directions  113 ,  114  respectively. Thus, the recording sheet  50 , which is nipped with the conveyer roller pair  59  and the discharge roller pair  64 , is carried in the conveying direction  17 . 
   Next, a feeding operation to pick up and feed the recording sheet  50  into the path  23  will be described with reference to  FIG. 5 .  FIG. 5  is a flowchart to illustrate the feeding operation to feed and convey the recording sheet  50  in the printer unit  11  in the MFD  10  according to the embodiment of the present invention. Each step in the flowchart shown in  FIG. 5  is conducted based on instructions issued by the control unit  100  according to a program stored in the ROM  102 . 
   When the feeding operation starts, in S 1 , the control unit  100  examines to judge as to whether an instruction to start printing is entered. Additionally, the control unit  100  examines to judge as to whether an instruction command to start printing along with printable data is transmitted from an external device and received. When no instruction to start printing is received (S 1 : NO), the control unit  100  sets the MFD  10  in a standby state. When an instruction to start printing is received (S 1 : YES), in S 2 , the control unit  100  resets the elapsed time t; i.e., the control unit  100  deletes a value representing the elapsed time t and stored in the RAM. 
   In S 3 , the control unit starts the feeding operation and the reverse rotating operation. More specifically, the control unit  100  rotates the feed roller  25  to feed the recording sheet  50  in the path  23  and at the same time rotates the conveyer roller  60  and the discharge roller  62  in the second directions  113 ,  114  respectively. In S 4 , the control unit  100  times an elapsed time period t since S 3 , i.e., the beginning of the feeding operation by the feed roller  25 . In the present embodiment, occurrence of a feeding error of the recording sheet  50  is judged based on the elapsed time t. The feeding error refers to failure of the feed roller  25  to feed the recording sheet  50  in the path  23  to the conveyer roller pair  59  and may include, for example, slipping and idle rotation of the feed roller  25  on the topmost recording sheet  50  and sheet jam in the path  23 . The control unit  100  examines to judge as to whether the feeding error has occurred based on the elapsed time t; i.e., it is determined that a feeding error has occurred when the elapsed time t is greater than a set time T and the register sensor  71  fails to detect the recording sheet  50  during the elapsed time t. 
   Following S 4 , in S 5 , the control unit  100  examines to judge as to whether the elapsed time t is greater than or equal to the set time T. More specifically, the control unit  100  reads the set time T stored in the EEPROM  104  and compares to the elapsed time t. In the present embodiment, the set time T is longer than a period A, which starts upon beginning of the feeding operation and ends when the recording sheet  50  is assumed to be detected by the register sensor  71  without being interrupted by a feeding error. The period A may be determined, for example, based on a distance between the slope board  22  and the position P 1  in the path  23  and a rotation speed of the feed roller  25  during the feeding operation. 
   In the present embodiment, as has been described above, when several dozen of recording sheets  50  are piled up on the discharge tray  21 , the height difference between the level of the topmost recording sheet  50  on the discharge tray  21  and the nipped position of the discharge roller pair  64  becomes small. When an additional recording sheet  50  is carried in the path  27  and discharged on the discharge tray  21 , the front end of the recording sheet  50  being discharged is subject to relatively large friction resistance caused by the topmost recording sheet  50  in the discharged sheet stack. Specifically, when the recording sheets  50  are gloss papers, the friction resistance becomes notably larger than a regular sheet of paper. Therefore, the rear end of the newly discharged recording sheet is approximated to the discharge roller  62  upon completion of the discharging operation. When the reverse registering operation is conducted, i.e., the conveyer roller  60  and the discharge roller  62  are rotated in the second directions  113 ,  114 , with the rear end of the newly discharged recording sheet close to the discharge roller  62 , the newly discharged recording sheet  50  may be caught in between the discharge roller pair  64  to be withdrawn in the path  27 . 
   In this regard, when the set time T is longer than necessary, and when the newly-discharged recording sheet  50  on the discharge tray  21  is withdrawn in the path  27  immediately after start of rotation of the conveyer roller  60  in the second direction  113  and the discharge roller  62  in the second direction  114 , the reversely carried recording sheet  50  may reach the conveyer roller pair  59  before the elapsed time t reaches the set time T. When the withdrawn recording sheet  50  is carried reversely further on the upstream side with respect to the conveyer roller  59  to reach the position P 1 , the register sensor  71  will detect the reversely carried recording sheet  50 . In other words, the resister sensor  71 , which is to detect the recording sheet  50  to be carried in the direction  17  from the feed tray  21  toward the conveyer roller pair  59 , can undesirably detect the reversely carried recording sheet  50 . In this occasion, the control unit  100  may not be able to correctly judge occurrence of a feeding error during the feeding operation with the feed roller  25 . In consideration of the above, the set time T is required to be longer than the period A, and shorter than a period B, which starts upon rotation of the conveyer roller  60  and the discharge roller  62  to reversely convey the recording sheet  50  on the discharge tray  21  and ends when the reversely conveyed recording sheet  50  is assumed to reach the conveyer roller pair  59 . The period B may be determined, for example, based on the length L (see  FIG. 2 ) between the conveyer roller  60  and the discharge roller  62  and a rotation speed of the conveyer roller  62  (and the discharge roller  62 ), which can be obtained based on the result detected by the rotary encoder  83 . 
