Patent Publication Number: US-6213464-B1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an image forming apparatus provided with a recording apparatus for recording an image on a sheet, and a sheet feeding apparatus detachably mounted on the recording apparatus for automatically feeding sheets in succession, and more particularly to a guide member for defining a conveying path of the sheet. 
     2. Related Background Art 
     There have conventionally been proposed various image forming apparatuses for forming an image on a sheet. 
     Among such apparatuses, there has been proposed an apparatus provided with a recording apparatus for image recording (hereinafter called “printer”) and an auto sheet feeder (hereinafter referred to as “ASF”) detachably mounted on a sheet feeding port of the printer, wherein, through such sheet feeding port: 
     sheets are manually fed one by one when the ASF is not mounted; and 
     sheets are automatically fed in succession when the ASF is mounted (see the Japanese Patent Application Laid-Open No. 6-183582). 
     In the image forming apparatus of the above-described type, the printer is provided with a guide member for guiding the sheet in case of sheet feeding with manual insert, while the ASF is provided with a guide member for guiding the sheet in case of automatic sheet feeding. These guide members are provided in approximately the same position in the transverse direction of the sheet, such that the image recording position in the transverse direction of the sheet (namely the position of image formation on the sheet, in the transverse or width direction thereof) remains the same in the sheet feeding with manual insert and in the automatic sheet feeding. 
     In the image forming apparatus of the above-described type, however, if the guide member of the printer is positioned at the inner side of the sheet compared to the guide member of the ASF because of the dimensional tolerance at the manufacture thereof, the guide member of the printer becomes an obstacle for the automatically fed sheets and may induce skewing or jamming of the sheet or damage to the sheet end. 
     Such drawback can be resolved by precisely assembling the image forming apparatus with highly precise parts, but such assembling is difficult and the use of the highly precise parts results in an increased cost. 
     Furthermore, even if the guide member of the printer and that of the ASF are provided in substantially the same position in the transverse direction of the sheet, a sheet eventually skewed will interfere with the guide member of the printer, thus inducing skew or jamming of the sheet or damage to the sheet end. 
     SUMMARY OF THE INVENTION 
     In consideration of the foregoing, an object of the present invention is to provide an image forming apparatus for preventing skewing or jamming of a sheet or damage to an end of the sheet. 
     Another object of the present invention is to provide an inexpensive image forming apparatus. 
     Still another object of the present invention is to provide an image forming apparatus for matching an image recording position in a transverse direction of the sheet, regardless of whether a sheet feeding apparatus is used or not. 
     The above-mentioned objects can be attained, according to the present invention, by an image forming apparatus provided with a recording apparatus having a feeding port for feeding a sheet and recording an image on the sheet fed from the feeding port, and a sheet feeding apparatus detachably mountable on the feeding port and automatically feeding the sheets in succession to the recording apparatus. 
     In the present invention, the recording apparatus includes a first guide member for guiding an edge of the sheet in the transverse direction of the sheet. 
     The sheet feeding apparatus includes a second guide member for guiding the edge of the sheet in the transverse direction of the sheet. 
     The second guide member is disposed and displaced toward an inner side of the sheet with respect to the first guide member. 
     In such a case, the image recording position in the transverse direction of the sheet is preferably displaced, in case the sheet is fed by the sheet feeding apparatus, toward the inner side of the sheet, in comparison with the case in which the sheet is not fed by the sheet feeding apparatus, by an amount substantially equal to the amount of displacement between the first guide member and the second guide member. 
     There may also be provided mode discrimination means for discriminating whether the sheet feeding is executed by the sheet feeding apparatus, and the image recording position in the transversal direction of the sheet may be displaced according to the result of discrimination by the mode discrimination means. 
     In such case, the recording apparatus and the sheet feeding apparatus may be respectively provided with connectors allowing mutual electrical connection, and the mode discrimination means may electrically detect the connection state of the connectors. 
     On the other hand, the recording apparatus may be provided, together with the first guide member, with a third guide member for guiding an edge of the sheet in the transverse direction thereof, while a conveying path of the sheet when the sheet feeding apparatus is connected to the recording apparatus is disposed to make a detour to avoid the third guide member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are perspective views showing embodiments of the present invention; 
     FIGS. 3 and 4 are cross-sectional views showing embodiments of the present invention; 
     FIGS. 5 and 6 are perspective views showing embodiments of the present invention; 
     FIG. 7 is a schematic plan view showing an embodiment of the present invention; 
     FIG. 8 is a cross-sectional view showing an embodiment of the present invention; 
     FIGS. 9 and 10 are perspective views showing embodiments of the present invention; 
     FIG. 11 is a perspective view showing an arrangement of parts relating to a printer mounting and dismounting mechanism of the ASF of the present invention; 
     FIG. 12 is a perspective view showing an arrangement of parts, associated with the mounting and dismounting mechanism of the ASF, of a printer to be connected with the ASF of the present invention; 
     FIGS. 13,  14 ,  15 ,  16 ,  17  and  18  are cross-sectional views seen from the left hand side and showing the mounting and dismounting mechanism of the printer and the ASF of the present invention; 
     FIG. 19 is a perspective view showing the arrangement of parts and relationship of forces in symbols, relating to the mounting and dismounting mechanism of the printer and the ASF of the present invention; 
     FIGS. 20,  21 ,  22  and  23  are plan views showing the mounting and dismounting mechanism of the printer and the ASF of the present invention; 
     FIG. 24 is a block diagram showing a connection of a printer  101  and an ASF  1  of the present invention; 
     FIG. 25 is a schematic cross-sectional view showing the printer  101  and the ASF  1  of the present invention in a connected state; 
     FIG. 26 is a schematic view showing the connection between a connector  117  and an ASF connector  44 ; 
     FIGS. 27 and 28 are schematic views showing the connection and the operating direction of a driving mechanism of the ASF  1 ; 
     FIG. 29 is a flowchart showing the control sequence for the sheet feeding operation in a printer control unit  202  in a first embodiment; 
     FIG. 30 is a flowchart showing the main control sequence of an ASF control unit  201 ; 
     FIG. 31 is a sub flowchart C 2  for controlling a sheet feeding operation by the ASF control unit  201  in the first embodiment; 
     FIG. 32 is a sub flowchart C 3  for controlling an initializing operation by the ASF control unit  201 ; 
     FIG. 33 is a sub flowchart C 1  for controlling a device discriminating operation in the printer control unit  202 ; 
     FIG. 34 is a flowchart for controlling the sheet feeding operation by the printer control unit  202  in a second embodiment; 
     FIG. 35 is a sub flowchart C 2  for controlling the sheet feeding operation by the ASF control unit  201  in the second embodiment; 
     FIG. 36 is a schematic cross-sectional view showing a state after completion of a step S 22  in the sheet feeding operation; 
     FIG. 37 is a timing chart showing the outline of the operation flow of the printer  101  and the ASF  1  in the second embodiment; and 
     FIG. 38 is a chart showing the content of a drive table T for a sheet feeding motor  27 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the present invention will be described in detail by preferred embodiments thereof, with reference to the attached drawings. 
     [First Embodiment] 
     FIG. 1 is a perspective view showing a printer mounted to an ASF constituting a first embodiment of the present invention; FIG. 2 is a view showing the mode of mounting of the printer to the ASF; FIG. 3 is a cross-sectional view of the ASF; and FIG. 4 is a cross-sectional view of the ASF in a state in which the printer is mounted. 
     As shown in FIGS. 1 to  4 , the image forming apparatus  100  is provided with a printer (recording apparatus)  101  for recording an image on a sheet, and an ASF (automatic sheet feeding apparatus)  1  for automatically feeding sheets in succession to the printer  101 . The printer  101  is provided with a sheet feeding port (feeding port)  101 A (cf. FIG. 5) for feeding the sheets, and the ASF  1  is so constructed as to be detachably mounted to the feeding port  101 A. The printer  101  and the ASF  1  are respectively provided with connectors  117 ,  44  capable of mutual electrical connection, as will be explained later. 
     The above-mentioned printer  101  is a so-called mobile printer, which is compact, portable and is provided with a battery. In the present embodiment, the printer  101  is not provided with the ASF, so that the printer  101  alone can only achieve a sheet feeding in manual insert. Such configuration allows compactization, simplification and low cost in the printer  101 , optimum for the mobile printer. However, the present invention is naturally applicable even if the printer  101  is provided with a compact ASF. 
     Such compact, portable printer  101  is assumed to be particularly used outdoors, in an automobile or in a customer&#39;s office at the visit of a salesman. In such situations, the number of the required recording sheets is relatively small, so that the manual insert sheet feeding or the simple internal ASF of a low capacity is presumed to be enough, but, if the printer  101  is used in the ordinary office environment, there may be encountered a requirement of printing the various sheets of a relatively large quantity. 
     The ASF  1  separated from the printer  101  is suitable for such requirement. The ASF  1  has a so-called desk-top form which is commonly found on the desk of the ordinary office environment, and the printer  101 , when fitted to the ASF  1 , can have the character of a desk-top printer. The ASF can automatically feed various recording media, not only ordinary paper but also postcards, envelopes, plastic films, fabrics etc. owing to the configuration to be explained later. 
     Thus the present embodiment can provide an extremely valuable printer, in which a printer which is compact and mobile when used singly can also be used as a desk-top printer of high performance by being mounted to the ASF of the present invention. The ASF  1  functions also as so-called docking station, serving as a storage box for the printer  101  when it is not used, and also adding the automatic sheet feeding function when the printer is mounted. 
     The ASF  1  of the present invention can stably stand by itself when the printer  101  is not mounted, and can separate the printer  101  while supporting the sheets. Thus, the stand-by state for the operation of the desk-top printer can be attained by simply mounting the separated printer  101  to the self-standing ASF  1 . Consequently, there can be provided a docking station extremely convenient for use by the user. 
     In order to use the printer  101  both as the mobile printer and the desk top printer, it is important that the mounting and dismounting operations of the ASF  1  and the printer  101  can be easily achieved, because, for a user who carries the printer  101  without the ASF  1  almost everyday and combines the printer  101  with the ASF  1  whenever returning to his office, a complicated or time-taking operation of mounting and dismounting will be cumbersome. 
     In the present embodiment, as shown in FIG. 3, the ASF  1  is provided at the front face thereof with an aperture  1 A, for accommodating the printer  101 . The printer  101  is also provided with a substantially horizontal sheet passing path, and is so constructed as to be pushed into the front aperture  1 A of the ASF  1  with the sheet feeding side of the printer  101  being moved substantially horizontally toward the ASF  1  whereby a sheet path as will be explained latter is formed. 
     Thus, in the present embodiment, the printer  101  having the horizontal path is pushed substantially horizontally into the ASF  1  and is mounted thereto. When the printer  101  is pushed substantially horizontally into the ASF  1 , the printer  101  is automatically fixed thereto (method of mutual fixation when the printer  101  is mounted to the ASF  1  being explained later in detail). In order to separate the printer  101  from the ASF  1 , it is merely necessary to push a lever  40  provided on the upper face of the ASF  1 , whereby the printer  101  is unlocked from the ASF  1  and is pushed out toward the front side of the ASF  1 . 
     Such configuration allows the user to achieve extremely easily mounting and dismounting of the printer  101  and the ASF  1 , whereby the printer can be used as the mobile printer and also as the desk-top printer. 
     In the present embodiment, in order to facilitate the mounting and dismounting operations, the ASF  1  is provided at the front side thereof with a table portion  45   c . In case of mounting the printer  101  to the ASF  1 , the printer  101  is at first placed on the table portion  45   c . In this operation, the user grasps, with one hand, the top and bottom faces of the printer  101  at the approximate center portion of the front (a sheet discharging side) thereof and places the printer  101  in such a manner that the rear side (a sheet feeding side) of the printer  101  is lightly positioned on the table portion  45   c . (Otherwise the user may hold both ends of the printer  101  with both hands.) 
     Then the printer  101  placed on the table portion  45   c  is pushed deeper with a hand, whereby the lateral faces of the printer  101  are guided, by printer side guide portions  45   a  provided at both ends of the table portion  45   c , to a positioning boss (to be explained later), which is thus fitted with a positioning hole (to be explained later) of the printer  101  and the positioning is achieved. In this operation, the user is only required to place the printer  101  at the approximate center of the table portion  45   c  and to press in the printer  101 , and any precise positioning operation is not required. 
     The table portion  45   c  is provided, at both lateral portions, with printer sliding areas  45   b  on which the bottom face of the printer slides. The printer  101  is provided, on the bottom face thereof, with plural rubber feet (not shown), in order that the printer  101  is less easily moved by the external force when the printer  101  is singly used for example on a desk. 
     However, in mounting the printer  101  to the ASF  1 , there will be required a large pressing force and the pressing operation of the printer  101  will become difficult if the rubber feet are in contact with the table portion  45   c . Consequently, the printer sliding portions  45   b  are formed with a step difference larger than the height of the rubber feet, in order that the rubber feet do not come into contact with the table portion  45   c.    
     On the other hand, the upper case  47  of the ASF is provided with an eaves portion  47   a  substantially parallel to the table portion  45   c , and forms, in cooperation with the table portion  45   c , a pocket for accommodating the printer  101 . The pocket thus formed indicates to the user, by its form, the direction of substantially parallel pushing of the printer  101  toward the ASF  1 , and the user can push the printer  101  only in such direction. 
     This pushing direction coincides with the connection direction of the connectors to be explained later for electrically connecting the printer  101  and the ASF  1 , and the connectors are mutually connected in the course of pushing the printer  101  into the ASF  1 . Such configuration improves operability by eliminating other separate operations for connecting the connectors, and prevents destruction of the connectors resulting from abnormal interference thereof caused by pushing from a different direction. 
     Also, if the front portion (sheet discharging side) of the printer  101  receives an upward force after the printer  101  is mounted to the ASF  1 , the eaves portion  47   a  prevents that the printer  101  is lifted upwards with respect to the ASF  1  to cause destruction of the mounting portion or release of the mounting. 
     Also in the present embodiment, the eaves portion  47   a  shows a largest protruding amount at both ends and has a recessed eaves portion  47   b  at the center. Such recessed eaves portion  47   b  avoids covering of an operation unit, such as a power switch, provided on the top face of the printer  101 . The effect of preventing the above-mentioned upward lifting motion of the printer can be sufficiently obtained in case the clearance between the eaves portion  47   a  and the top face of the printer is within a range of 0.5 to 2 mm, but the desired effect cannot be obtained if the clearance is excessively large. 
     In the present embodiment, the depth L 1  of the printer  101 , the depth L 2  of the table portion  45   c  and the depth L 3  of the eaves portion  47   a  shown in FIG. 4 satisfy the following relation: 
     
       
           L   1 / 2 ≲ L   2 ≲ L   1 −15 mm  
       
     
     The printer  101  can be stabilized when it is mounted in the ASF  1 , by selecting the depth L 2  of the table portion  45   c  larger than a half (L 1 / 2 ) of the depth L 1  of the printer. Such relationship need only be satisfied in a part of the table portion  45   c  but not in the entire area of the table portion  45   c . 
     If L 1 / 2 ≳L 2 , the printer  101  protrudes significantly from the ASF  1  in the mounted state, and the entire apparatus becomes very unstable, as the rear part thereof may be lifted for example by a downward external force applied to such protruding portion. 
     On the other hand, a finger inserting space can be secured under the front side of the printer  101  by selecting the depth L 2  of the table portion  45   c  smaller, by at least 15 mm, than the depth L 1  of the printer  101 . Thus the user can hold the top and bottom faces of the printer  101  with a hand, in mounting and dismounting the printer  101 . (Naturally the user may hold the printer with both hands.) Such relationship need not be satisfied over the entire width of the table portion  45   c  but a recess or recesses may be formed at the central portion or at the end portions so as to satisfy the above-mentioned relationship. 
     Furthermore, as a space is provided under the front face of the printer  101 , there can be realized a design without a large height in the visual impression. The thickness (height) of the table portion  45   c  is preferably at least about 10 mm in order that the user can insert a finger under the printer  101 . 
     The present embodiment further satisfies the following relationship: 
     
       
         L 1 / 4 ≲L 3 ≲L 1 / 2   
       
     
     It is found possible to prevent the upward lifting the printer  101  and there can be obtained sufficient effect for limiting the pressing direction of the printer  101  if the depth L 3  of the eaves portion  47   a  is equal to or more than ¼ of the depth L 1  of the printer  101 . It is also found, if the depth L 3  of the eaves portion  47   a  exceeds ½ of the depth L 1  of the printer  101 , the pushing amount of the printer  101  is excessively large with respect to the depth of the printer  101  and the feeling for operation becomes unsatisfactory. 
     Also, the large eaves portion  47   a  results in visual disadvantages that the entire apparatus appears larger and oppressive to the user. It may also interfere with the manipulation of the printer  101  on the top face of the printer  101 , so that the depth L 3  of the eaves portion  47   a  preferably does not exceed ½ of the depth of the printer  101 . The protruding amount within the above-mentioned range can maintain a sufficient strength in thus protruding eaves portion  47   a , thus providing sufficient toughness in the entire apparatus. 
     The configuration of the table portion  45   c  and the eaves portion  47   a  under the above-mentioned conditions allows a form capable of fully exhibiting the effects such as extremely good operability, limitation of the pressing direction and prevention of the upward lifting of the printer  101 . 
     Between the table portion  45   c  and the eaves portion  47   a  there are laterally formed large apertures, as the height of the printer side guide portions  45   a  need only be larger than the clearance between the eaves portion  47   a  and the top face of the printer  101 . Such a large aperture avoids interference with a power supply cord, an interface connector or an infrared communication unit eventually provided on the lateral faces of the printer  101 . Thus, the printer  101  with the power supply cord or the interface connector mounted thereon may be mounted to or detached from the ASF  1 . 
     Below there will be explained the connectors  117 ,  44  for enabling mutual electrical connection of the printer  101  and the ASF  1 , and connector covers  119 ,  59  for protecting these connectors. 