   In S 5 , when the control unit  100  determines that the elapsed time t is smaller than the set time T (S 5 : NO), in S 6 , the control unit  100  judges as to whether the front end of the recording sheet  50  is detected at the position P 1  based on the level of the sensor signal, which is output from the register sensor  71 . When the front end is not detected at the position P 1  (S 6 : NO), the control unit  100  returns to S 5 . In the present embodiment, as has been described above, the set time T is longer than the period A; therefore, when the elapsed time t is greater than or equal to the set time T and when no recording sheet  50  is detected by the register sensor  71  since the feeding operation with the feed roller  25  started (S 5 : YES), the control unit  100  determines that a feeding error has occurred and proceeds to S 12 . 
   When the front end is detected (S 6 : YES), the control unit  100  judges that no feeding error has occurred in the feeding operation with the feed roller  25  and proceeds to S 7 . In S 7 , the control unit  100  judges as to whether the elapsed time t is greater than or equal to the set time T. When the elapsed time t is greater than or equal to the set time T (S 7 : YES), the control unit  100  determines that a feeding error has occurred and proceeds to S 12 . 
   When the elapsed time t is smaller than the set time T (S 7 : NO), in S 8 , the control unit  100  judges as to whether the feed roller  25  has been driven to rotate for a predetermined amount (or a predetermined rotation angle), by which the recording sheet  50  could be carried beyond the conveyer roller pair  59 , after the affirmative judgment in S 6  (S 6 : YES); i.e., detection of the recording sheet  50  at the position P 1  by the register sensor  71 . Within the predetermined amount of rotation (or the predetermined rotation angle), according to the present embodiment, it is assumed that the recording sheet  50  could be transported beyond the conveyer roller pair  59  in the path  27 ; however, the front end of the recording sheet  50  comes in contact with the conveyer roller pair  59 , which is rotated in the reverse rotating operation activated in S 3 . Therefore, the recording sheet  50  cannot be carried beyond the conveyer roller pair  59  but pushed backwards with respect to the conveying direction  17  toward the upstream side of the paths  23 ,  27 . 
   In S 8 , when the control unit  100  determines that the feed roller  25  has been driven to rotate for the predetermined amount, by which the recording sheet  50  could be carried beyond the conveyer roller pair  59  (S 8 : NO), the control unit  100  returns to S 7 . When the feed roller  25  has been driven to rotate for the predetermined amount (S 8 : YES), in S 9 , the control unit  100  controls the drive circuits  80 ,  81  to inactivate the motors  84 ,  85  so that the feeding operation and the reverse rotating operation activated in S 3  are ceased. 
   In this regard, the conveyer roller  60  and the discharge roller  62  have been rotated in the second directions  113 ,  114  until the rotation is ceased in S 9 . Therefore, the recording sheet  50  picked up from the feed tray  20  and carried in the path  23  is pushed backwards with respect to the conveying direction  17  toward the upstream side of the path  23  by the conveyer roller pair  59  in the reverse registering operation during the predetermined amount of rotation, in which the recording sheet  50  could be carried beyond the conveyer roller pair  59  toward the downstream side of the conveying direction  17 . Due to the backward pressure by the conveyer roller pair  59  in the reverse registering operation, the orientation of the recording sheet  50  can be corrected with respect to the conveying direction  17  even when the recording sheet  50  is gloss paper with firmness. 
   Following S 9 , in S 10 , the control unit  100  controls the drive circuit  81  to activate the motor  85  and rotate the conveyer roller  60  and the discharge roller  62  in the first and normal directions  111 ,  112  respectively. Thus, the recording sheet  50  is nipped with the conveyer roller pair  59  and the discharge roller pair  64  and carried in the paths  23 ,  27  in the conveying direction  17 . When the recording sheet  50  passes above the platen and below the image forming unit  24 , an image is formed on the upper surface thereof, and the recording sheet  50  is discharged on the discharge tray  21 . 