     The printer  101  and the ASF  1  are respectively provided with detachable and attachable connectors  117 ,  44 , which are electrically connected for exchanging the power supply or the control signals. (In the following description, the connector  117  at the side of the printer  101  will be referred to as “printer connector  117 ”, while the connector  44  of the ASF  1  will be referred to as “ASF connector  44 ”. 
     The printer connector  117  is provided, as shown in FIG. 5, in the upper part of a face opposed to the ASF  1  on the mounting operation of the printer  101  to the ASF  1 , and the ASF connector  44  is provided, as shown in FIG. 11, in a position opposed to the printer connector  117  when the printer  101  is mounted. 
     The printer  101  and the ASF  1  are respectively provided with connector covers  119 ,  59  detachably mountable on the connectors  117 ,  44 . (In the following description, the connector cover for protecting the printer connector  117  will be referred to as “printer connector cover  119 ”, while the connector cover for protecting the ASF connector  44  will be referred to as “ASF connector cover  59 ”. See FIGS. 5 and 4 respectively for the printer connector cover  119  and the ASF connector cover  59 ). When the printer  101  and the ASF  1  are mutually separated, the connector covers  119 ,  59  are respectively fitted on the connectors  117 ,  44  for protecting the same. Thus the connectors  117 ,  44  are protected from dust deposition, whereby the conductivity in the connected state can be satisfactorily maintained. It is also rendered possible to prevent application of excessive large static electricity to the internal electrical circuits through the connectors  117 ,  44 , thereby preventing destruction of such electrical circuits. Furthermore, such detachable connector covers  119 ,  59  allows to achieve a lower cost and space saving, and are particularly suitable for an ultra compact printer such as the mobile printer. 
     On the other hand, on the upper face of the table portion  45   c  of the ASF  1  (namely the surface on which the printer  101  rests), there are provided connector cover storage areas  45   d ,  45   e  for storing the connector covers  119 ,  59  detached from the connectors  117 ,  44 , whereby, in the mutually connected state of the printer  101  and the ASF  1 , the connector covers  119 ,  59  detached from the connectors  117 ,  44  are placed in such storage areas  45   d ,  45   e  (cf. FIG.  4 ). The storage areas  45   d ,  45   e  are composed of projections corresponding to the dimensions of the connectors, within the thickness of the table portion  45   c.    
     The connector covers  119 ,  59  stored in the storage areas  45   d ,  45   e  are supported between the printer  101  and the ASF  1 , thus being protected from being lost. Such configuration is also preferable from an aesthetic standpoint as the connector covers  119 ,  59  become no longer visible from the outside. Furthermore, in detaching the printer  101  from the ASF  1 , the connector covers  119 ,  59  stored in the storage areas  45   d ,  45   e  become easily visible so that the user does not forget fitting of the connector covers  119 ,  59  on the connectors  117 ,  44 . 
     With respect to the present embodiment relating to the connector covers, the present invention is applicable to the printer and the ASF even when they are for example a notebook personal computer and a station therefor. 
     Furthermore, the printer connector  117  and the ASF connector  44  of the present embodiment are both protected by the connector covers  119 ,  59 , but either of the connectors  117 ,  44  may be protected by a connector cover. 
     Furthermore, the connector cover storage areas are provided, in the present embodiment, on the upper face of the table portion  45   c  of the ASF  1 , but they may be provided in another part of the ASF  1 . Also, the connector cover storage areas may be provided in the printer  101  instead of the ASF  1 . 
     Below there will be outlined how the recording sheet is fed and recorded in a state where the printer  101  is mounted to the ASF  1  (details being given later). 
     FIG. 4 is a cross-sectional view showing a state in which the printer  101  is mounted to the ASF  1 , wherein a pressure plate  26  is provided for setting a predetermined number of sheets to be explained later. The pressure plate  26  is rotatably supported at an end thereof by an ASF chassis  11  and is clockwise biased, by a pressure plate spring  13 , toward a pickup rubber member  23 , wound around a pickup roller  19 . 
     At the sheet setting, the pressure plate  26  is displaced and retained in a direction separated from the pickup rubber member  23 , by means of a cam to be explained later. In such state, a predetermined clearance is maintained between the pickup rubber member  23  and the pressure plate  26 , and the sheets are inserted and set in such clearance. 
     The leading end of the sheet impinges on and is defined in position by a bank sheet  37 , composed of a plastic film, provided on a bank  36 . A major portion of a trailing end of the sheet is supported by an ASF sheet feeding tray  2 , which is rotatably supported at an end thereof by an upper case  47  and is supported at a certain angle in a sheet supporting state. 
     When the ASF  1  receives a sheet feeding command from the printer  101 , the pickup roller  19  starts clockwise rotation and the cam at the same time releases the pressure plate  26  from the supported state. Thus, the pressure plate  26  comes into contact with the pickup rubber member  23  whereby the sheet starts to move by the surface friction of the pickup rubber member  23 . A sheet is then separated by the bank sheet  37  and is transported in an ASF sheet path  58  (cf. FIG. 3) formed by the bank  36  and a positioning base  39 . 
     Thereafter, the sheet is conveyed from an ASF sheet discharge portion  56  (cf. FIG. 3) to a sheet path, formed by a platen  105  and a bottom surface of a battery  107  in the printer and constituting a manual insertion port in the printer  101  alone. 
     When a sheet end sensor  108  detects the sheet conveyed in the above-mentioned sheet path, the printer  101  recognizes the sheet conveyance from the ASF  1 , and a leading end of the sheet impinges on a nip between an LF roller  109  and a pinch roller  110 . Also in response to the information from the sheet end sensor  108  of the printer  101 , the ASF  1  transmits, at a predetermined timing, a response signal indicating the completion of sheet feeding to the printer. 
     In this state the sheet is pressed, by the rigidity thereof, toward the nip between the LF roller  109  and the pinch roller  110 , thereby achieving so-called registration of the leaving end of the sheet. Upon receiving the response signal indicating the completion of the sheet feeding from the ASF  1 , the printer  101  rotates the LF roller  109  at a predetermined timing, thereby advancing the sheet toward a recording unit provided with a head  115 . Thus, the sheet is advanced by a predetermined manner and the head  115  executes the recording on the sheet surface. Subsequently, the sheet is conveyed between a discharging roller  112  and a spur  111  and is discharged. 
     In the present embodiment, the sheet path is formed in the above-described manner when the printer  101  is mounted to the ASF  1 , and the mounting direction of the connectors  44 ,  117  is substantially parallel to the direction of such sheet path of the printer  101 . 
     In the case the sheet conveyed from the ASF  1  to the printer  101  and present over the ASF  1  and the printer  101  is jammed in any part, it becomes necessary to separate the printer  101  from the ASF  1 . The substantially parallel configuration of the sheet path and the connecting direction of the connectors enables mutual separations of the sheet path and the connection of the connectors in such situation. 
     If the sheet path is perpendicular to the connecting direction of the connectors, the sheet has to be moved in a direction of a thickness of the sheet for separating the printer  101  in the connecting direction of the connectors, whereby the sheet may be broken or the broken sheet may remain in the apparatus. Furthermore, if the sheet is thick enough and cannot be easily broken, the separation itself of the printer  101  becomes impossible. 
     However, in the configuration of the present embodiment in which the sheet path is substantially parallel to the connecting direction of the connectors, the printer  101  can be separated in the case of a sheet jamming by a movement along the sheet, whereby the sheet jamming can be extremely easily handled without the sheet breaking or without a broken sheet remaining in the apparatus. 
     Below there will be explained the method of guiding the conveyed sheet (method of positioning the sheet in the transverse direction thereof). 
     In the present embodiment, as the ASF  1  is so constructed as to be detachably mountable on the sheet feeding port of the printer  101 , there can be achieved both: 
     sheet feeding without the ASF  1 ; and 
     automatic feeding of sheets in succession with the mounted ASF  1 . 
     Thus, there can be enabled both the manual insert sheet feeding and the automatic sheet feeding, and the apparatus can be made more compact in comparison with a configuration having a manual insert sheet feeding port and an automatic sheet feeding port separately. 
     The printer  101  is provided, as shown in FIG. 5, with a sheet feeding tray  116 , which is pivotably supported at an end thereof and is rendered openable and closable. The sheet feeding tray  116  constitutes the sheet path and stabilizes the sheet feeding operation, in case of sheet feeding in manual insertion without the mounting of the ASF  1 . The sheet feeding tray  116  (or sheet path) is supported substantially horizontally in case of the manual insert sheet feeding. 
     At one end of the upper face of the sheet feeding tray  116 , there is perpendicularly formed a reference guide (third guide member)  116   a  parallel to the edge thereof, and, at the other end of the upper face, there is provided a right edge guide  122  which is slidably movable in the transverse direction of the sheet. These guides  116   a ,  122  guide both lateral edges of the manually inserted and fed sheet. These guides  116   a ,  122  have a substantially same shape (seen in the transverse direction of the sheet). 
     On the other hand, the ASF  1  is provided, as shown in FIG. 4, with a reference guide accommodating portion  36   b , formed by a reference guide guiding portion  36   c  positioned thereabove. When the printer  101  is pressed into the ASF  1 , the reference guide  116   a  of the printer is pressed downwards by the guiding portion  36   c  and is rotated further downwards, and is accommodated, together with the right edge guide  122 , in the accommodating portion  36   b . Above the reference guide accommodating portion  36   b , there is formed a sheet path for automatic sheet feeding so as to make a detour to avoid the reference guide (third guide member)  116   a . In the present embodiment, as the sheet feeding tray  116  is accommodated, in a downward rotated state, in the reference guide accommodating portion  36   b , the sheet path on the automatic sheet feeding can be formed horizontally (particularly in the vicinity of the accommodating portion  36   b ), like the sheet path on the manual insert sheet feeding, there avoiding drawbacks (such as a backward tension on the sheet) resulting from an unnaturally shaped sheet path. The reference guide accommodating portion  36   b  is so formed as to accommodate the right edge guide  122  in any sliding position. On the automatic sheet feeding, a lateral edge of the sheet in the transverse direction thereof is guided by a sheet reference guide (second guide member)  26   b  of the ASF. 
     If the sheet automatically fed by the ASF  1  is guided both by the guide  26   b  of the ASF and the guide  116   a  of the printer, and if the guide  116   a  of the printer is eventually positioned at the inner side of the sheet than the guide  26   b  of the ASF because of the dimensional tolerance in the manufacture, the guide  116   a  of the printer constitutes an obstacle for the automatically fed sheet, resulting in skewed sheet advancement, damage to the sheet end or sheet jamming. 
     However, the present embodiment can avoid such drawbacks since the sheet automatically fed by the ASF  1  is guided solely by the guide  26   b  of the ASF. 
     Also, it is not necessary to precisely form the guide  26   b  of the ASF and the guide  116   a  of the printer or to employ precisely formed parts therefor in order to avoid such drawbacks, so that there can be avoided an increase in the cost resulting therefrom. 
     Furthermore, even if the sheet is somewhat skewed, the sheet can be protected from interference with the guide  116   a  of the printer, whereby skewed sheet advancement, damage to the sheet end or sheet jamming resulting from such interference can be avoided. 
     In the printer, the sheet is guided by the guide (third guide member)  116   a  of the sheet feeding tray  116 , but it is also possible to provide the interior of the printer with a similar guide (first guide member  124 ) in a same position in the transverse direction of the sheet to guide a lateral edge of the manually inserted and fed sheet  200  with the guide  116   a  on the sheet feeding tray and such internal guide. The skewed sheet advancement can be further prevented by defining the sheet conveying direction with a longer section along the sheet conveying direction. 
     In the case the guide (first guide member  124 ) is provided in the interior of the printer, the sheet reference guide (second guide member)  26   b  of the ASF can be formed, as shown in FIG. 7, at a position displaced, by a predetermined amount t, toward the inner side of the sheet (namely toward the recording position by the head). Thus, in case of automatic sheet feeding, the sheet can be prevented from interference with the internal guide of the printer, whereby skewed sheet advancement, damage to the sheet end or sheet jamming resulting from such interference can be avoided. The displacement amount t is determined to be equal to or more than the positioning tolerance between the printer  101  and the ASF  1  in the transverse direction of the sheet. In consideration of eventual skewed sheet feeding from the ASF, the displacement amount t may be, for example, about 0.6 mm. 
     Furthermore, in the case the guide of the ASF is displaced by t from that of the printer as explained above, the image recording position on the sheet in the transverse direction of the sheet in the case of sheet feeding by the ASF  1  (namely in case of automatic sheet feeding) may be displaced by an amount approximately equal to t (amount of displacement between the first and second guide members), in comparison with a case where the sheet feeding is not executed by the ASF  1  (namely in case of sheet feeding in manual insert). In this manner, the image is recorded in the same position regardless of the automatic or manual insert sheet feeding, thereby avoiding a drawback resulting from the difference in the image recording position (for example difference in the image recording position on a pre-printed sheet). 
     In the case the recording position is automatically displaced according to whether the sheet feeding is executed or not by the ASF, there may be provided mode discrimination means for discriminating whether the sheet feeding is executed by the ASF  1 , and the recording position may be displaced according to the result of discrimination by the mode discrimination means. Such mode discrimination means can be composed, for example, of: 
     means for electrically detecting the connection state of the printer connector  117  and the ASF connector  44 ; or 
     a switch or a sensor provided on the printer for exclusively detecting the presence or absence of the ASF  1  (namely detecting the automatic/manual insert sheet feeding). 
     The amount of displacement between the guides of the ASF and of the printer and the amount of displacement in the recording position between the automatic and manual insert sheet feedings need not be exactly same, but have to be selected same in such a level that an ordinary person observes that “the image is recorded in a same recording position regardless whether the sheet is automatically fed or manually inserted and fed”. 
     Below there will be explained an ASF sheet feeding tray  2  for supporting the stacked sheets. 
     As shown in FIGS. 1 to  4 , the ASF sheet feeding tray  2  is supported at an end thereof by the upper case  47  of the ASF and is rendered rotatable about the supporting portion. Thus, the ASF sheet feeding tray  2  is opened with a predetermined angle when supporting sheets and can be closed, as shown in FIG. 8, in the absence of the stacked sheets thereon. 
     Such configuration is not for using the mobile printer  101  as the desk-top printer in combination with the ASF  1  but indicates that the printer  101  is very compact and portable even in a state mounted in the ASF  1 . 
     In order to enable such use, the ASF sheet feeding tray  2  needs to be closed, as far as possible in a form along the external form of the ASF  1  mounted with the printer. For this reason, the ASF sheet feeding tray  2  is composed of a thin plate. 
     Also in the present embodiment, the sheet feeding tray  2  is so shaped as to cover, in the closed state, the operation unit of the printer  101  as shown in FIG. 9, in order to prevent the function of the printer  101  caused by an unexpected manipulation of the operation unit when the ASF  1  is carried with the closed sheet feeding tray  2  and with the printer  101  mounted therein. Furthermore, the sheet feeding tray  2  preferably engages with an arbitrary portion of the upper case  47  of the ASF  1 , in order to prevent unexpected opening of the tray  2  when the ASF is carried. 
     On the other hand, in case of feeding an envelope E in the longitudinal position as shown in FIG. 10, the tab E 1  of the envelope E is usually positioned at the left hand side, and the ASF  1  of the present embodiment receives a strong resistance at the tab side (left side) for example by the swelling of the tab portion by moisture, whereby the envelope E receives a clockwise rotating force. 
     In the present embodiment, in order to prevent such clockwise rotation of the envelope E, the ASF sheet feeding tray  2  is provided, at an upstream position in the sheet feeding direction, with an ASF sheet feeding tray side guide  2   a  (hereinafter simply referred to as side guide). Thus, when the envelope E is set in the longitudinally oblong position on the ASF  1 , a right edge of a trailing end of the envelope lies along the side guide  2   a  and is prevented from the clockwise rotation. 
     The envelope in the longitudinally oblong position is subjected to a resistance of the tab portion E 1  at the timing of feeding the envelope E, particularly in the present embodiment when the envelope E proceeds over the bank sheet  37  and when a leading end of the envelope E is lifted immediately thereafter along the inclined surface of the bank  36 . After these situations the resistance of the envelope tab E 1  becomes smaller so that the clockwise rotation is not generated even without the side guide  2   a.    
     For these reasons, in the present embodiment, the side guide  2   a  is provided in a part in the vicinity of a trailing end of the envelope E for preventing the clockwise rotation of the envelope E, but not in the entire longitudinal range of the envelope. The side guide  2   a  is so shaped, when the ASF sheet feeding tray  2  is closed, as to be accommodated in a step difference G formed between the upper case  47  of the ASF and the printer  101  (cf. FIG.  8 ), whereby, when the sheet feeding tray  2  is closed, the side guide  2   a  does not interfere with other portions and the portability is not deteriorated as the sheet feeding tray  2  can be accommodated in a form matching the external shape of the ASF. 
     The side guide  2   a  can be effective if the height thereof is equal to or more than the thickness of the stacked sheets such as the envelopes, and a step difference at least equal to the thickness of the stacked sheets is formed between the upper case  47  of the ASF and the printer  101 . 
     Furthermore, the configuration of the present embodiment is effective for preventing not only the clockwise rotation of the envelope in the longitudinal feeding but also eventual clockwise rotation of any other sheet of a length comparable to that of the envelope, caused by any reason. The side guide  2   a , being integral with the ASF sheet feeding tray  2 , can also be very inexpensive in cost. The side guide  2   a  may also be so formed as to be accommodated, in the closed state, in a recess formed in the printer  101  or the ASF  1 , instead of the step difference G mentioned above. 
     Below there will be explained a mounting and dismounting mechanism of the ASF  1  and the printer  101 . 
     FIG. 11 is a perspective view showing the mounting and dismounting mechanism of the ASF  1 ; FIG. 12 is a perspective view showing the mounting and dismounting mechanism of the printer  101 ; and FIG. 13 is a cross-sectional view showing the mounting and dismounting mechanism of the ASF  1 . 