   In S 11 , the control unit  100  examines to judge as to whether printable data for a next page is present and stored in the RAM  103 . When the printable data for the next page is present (S 11 : YES), the control unit  100  returns to S 2 . When no printable data for a new page is present (S 11 : NO), the control unit  100  terminates the flow. 
   In the present embodiment, when the control unit  100  judges that the feeding error has occurred (S 5 : YES), the feeding operation is paused and resumed. That is, rotation of the feed roller  25  is stopped and resumed so that the feed roller  25  retries to pick up the topmost recording sheet  50  in the feed tray  20  and the slipping on the topmost recording sheet  50  is resolved. Thus, the topmost recording sheet  50  can be fed into the path  23 . 
   In the present embodiment, however, according to the reverse rotating operation of the conveyer roller  60  and the discharge roller  62 , which is conducted at the same time when the feed roller  25  rotates in the feeding operation, the recording sheet  50 , which has been withdrawn from the discharge tray  21  and carried in the reverse direction, may remain in the path  27  due to a feeding error occurred during the feeding operation by the feed roller  25 . In this occasion, when the feeding operation by the feed roller  25  is resumed to stop idle rotation of the feed roller  25  on the recording sheet  50 , the recording sheet  50  once withdrawn and remaining in the path  27  is further carried in the reverse direction for a second time. Therefore, the recording sheet  50  is reversely carried twice for a period longer than the set time T. Accordingly, in the second feeding operation, the reversely carried recording sheet  50  may reach the conveyer roller pair  59  before the recording sheet  50  fed by the feed roller  25  reaches the conveyer roller pair  59 . Thus, the reversely carried recording sheet  50  may interfere the feeding operation and the reverse registering operation of the recording sheet  50  fed by the feed roller  25 . 
   In consideration of the above, in the present embodiment, when the control unit  100  determines that the elapsed time t is greater than or equal to the set time T (i.e., a feeding error has occurred) (S 5 : YES), in S 12 , the control unit  100  controls the drive circuits  80 ,  81  to inactivate the motors  84 ,  85  so that the feeding operation and the reverse rotating operation activated in S 3  are ceased. Thereafter, in S 13 , the control unit  100  controls the drive circuit  81  to activate the motor  85  and rotate the conveyer roller  60  and the discharge roller  62  in the first and normal directions  111 ,  112  respectively. In the above steps S 5 , S 12 , and S 13 , the control unit  100  rotates the conveyer roller  60  and the discharge roller  62  in the first and normal direction  111 ,  112  simultaneously upon occurrence of a feeding error. Therefore, even when the withdrawn recording sheet  50  is present in the path  27 , the withdrawn recording sheet  50  is carried in the conveying direction  17  to be discharged on the discharge tray  21 . Following the discharging operation in S 13 , the control unit  100  returns to S 2 , in which a feeding operation is resumed. 
   When the once withdrawn recording sheet  50  is carried in the discharging operation in S 13 , no image forming operation is conducted to the withdrawn recording sheet  50  when the withdrawn recording sheet  50  passes above the platen  42 , since the withdrawn recording sheet  50  has been already through the image forming operation. Further, the control unit  100  executes S 12  and S 13  following the affirmative judgment in S 7  (S 7 : YES). This is because of that a recording sheet  50  once discharged on the discharge tray  21  may be withdrawn in the path  27  and carried to reach the conveyer roller pair  59  even without a feeding error. 
   According to the above embodiment, the conveyer roller  60  and the discharge roller  62  having been rotated in the second directions  113 ,  114  are rotated in the first directions  111 ,  112  upon judgment of the control unit  100  that the feeding error has occurred. Therefore, the reversely carried recording sheet  50  remaining in the path  27  is discharged by the discharge roller pair  64  onto the discharge tray  21 . Thus, the feeding operation resumed after a feeding error and the reverse registering operation can be prevented from being interfered by the reversely carried recording sheet  50  remaining in the path  27 . 
   Further, according to the above embodiment, the conveyer roller  60  and the discharge roller  62  are rotated in the first directions  111 ,  112  when the elapsed time t is greater than or equal to the set time T. Therefore, when the set time T is set to have an effective length, the feeding operation resumed after a feeding error and the reverse registering operation can be prevented from being interfered by the reversely carried recording sheet  50  remaining in the path  27  even without having the reversely carried recording sheet  50  detected by a sensor and the like. 