     As shown in FIG. 11, the ASF  1  is provided with a positioning base  39  which is provided with two positioning bosses  39   d ,  39   e . On the other hand, the printer  101  is provided, as shown in FIG. 12, with a board holder  118  so positioned as to oppose to the positioning base  39  and provided with a positioning hole  118   a  opposed to a first positioning boss  39   d  and a positioning oblong hole  118   b  opposed to a second positioning boss  39   e . In connecting the printer  101  with the ASF  1 , before the connection is made between the ASF connector  44  and the printer connector  117 , the bosses  39   d ,  39   e  are fitted with the positioning holes  118   a ,  118   b  (oblong hole) to define the relative position of the printer  101  and the ASF  1  in the x and z directions. Thus the ASF connector  44  and the printer connector  117  can be exactly connected without damage by the misalignment of the connectors. Also, the sheet path of the ASF  1  can be exactly connected with the sheet path in the printer  101 . 
     On the other hand, the ASF  1  is provided, as shown in FIG. 11, with a horizontal printer sliding portion  45   b  for defining the moving direction of the printer  101  on the connecting operation. Also, there are provided hooks  16 ,  17  (more exactly hook claws  16   a ,  17   a  of the hooks  16 ,  17 ) so as to be protrudable upwards from the printer slidable portion  45   b . These hooks  16 ,  17  (in the following they are distinguished if necessary as a left hook  16  and a right hook  17 ) are both fixed on a hook shaft  18  as shown in FIG.  13  and are rotatably mounted on the chassis  11  so as to rotate integrally. Between the hook  16  and the ASF base  45 , there is provided a hook spring  3 , composed of a compressed coil spring, to bias the hooks  16 ,  17  upwards (namely in a direction that they engage with hook fixing holes  103   y ,  103   z  to be explained in the following). 
     On the other hand, the base  103  of the printer  101  is provided with, as shown in FIG. 12, hook fixing holes  103   y ,  103   z  in positions corresponding to the claws  16   a ,  17   a  of the hooks  16 ,  17  when the ASF  1  is mounted, and the engagement of the claws  16   a ,  17   a  with the fixing holes  103   y ,  103   z  defines the relative position of the ASF  1  and the printer  101  in the y direction. 
     On the other hand, on the positioning base  39  of the ASF, there is fixed, as shown in FIG. 13, a lever shaft  42  supporting a push lever  40  so as to be movable in directions  40 A and  40 B and rotatable in a direction  40 C. Between the push lever  40  and the chassis  11  there is provided a push lever spring  7  for clockwise biasing the push lever  40 . Between the push lever  40  and the left hook  16  there is provided a connecting spring  9  so as to maintain the upper face of the left hook  16  and the lower end  40   d  of the push lever  40  in constant contact (engagement). 
     Furthermore, the push lever  40  is provided with a boss  40   c  for limiting the rotation thereof, and the positioning base  39  is provided with slide faces  39   a ,  39   b ,  39   c  for impinging on the boss  40   c . The slide faces  39   a ,  39   b ,  39   c  are represented by chain lines in order to clarify the configuration. In the above-described configuration, the rotation of the push lever  40  about the lever shaft  42  is limited by impingement of the boss  40   c  of the push lever  40  against the guide face  39   a.    
     In the foregoing description, the hooks  16 ,  17  and the push lever  40  are provided on the ASF  1  while the hook fixing holes  103   y ,  103   z  are provided on the printer  101 , but it is also possible to provide the printer  101  with the hooks and a push lever and the ASF  1  with the hook fixing holes. Also, there are provided two hooks  16 ,  17  and the corresponding fixing holes  103   y ,  103   z , but such number is not restrictive and there may be provided three or more hooks and the corresponding fixing holes. Furthermore, the hooks  16 ,  17  do not need to be rotatable as explained in the foregoing but they only need to be displaceable. Furthermore, the hooks  16 ,  17  are so formed as to rotate integrally by fixing on the hook shaft  18 , but it is also possible to press the hooks  16 ,  17  by the lever shaft  42  and to achieve integral rotation by such configuration. 
     Thereafter, popup members  43   a ,  43   b  provided on the ASF  1  press an upper part  102   a  of the printer  101  in the sheet feeding side thereof in a direction  43 A (y direction) to release the connection between the connectors  117 ,  44 . The popup members  43   a ,  43   b  are biased by an elastic member (not shown), in the direction  43 A (y direction) and are rendered slidable in the y direction. 
     The biasing force for the popup members  43   a ,  43   b  is selected at a suitable level, because such biasing force, functioning as a repulsive force in mounting the printer  101  to the ASF  1 , renders such mounting impossible if it is excessively strong (for example at a level with which the ASF  1  is not moved by the biasing force at the mounting of the printer  101  thereto). 
     However, the force required for detaching the connectors may exceed the biasing force of the popup members  43   a ,  43   b , and, in such case, the connection between the connectors cannot be released solely by the popup members  43   a ,  43   b . For this reason, in the present embodiment, a protruding portion  40   b  of the push lever  40  protrudes in the y direction by a movement of the push lever  40  in a direction of the arrow  40 A. 
     The protruding portion  40   b  of the push lever  40  presses a lower (or central) portion  102   b  of the printer  101  at the sheet feeding side thereof, thereby releasing the connection between the connectors  44 ,  117 . It is thus rendered possible, for the user, to easily extract the printer  101  in the y direction from the ASF  1 . 
     Now there will be explained, with reference to FIGS. 14 to  16 , the operations in connecting the printer  101  and the ASF  1  and the functions of such operations. FIG. 14 is a view showing a state in which the printer  101  is rested on the printer sliding portion  45   b ; FIG. 15 is a view showing a state in which the printer  101  is pressed in; and FIG. 16 is a view showing a state in which the printer  101  is connected to the ASF  1 . 
     At first referring to FIG. 14, as the printer  101  is pressed in a direction indicated by the arrow A along the printer sliding portion  45   b  of the ASF base  45 , the hooks  16 ,  17  are rotated clockwise and the claws  16   a ,  17   a  are pressed downwards in a direction indicated by the arrow  16 A (hook  17  and claw  17   a  being omitted in FIG.  15 ). In this operation, the push lever  40  is moved downwards through the connecting spring  9 . The printer  101  is pressed further in this state and, the bosses  39   d ,  39   e  of the ASF engage with the positioning holes  118   a ,  118   b  (oblong hole) of the printer to define the relative position in the x and z directions. Thereafter, the ASF connector  44  and the printer connector  117  are mutually connected. 
     When the hook fixing holes  103   y ,  103   z  reach the positions of the claws  16   a ,  17   a , they are moved counterclockwise (in a direction indicated by the arrow  16 B) by the biasing force of the hook spring  3 , whereby the fixing holes  103   y ,  103   z  respectively engage with the claws  16   a ,  17   a . Also the push lever  40 , already moved downwards, is pushed up to the normal position by the hook spring  3 , through the hooks  16 ,  17 . In this manner the connection between the printer  101  and the ASF  1  is completed. As the hooks  16 ,  17  are so constructed as to rotate integrally, they do not rotate unless both claws  16   a ,  17   a  match and engage with the fixing holes  103   y ,  103   z  and the push lever is not pushed up. Consequently, for example if the printer  101  is mounted in an inclined state to the ASF  1 , the push lever  40  is not pushed up to the normal position, and the user can easily know whether the printer  101  is properly mounted to the ASF  1 , by observing the state of the push lever  40 . 
     Furthermore, if the height of the claws  16   a ,  17   a  in a state engaging with the fixing holes  103   y ,  103   z  is selected substantially same as or somewhat higher than the height of the hook shaft  18  (constituting the rotary center of the hooks  16 ,  17 ), the hooks  16 ,  17  do not rotate under the application of a force in an opposite direction (or in a direction opposite to the direction indicated by the arrow A) to the printer  101 , whereby the printer  101  can be protected from detachment from the ASF  1 . 
     Below there will be explained the operations in separating the printer  101  and the ASF  1  and the functions of such operations. 
     For separating the printer  101  and the ASF  1 , a push portion  40   a  of the push lever  40  is pressed down (in the direction indicated by the arrow  40 A) as shown in FIG.  17 . The push lever  40 , of which boss  40   c  is sandwiched between guide faces  39   a ,  39   b  formed on the positioning base  39 , is incapable of rotation about the lever shaft  42  until the end of the guide face  39   b  and descends in the direction indicated by the arrow  40 A. Thus, the hooks  16 ,  17  integrally rotate in a downward direction indicated by the arrow  16 A about the hook shaft  18  whereby the claws  16   a ,  17   a  are disengaged from the fixing holes  103   y ,  103   z . In the present embodiment, as the hooks  16 ,  17  are so constructed as to rotate integrally, the manipulation of the push lever  40  allows for simultaneous disengagement of both claws  16   a ,  17   a  thereby achieving a simple separating operation. Also, in disengaging the claws  16   a ,  17   a  from the fixing holes  103   y ,  103   z , it is not necessary to unmovably hold the image forming apparatus  100  itself, and a simple separating operation is realized by merely depressing the push lever  40  with one hand. 
     When the claws are disengaged as explained above, the popup members  43   a ,  43   b , shown by broken lines in FIGS. 16 and 17, push the upper portion  102   a  of the printer  101  in the sheet feeding side thereof, thereby pushing out the printer  101  in a direction indicated by the arrow B. At the same time, the ASF connector  44  and the printer connector  117  are mutually disengaged. 
     A state shown in FIG. 15 is reached when the user terminates the depression of the push lever  40  in the direction indicated by the arrow  40 A. In this state, the connectors  44 ,  117  are disconnected and the hook  16  and the printer  101  are disengaged, whereby the user can easily remove the printer  101  from the ASF  1 . 
     However, if the force required for disengaging the connectors exceeds the pushing force of the popup member  43   a ,  43   b  as explained in the foregoing, the state shown in FIG. 15 is not reached because the printer  101  does not move when the hook  16  is disengaged from the printer  101 , so that the user cannot remove the printer  101  from the ASF  1 . 
     Consequently, in the present embodiment, there is added the above-described pushing function by the user. 
     FIG. 17 shows a state in which the printer  101  does not move even after the hook  16  is disengaged from the printer  101 . In this state, the (left) hook  16  is disengaged from the fixing hole  103   y  while the boss  40   c  of the push lever  40  is released from the limitation in the moving direction by the guide face  39   b  of the positioning base  39 . 
     Also the lever shaft  42  is pressed to the upper end face of a sliding hole  40   e  of the push lever  40 , thereby limiting the downward movement of the (left) hook  16 . Furthermore, as a contact face  40   d  of the push lever  40  with the (left) hook  16  is formed as an arc around the lever shaft  42 , the position of the (left) hook  16  does not change by the rotation of the push lever  40 . 
     If the user continues to depress the push portion  40   a  of the push lever  40 , it rotates in a direction indicated by the arrow  40 D about the lever shaft  42 , and such rotation brings the protruding portion  40   b  of the push lever  40  in contact with the lower portion  102   b  of the printer  101  in the sheet feeding side thereof while the (left) hook  16  is disengaged from the printer  101 , whereby the printer  101  is pushed out in a direction indicated by the arrow B. 
     If the push lever  40  continues to be depressed thereafter, a contact face  40   f  of the push lever  40  impinges on a stopper portion  39   f  of the positioning base  39  as shown in FIG. 18, whereupon the rotation of the push lever  40  is limited. The push-out (shifting) amount of the printer  101  by the push lever  40  is so selected as to release the engagement between the (left) hook  16  and the printer  101  and the engagement between the connectors. 
     After the printer  101  is thus pushed out, the user terminates the depression of the push portion  40   a  of the push lever  40 , whereupon the (left) hook  16  is elevated in a direction indicated by the arrow  16 B by the function of the hook spring  3 . At the same time the push lever  40  is pushed up by the (left) hook  16 , whereby the boss  40   c  of the push lever  40  impinges on the guide face  39   c  of the positioning base  39  and the push lever  40  rotates thereafter in a direction indicated by the arrow  40 E by the tension of the spring  7 . When the boss  40   c  of the push lever  40  impinges on the guide face  39   a  of the positioning base  39 , the push lever  40  is limited in rotation and is elevated in a direction indicated by the arrow  40 B by the force of the hook spring  3 . 
     Thus, the connectors are eventually disconnected as shown in FIG. 15, while the (left) hook  16  is disengaged from the printer  101 , and the user can easily remove the printer  101  from the ASF  1 . 
     In the present embodiment, as explained in the foregoing, the push lever  40  is depressed substantially vertically in detaching the printer  101  from the ASF  1 , so that a vertical force is applied to the ASF itself. For this reason, the ASF  1  is not displaced when the printer  101  is pushed out substantially horizontally. Also, since the printer  101  is pushed out substantially horizontally, there will not result a failure in the detachment, caused by the movement of the printer  101  in the mounting direction by the weight thereof. 
     FIG. 19 is a view showing the arrangement of the push lever  40 , the popup members  43   a ,  43   b , the positioning bosses  39   d ,  39   e , the (left) hook  16 , the (right) hook  17 , and the ASF connector  44  and the relationship of forces thereof, and FIG. 20 is a partial cross-sectional view of the upper face of the ASF  1 . 
     As shown in FIGS. 19 and 20, the positioning bosses  39   d ,  39   e  of the printer  101  and the hooks  16 ,  17  are provided in the vicinity of both ends of the printer  101  in the width thereof. The ASF connector  44  is positioned between the positioning bosses  39   d ,  39   e , close to the second positioning boss  39   e . The push lever  40  and the second popup member  43   b  are positioned farther, than the ASF connector  44 , from the first positioning boss  39   d.    
     In the above-described configuration, the detachment of the printer  101  from the ASF  1  is achieved by the depression of the push lever  40  in the direction indicated by the arrow  40 A as explained in the foregoing, whereupon the hooks  16 ,  17  are disengaged from the fixing holes  103   y ,  103   z  (cf. FIG. 14) while the protruding portion  40   b  of the push lever  40  impinges on and pushes out the printer  101 . In this manner there can be achieved disconnection of the connectors and disengagement of the hooks  16 ,  17  from the fixing holes  103   y ,  103   z.    
     The popup members  43   a ,  43   b  are auxiliary members for reducing the force required for depressing the push lever  40  by the user, and are slidably biased, by an elastic member (not shown), at a predetermined position. 
     In the present embodiment, the printer  101  is pushed out, while sliding on the printer sliding portion  45   b  by rotation about the positioning boss  39   d  or  39   e.    
     The positioning hole  118   a  at the side of the first positioning boss constituting the center of rotation is formed as a circular hole while the positioning hole  118   b  at the side of the second positioning boss is formed as an oblong hole (cf. FIG.  12 ), so that, in case of detaching the printer  101  from the ASF  1  by rotation about the first positioning boss  39   d  starting from the state shown in FIG. 20, there is reached a positional relationship between the printer  101  and the ASF  1  as shown in FIG.  21 . 
     In such state, however, the printer  101  cannot be moved by the pushing force of the first popup member  43   a  alone, because of the sticking engagement between the first positioning boss  39   d  and the positioning hole  118   a . Also, in case the user forcedly remove the printer  101  from the ASF  1 , there may result deformation or destruction of the first positioning boss  39   d . 
     Therefore, in order to avoid such sticking engagement, the present embodiment adopts a configuration in which, before the printer  101  is pushed out by the push lever  40  and the second popup member  43   b , the engaging position between the first positioning boss  39   d , constituting the center of rotation of the printer  101 , and the positioning hole  118   a  is displaced toward the connector disengaging direction by the pushing force of the first popup member  43   a.    
     More specifically, in the dimensional relationship shown in FIG. 19, the force required to push out the printer  101  by the pushing force of the first popup member  43   a  by rotation about the first positioning boss  39   d  is represented by: 
     
       
           F   1 ≳( X   1 / X   2 )× P   1 + P   2   
       
     
     wherein F 1  is the printer pushing force of the first popup member  43   a , P 1  is the force required for detaching the connectors, P 2  is the frictional force between the printer  101  and the printer sliding portion  45   b  of the ASF  1 , X 1  is the distance from the second positioning boss  39   e  constituting the center of rotation to the connector  44 , and X 2  is the distance from the second positioning boss  39   e  constituting the center of rotation to the first popup member  43   a.    
     As will be apparent from the foregoing relation, the pushing force F 1  of the first popup member  43   a  can be made smaller as the distance between the first popup member  43   a  and the ASF connector  44  becomes larger or as the ratio X 1 /X 2  becomes smaller. In consideration of the aforementioned fact that the pushing force F 1  of the first popup member  43   a  functions as a repulsive force in mounting the printer  101  to the ASF  1  and a fact that the force required for disconnecting the connectors is generally within a range of 1 to 2 kgf, the ratio X 1 /X 2  is advantageously selected at 0.5 or smaller. 
     On the other hand, in the present embodiment, the claw of the (right) hook  17  is formed lower than that of the (left) hook  16 , whereby the (right) hook  17  is disengaged from the fixing hole  103   z  (cf. FIG. 12) earlier than the disengagement of the (left) hook  16  from the fixing hole  103   y.    
     Therefore, at the moment when the (right) hook  17  is disengaged from the fixing hole  103   z  of the printer  101 , the printer  101  rotates about the second positioning boss  39   e  by the pushing force of the first popup member  43   a , whereby the engaging position between the first positioning boss  39   d  and the positioning hole  118   a  moves toward the connector disengaging side as shown in FIG.  22 . 
     Subsequently the (left) hook  16  is disengaged from the fixing hole  103   y  of the printer  101  whereupon the printer  101  is pushed out by the push lever  40  and the second popup member  43   b . Thus it is rendered possible to detach the printer  101  from the ASF  1  in a state shown in FIG. 23 without the sticking engagement between the first positioning boss  39   d  and the positioning hole  118   a.    
     If the push lever  40  and the second popup member  43   b  are provided between the ASF connector  44  and the first positioning boss  39   d  constituting the center of rotation of the printer  101 , and if the connectors has a large connecting force, the connector  44  becomes the center of rotation of the printer  101  whereby a sticking engagement is generated between the first positioning boss  39   d  and the circular positioning hole  118   a  of the printer  101 , eventually resulting in deformation and/or destruction of the boss  39   d.    
     Based on these facts, the push lever  40  and the second popup member  43   b  have to be positioned farther, than the ASF connector  44 , from the first positioning boss  39   d  constituting the center of rotation of the printer  101 . 