   In the above embodiment, the conveyer roller  60  and the discharge roller  62  having been rotated in the reverse direction are switched to rotate in the normal direction based on occurrence of a feeding error. In another embodiment, for example, the rotating direction of the conveyer roller  60  and the discharge roller  62  may be switched to the normal direction based on occurrence of a feeding error and detection of the reversely carried recording sheet  50  by an optical sensor  75  (see  FIG. 6 ). The modified embodiment with the optical sensor  75  will be described hereinbelow. 
     FIG. 6  illustrates an internal configuration of the printer unit  11  according to the modified embodiment of the present invention. In the present embodiment, the printer unit  11  includes the optical sensor  75  in the path  27 . The optical sensor  75  is to detect presence of the reversely carried recording sheet  50  in a position P 2 , which is in between the conveyer roller pair  59  and the discharge roller  64  in the path  27 . In order to detect the recording sheet  50  withdrawn from the discharge tray  21 , the optical sensor  75  is arranged in the position P 2 , which is on the upstream side with respect to the discharge roller pair  64  in the path  27  and in the vicinity of the discharge roller pair  64 . The optical sensor  75  is a reflective photo-sensor including a light-emitting diode (not shown) and a light receiver (not shown). The light-emitting diode emits light downward, and the light is reflected on the surface of the recording sheet  50  when the withdrawn recording sheet  50  is present in the path  27  at the position P 2 . Meanwhile, the emitted light is reflected on a part of a frame (not shown) of the printer unit  11 , which is located directly below the light-emitting diode when no recording sheet  50  is in the position P 2 . The reflected light is received by the light receiver of the optical sensor  75 . When the reflected light is received by the light receiver, a sensor signal depending on a level of brightness being received is generated. The part of the frame on which the emitted light reflects is colored in a color darker with lower reflectance than the surface of the recording sheet  50 ; therefore, when no recording sheet  50  is present in the position P 2  and the emitted light is reflected on the part of the frame, the light reflected in the lower reflectance is received by the light receiver. Thus, a sensor signal according to the lower reflectance is generated in the optical sensor  75 . Meanwhile, when the recording sheet  50  is present in the position P 2  and the emitted light is reflected on the surface of the recording sheet  50 , the light reflected with the higher reflectance is received by the light receiver, and the a sensor signal according to the higher reflectance is generated in the optical sensor  75 . 
   The optical sensor  75  is connected to the ASIC  109 , although the connection is not shown. The sensor signal generated in the optical sensor  75  is output to the control unit  100 , which detects presence of the recording sheet  50  in the position P 2  based on the levels of the sensor signals output from the optical sensor  75 . 
   Next, a feeding operation in cooperation with the optical sensor  75  will be described with reference to  FIG. 7 .  FIG. 7  is a flowchart to illustrate the feeding operation to feed and convey the recording sheet  50  in the printer unit  11  in the MFD  10  according to the modified embodiment of the present invention. The flow according to the modified embodiment shown in  FIG. 7  is identical with the flow according to the previous embodiment shown in  FIG. 5  except S 15 , which is conducted prior to S 12 . Therefore, description of steps in the modified embodiment identical with the steps in the previous embodiment is herein omitted. 
   When several dozen of recording sheets  50  are piled up on the discharge tray  21 , the height difference between the level of the topmost recording sheet  50  on the discharge tray  21  and the nipped position of the discharge roller pair  64  becomes small. Therefore, the topmost recording sheet  50  in the discharged sheet stack can be withdrawn from the discharge tray  21  and carried backwards in the path  27  when the feeding operation and the reverse rotating operation start. On the contrary, when a small amount of recording sheets  50  is piled up on the discharge tray  21 , the recording sheet  50  on the discharge tray  21  tends not to be withdrawn in the path  27 . Thus, the reverse conveyance of the once discharged recording sheet  50  may not necessarily occur upon the feeding operation and the reverse rotating operation. Therefore, the normal rotation of the conveyer roller  60  and the discharge roller  62  in the normal direction in S 13  can be performed limitedly when the reverse conveyance of the recording sheet  50  is detected in the path  27 . 
   During the feeding operation, the conveyer roller  60  and the discharge roller are rotated in the second directions  113 ,  114  (see S 3 ). Therefore, the recording sheet  50  fed by the feed roller  25  is pushed backwards in the reverse direction by the conveyer roller pair  59  and prevented from reaching the point P 2 . Thus, the recording sheet  50  fed by the feed roller  25  cannot be detected by the optical sensor  75 . Instead, when the optical sensor  75  detects presence of a recording sheet  50  in the path  27  at the position P 2 , it is determined that the detected recording sheet  50  is a reversely-carried recording sheet  50  withdrawn from the discharge tray  21 . 