     [Control Unit] 
     FIG. 24 is a block diagram of a main control unit for the printer and a control unit for the external ASF of the present invention. 
     A main control unit  202  for controlling the printer  101  is provided on a main body board  123  shown in FIG. 4, and is provided with a microcomputer in which a CPU  203 , a ROM  204 , and a RAM  205  are connected through buses. 
     In the recording operation by the printer  101 , the main control unit  202  drives a carriage motor  121  through a motor driver  208  and also drives a recording head  115  mounted on a carriage (not shown) connected to the carriage motor  121  through a head driver  210  according to a main control program stored in the ROM  204 , thereby effecting recording of a line. 
     Subsequently the main control unit  202  advances the sheet by driving a sheet feeding motor  120  through a motor driver  206 , and then repeats the driving of the carriage motor  121  and the recording head  115 , thereby completing the recording on the sheet. The connector  117  functions as a bothway communication port capable of transmitting command signals from the CPU  203  of the main control unit to the exterior and receiving response signals from the exterior into the CPU  203 , and is also capable of power supply to the exterior as will be explained later. A sheet end sensor  108  is provided in the printer body and has an optical or mechanical switch. When the sheet  200  is inserted into the printer main body, the output voltage of the sheet end sensor  108  changes from a LOW state to a HIGH state. A sheet discharge sensor  113  similar in configuration to the sheet end sensor  108  outputs a voltage of a HIGH state if the sheet  200  after recording remains in the printer body. 
     The output voltages of the sheet end sensor  108  and the sheet discharge sensor  113  can be monitored by the CPU  203 , and the output voltage of the sheet end sensor  108  can be directly outputted to the exterior through the connector  117 . 
     The ASF control unit  201  for controlling an external ASF  1  is provided, as in the main control unit  202 , with a microcomputer in which a CPU  213 , a ROM  214  and a RAM  215  are connected through buses. The CPU  213  controls a sheet feeding motor  27  through a motor driver  216 , based on an ASF control program stored in the ROM  214 . The ASF connector  44  functions as a bothway communication port for receiving signals from external equipment such as the printer  101  and transmitting signals from the CPU  213  of the ASF control unit. 
     [Communication Port] 
     FIG. 26 is a schematic view showing the detailed configuration of the connector  117  and the ASF connector  44  mentioned above. The connector  117  and the ASF connector  44  are respectively provided with eight ports  117   a  to  117   h  and  44   a  to  44   h , and ports having a same alphabetical suffix are mutually connected electrically when the printer  101  is mounted to the ASF  1 . 
     In the ASF side, there are provided a ground (GND) line  44   a ; a 5 V power supply line  44   b  for signals; a 24 V power supply line  44   e  for driving the sheet feeding motor  27 ; a transmission port  44   f  for transmitting signals to the printer; a reception port  44   g  for receiving signals from the printer; and a line  44   h  for receiving the output voltage of the sheet end sensor  108  provided in the printer body. Ports  44   c  and  44   d  are mutually short circuited, whereby the printer  101  can easily identify, utilizing ports  117   c  and  117   d , connection of an external equipment. 
     [Separating and Conveying Mechanisms of ASF] 
     FIG. 25 is a cross-sectional view showing a state in which the external ASF of the present invention is mounted on the printer body. 
     A sheet feeding roller  19  for feeding the sheet  200  is fitted with a sheet feeding rubber member  23 , and, when the sheet feeding roller  19  is rotated, the sheet  200  is conveyed, by the frictional force of the sheet feeding rubber member  23 . 
     A pressure plate  26  for supporting the stacked sheets  200  is pivotably supported by the ASF chassis  11  at the upstream end in the sheet conveying direction. The pressure plate  26  is biased toward the sheet feeding rubber member  23  by a pressure plate spring  13 . But, in the initial state, the sheet feeding rubber member  23  and the pressure plate  26  are mutually separated because cam portions  19   c  provided on both ends of the sheet feeding roller  19  engage with cam portions  26   a  provided on both ends of the pressure plate  26 , whereby the sheets  200  can be smoothly set. A bank  36  is provided with an impingement face  36   a  in the extension of the sheet conveying direction of the pressure plate  26 , and the sheets  200  are set in such a manner that the front end thereof impinges on the impingement face  36   a . The impingement face  36   a  is provided with a bank sheet  37  constituting a sheet separating member. The bank sheet  37  is composed of an elastic member such as a plastic sheet, and serves to separate the sheets one by one, utilizing an elastic force generated by bending. 
     [Conveying and Printing Mechanisms of the Printer] 
     In the following there will be explained, with reference to FIG. 25, a conveying mechanism and a printing mechanism in the printer. 
     An LF roller  109  for conveying the sheet  200  is composed of a metal pipe and a film of a material with a high friction coefficient such as urethane resin formed on the metal pipe. The LF roller  109  is rotated by the sheet feeding motor  120  shown in FIG.  24  and pinches and conveys the sheet  200  in cooperation with a pinch roller  110 . 
     A recording head  115  for recording image information on the sheet  200  conveyed by the LF roller  109  is mounted on a carriage (not shown), capable of reciprocating motion along the longitudinal direction of the LF roller  109 . The recording head  115  is driven together with the carriage by the carriage motor  121  shown in FIG. 24, and is capable of a reciprocating motion in the transverse direction of the sheet (a direction perpendicular to the plane of the drawing). 
     Spurs  111  and sheet discharge rollers  112  are positioned in two sets at the downstream side of the LF roller  109  and the recording head  115 , for conveying the sheet  200  after the recording. The sheet discharge rollers  112  are linked with the LF roller  109  through transmission members (not shown) and are rotated by the LF roller  109  so as to convey the sheet  200  in a direction the same as the conveying direction of the LF roller  109 . 
     Also the sheet end sensor  108  is provided in the sheet path of the upstream side in the sheet conveying direction with respect to the LF roller  109  while the sheet discharge sensor  113  is positioned between the two sets of sheet discharge rollers, and each sensor changes the output voltage from the LOW state to the HIGH state in response to the passing of the sheet  200 . 
     [Driving Mechanism of ASF] 
     FIGS. 27 and 28 show the driving mechanism of the external ASF of the present invention. 
     There are provided a sheet feeding motor  27  composed of a stepping motor capable of forward and reverse rotation; an idle gear  28  meshing with a motor gear  27   a  of the sheet feeding motor  27 ; an ASF double gear  29  having two gears different in diameter and meshing with the idle gear  28 ; a forward rotating planet gear  31  meshing with the smaller one of the above-mentioned double gear and rotating around the double gear; a reverse rotating sun gear  33  having two gears different in diameter and meshing with the smaller one of the above-mentioned double gear  29 ; and a reverse rotating plane gear  35  meshing with the smaller gear of the above-mentioned reverse rotating sun gear  33  and rotating around the reverse rotating sun gear. A sheet feeding roller gear  19   a  provided on the shaft end of the sheet feeding roller  19  has a toothless portion  19   b . The sheet feeding roller gear  19   a  is positioned on the rotating trajectory of the forward rotating planet gear  31  and the reverse rotating planet gear  35  so as to mesh with these gears. 
     In the following there will be explained the functions of these gears. Referring to FIG. 27, when the sheet feeding motor  27  rotates in a direction indicated by the arrow b (reverse rotation), the gears rotate respectively in directions indicated by arrows. More specifically, through the idle gear  28  and the ASF double gear  29 , the reverse rotating planet gear  35  moves about the reverse rotating sun gear  33  from a broken-lined position in FIG. 27 to a solid-line position as indicated by an arrow, and meshes with the sheet feeding roller gear  19   a , thereby rotating the sheet feeding roller  19  in a direction indicated by an arrow (namely in a direction to advance the sheet  200  on the pressure plate  26  toward the printer  101 ). The sheet feeding roller gear  19   a , rotating by meshing with the reverse rotating planet gears  35 , is released from the meshing when the toothless portion  19   b  reaches a position opposed to the reverse rotating planet gear  35 , whereby it is no longer rotated by the reverse rotation of the sheet feeding motor  27 . 
     In this operation, the forward rotating planet gear  31  moves from a broken-lined position in FIG. 27 to a solid-line position in a direction indicated by the arrow and is stopped therein by impinging on a stopper (not shown), so that it does not influence the rotation of the sheet feeding roller  19 . 
     Then, referring to FIG. 28, when the sheet feeding motor  27  rotates in a direction indicated by the arrow f (forward rotation), the gears rotate respectively in directions indicated by arrows. More specifically, through the idle gear  28  and the ASF double gear  29 , the forward rotating planet gear  31  moves about the ASF double gear  29  from a broken-lined position in FIG. 28 to a solid-line position as indicated by an arrow, and meshes with the sheet feeding roller gear  19   a , thereby rotating the sheet feeding roller  19  in a direction indicated by an arrow in FIG. 28 (namely in a direction to advance the sheet  200  on the pressure plate  26  toward the printer  101 ). The sheet feeding roller gear  19   a , rotating by meshing with the forward rotating planet gear  31 , is released from the meshing when the toothless portion  19   b  reaches a position opposed to the forward rotating planet gear  31 , whereby it is no longer rotated by the forward rotation of the sheet feeding motor  27 . 