   As shown in  FIG. 6 , when the control unit  100  determines that the elapsed time t is greater than or equal to the set time T in S 5  (S 5 : YES) and in S 7  (S 7 : YES), in S 15 , the control unit  100  judges as to whether the recording sheet  50  is detected at the position P 2  based on the level of the sensor signal, which is output from the optical sensor  75 . When the recording sheet  50  is not detected at the position P 2  (S 15 : NO), it is determined that no recording sheet  50  has been withdrawn in the path  27 . Therefore, the control unit  100  skips S 12  and S 13  and returns to S 2 . 
   In S 15 , when the recording sheet  50  is detected at the position P 2  (S 15 : YES), it is determined that the recording sheet  50  has been withdrawn from the discharge tray  21  and remaining in the path  27 . Thus, upon occurrence of a feeding error (S 5 : YES) and detection of the recording sheet  50  by the optical sensor  75  (S 15 : YES), the conveyer roller  60  and the discharge roller  62  are rotated in the first directions  111 ,  12  (S 13 ). In this configuration, an unnecessary discharging operation, in which the conveyer roller  60  and the discharge roller  62  are rotated in the first directions  111 ,  112  in vain when no recording sheet  50  is in the path  27 , can be omitted. Further, a period between occurrence of the feeding error and retry of the feeding operation can be effectively shortened. 
   According to the modified embodiment, the optical sensor  75  is arranged on the upstream side with respect to the discharge roller pair  64  in the path  27 ; therefore, the reversely carried recording sheet  50  may be off the position P 2  but nipped between the discharge roller pair  64  and present in the path  27  without being detected by the optical sensor  75 . When the withdrawn recording sheet  50  is in such a position, and when the set time T is substantially close to the period B, the withdrawn recording sheet  50  nipped with the discharge roller pair  64  is reversely conveyed and may reach the conveyer roller pair  59  before the elapsed time t becomes greater than or equal to the set time T. In order to avoid the situation, in the modified embodiment, the period B is set to have a length, in which the withdrawn recording sheet  50  can be conveyed by the reversely rotated discharge roller pair  64  in the reverse direction no further beyond distance M (see  FIG. 6 ), whilst the distance M is a length between the optical sensor  75  and a contact position for the withdrawn recording sheet  50  with the conveyer roller pair  59 . The period B can be determined based on, for example, the distance M and a rotation speed of the conveyer roller  60  (and the discharge roller  62 ), which can be obtained based on the result detected by the rotary encoder  83 . 
   Although examples of carrying out the invention has been described, those skilled in the art will appreciate that there are numerous variations and permutations of a sheet conveyer device and an image reading apparatus that fall within the spirit and scope of the invention as set forth in the appended claims. It is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or act described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 
   For example, in the above embodiments, the conveyer roller  60  is arranged on the opposite side from the discharge roller  62  with respect to the path  27 , and vice versa. Accordingly, the first direction  111  of the conveyer roller  60  is opposite from the first direction  112  of the discharge roller  62 . Further, the second direction  113  of the conveyer roller  60  is opposite from the second direction  114  of the discharge roller  114 . It is to be noted that, when the conveyer roller  60  and the discharge roller  62  are arranged on a same side with respect to the path  27 , the first direction of the conveyer roller  60  and the first direction of the discharge roller  62  coincide with each other. Similarly, the second direction of the conveyer roller  60  and the second direction of the discharge roller  62  coincide with each other. 
   Further, in the above embodiments, the feed roller  25  is driven by the motor  84  whilst the conveyer roller  60  and the discharge roller  62  are driven by the motor  85 . However, the feed roller  25 , the conveyer roller  60 , and the discharge roller  62  may be driven by a same motor. In such a case, a drive circuit may be configured such that the feeding operation of the feed roller  25  and the reverse rotation of the conveyer roller  60  and the discharge roller  62  are generated by, for example, normal rotation of the motor, whilst normal rotation of the conveyer roller  60  and the discharge roller  62  are generated by reverse rotation of the motor. 
   Furthermore, the register sensor  71  to detect presence of the recording sheet  50  fed by the feeding roller  25  may be replaced with a different type of sensor. 
   The sheet conveyer device in the above embodiments is installed in the printer unit  11 . However, the sheet conveyer device may replace the ADF  28  in the scanner unit  12 . In this configuration, orientation of the original document sheets to be fed in the scanner unit  12  may be corrected with respect to a scanning path so that scanned and discharged documents are prevented from being withdrawn in the scanning path. Thus, a succeeding document to be scanned can be prevented from being interfered by the withdrawn document in the scanning operation.