     In this operation, the reverse rotating planet gear  33  moves from a broken-lined position in FIG. 28 to a solid-line position and is stopped therein by impinging on a stopper (not shown), so that it does not influence the rotation of the sheet feeding roller  19 . 
     In a position where the toothless portion  19   b  of the sheet feeding roller gear  19   a  is opposed to the forward rotating planet gear  31 , the cam portion  19   c  of the sheet feeding roller engages with the cam portion  26   a  of the pressure plate  26  as in the initial state, thereby separating the pressure plate  26  from the sheet feeding rubber member  23 . 
     Consequently, when the sheet feeding motor  27  is continuously rotated in the forward direction, the cam portion  19   c  of the sheet feeding roller  19  engages with the cam portion  26   a  of the pressure plate  26  whereby the sheet feeding roller  19  terminates rotation in a phase the same as in the initial state where the pressure plate  26  is separated from the sheet feeding rubber member  23 , and the forward rotating planet gear  33  and the reverse rotating planet gear  35  thereafter rotate idly in the solid-lined positions in FIG. 28 whereby the mechanism is stabilized in a state not transmitting the rotation to the sheet feeding roller  19 . 
     As explained in the foregoing, the sheet feeding roller  19  rotates only in a direction for advancing the sheet  200  toward the printer  101  regardless whether the sheet feeding motor  27  is rotated in the forward (normal) or reverse direction, and never rotates in the opposite direction. 
     [Sheet Feeding Operation and Recording Operation (in the Printer)] 
     In the following there will be explained a series of operations for feeding, conveying and recording the sheet  200  and thereafter discharging the sheet  200 , to be executed by the printer and the ASF of the present invention. 
     In response to a recording command received from an external information device such as a computer, the printer  101  executes at first a sheet feeding operation and then a recording operation. 
     FIG. 29 is a flowchart showing the control sequence in case the printer  101  executes a sheet feeding operation. At first the main control unit  202  of the printer  101  executes a sub flow C 1  of which details will be explained later with reference to FIG.  33 . The sub flow C 1  is to discriminate, a type of the external device mounted to the printer, through the ports  117   f ,  117   g  shown in FIG.  26 . 
     Then the sequence proceeds to a step S 1 , and, if the result of the sub flow C 1  indicates that the ASF is mounted to the printer  101 , the sequence proceeds to a step S 2  as the sheet feeding is to be executed by the ASF. In a step S 2 , the main control unit  202  transmits an initializing command signal to the ASF, and the sequence proceeds to a step S 3 . 
     If a response signal indicating the completion of initialization in the ASF is not received in the step S 3 , the sequence repeats the step S 3 . Upon receiving such response signal, the sequence proceeds to a step S 4  in which the main control unit  202  transmits a sheet feed command signal and a sheet type signal indicating the kind of the sheet (such as ordinary paper, coated paper, postcard, glossy film etc.) to the ASF, and the sequence proceeds to a step S 5 . 
     The sequence proceeds to a step S 8  if the response signal is not received from the ASF in the step S 5 , but repeats the step S 5  if a predetermined limit time t 2  has not elapsed. If a step S 8  identifies that the limit time t 2  has elapsed, the sequence proceeds to a step S 9  in which the main control unit  202  issues a sheet feeding error and terminates the sheet feeding operation. If a step S 5  receives a response signal from the ASF, indicating the completion of the sheet feeding, the sequence proceeds to a step S 7 . A step S 7  executes so-called head feeding operation for the sheet  200  (an operation for feeding a leading end of a sheet to an initial position), whereby the main control unit  202  drives the sheet feeding motor  120  to rotate the LF roller  109  by a predetermined amount R 3  in the sheet conveying direction in the recording operation (forward (normal) rotation), thereby terminating the sheet feeding operation. The predetermined amount R 3  is so selected that the leading end of the sheet  200  does not reach the detecting area of the sheet discharge sensor  113  but reaches a position directly under the recording head  115 . Consequently, when the printer  101  starts recording on the sheet  200 , it need not be reversed toward the upstream side in the conveying direction, so that the trailing end of the sheet  200  does not collide with the internal components of the ASF and that the sheet  200  can be protected from creasing or misfeeding. 
     Also if the step S 5  receives a response signal from the ASF, indicating a sheet feeding error, the sequence proceeds to a step S 9  in which the main control unit  202  issues a sheet feeding error and terminates the sheet feeding operation. 
     If, in the step S 1 , the result of the sub flow C 1  indicates that the ASF is not mounted on the printer  101 , the sequence proceeds to a step S 10  as the sheet feeding is to be executed by manual insert. 
     If the user does not insert a sheet in the step S 10 , the sheet is not detected and the sheet end sensor  108  provides a low output voltage, whereupon the sequence repeats the step S 10 . When the user inserts the sheet  200  in the printer  101  to impinge the sheet  200  on the LF roller  109 , the sheet end sensor  108  releases a High output voltage, indicating the sheet detection, whereupon the sequence proceeds to a step S 11 . In the step S 11 , the main control unit  202  drives the sheet feeding motor  120  through the sheet feeding motor driver  206 , so as to rotate the LF roller  109  normally by a predetermined amount R 4  (in the forward direction for conveying the sheet in the conveying direction in the recording operation). The predetermined amount R 4  is so selected that the leading end of the sheet  200  reaches the detection area of the sheet discharge sensor  113 . Then a step S 12  identifies that the sheet feeding is successful if the sheet discharge sensor  113  detects the sheet  200 , and the sequence proceeds to a step S 13 . In the step S 13 , the main control unit  202  drives the sheet feeding motor  120  through the sheet feeding motor driver  206 , so as to rotate the LF roller  109  reversely by a predetermined amount R 5  (in the reverse direction for conveying the sheet in a direction opposite to the conveying direction in the recording operation). The predetermined amount R 5  is so selected that the sheet  200  conveyed to the detection area of the sheet discharge sensor  113  is returned to the recording start position and that the leading end of the sheet  200  does not come out of the nip between the LF roller  109  and the pinch roller  110 . 
     In the step S 12 , if the sheet discharge sensor  113  does not detect the sheet  200 , for example if the sheet  200  does not impinge strongly enough on the LF roller  109  and is not pinched between the LF roller  109  and the pinch roller  110  or if the leading end of the sheet  200  does not reach the detection area of the sheet discharge sensor  113  even after sheet conveyance by the amount R 4  because of a skewed impingement of the sheet  200  on the LF roller  109 , the main control unit  202  identifies a failure in the sheet feeding in manual insert and the sequence proceeds to a step S 14 . In the step S 14 , the main control unit  202  drives the sheet feeding motor  120  through the sheet feeding motor driver  206  so as to rotate the LF roller  109  reversely by a predetermined amount R 6 . The predetermined amount R 6  is so selected that the leading end of the sheet  200 , conveyed to the detection area of the sheet discharge sensor  113 , can satisfactorily escape from the nip between the LF roller  109  and the pinch roller  110 . 
     Thus, in the manual insert operation, the successful feeding can be securely confirmed by discriminating whether the sheet discharge sensor  113  detects the sheet  200 , and, in case of a failure in the sheet feeding, the sheet  200  is returned to a position where it is not pinched by the LF roller  109  whereby the sheet  200  can be easily removed and manually inserted anew. 
     In contrast to the ASF sheet feeding, there are no mechanical components colliding with the returning sheet  200  in case of manual insert, so that the returning conveyance thereof does not result in creasing or misfeeding. 
     After the completion of the sheet feeding operation by the above-described sheet feeding control sequence, the printer  101  executes the recording operation. The main control unit  202  drives the carriage motor  121  through the motor driver  208  and the recording head  115 , mounted on the carriage (not shown) connected to the carriage motor  121 , through the head driver  210 , thereby affecting recording of a line. Subsequently the main control unit  202  drives the sheet feeding motor  120  through the motor driver  206  to advance the sheet  200  by a line, and then repeats the driving of the carriage motor  121  and the recording head  115 , thereby completing the recording on the sheet. After completion of the recording, the main control unit  202  drives the sheet feeding motor  120 , thereby rotating the LF roller  109  normally. Thus, the sheet discharge roller  112  is driven to discharge the sheet  200  from the printer  101 . 
     [Sheet Feeding Operation (in the ASF)] 
     FIG. 30 is a flowchart showing the main control sequence of the ASF which can be externally attached to the printer of the present invention. The control unit  201  of the ASF  1  of the present invention is normally in a stand-by state when connected to the printer  101 , and repeats a step S 37 , if a command signal is not received from the printer  101 , until the command signal is received. When a command signal from the printer  101  is received through the serial receiving port  44   g  shown in FIG. 26, the sequence proceeds to following sub flows or steps according to the content of the command signal. If the command signal from the printer  101  is a “sheet feed command” or an “initializing command”, the sequence respectively proceeds to a sub flow C 2  for controlling the ASF sheet feeding operation or a sub flow C 3  for controlling the initializing operation, and, after the completion of each sub flow, the sequence returns to the step S 37  to enter the stand-by state. If the command signal from the printer  101  is a “type of device discriminating command”, the sequence proceeds to a step S 6  for transmitting an ID code indicating the type of the ASF  1  itself to the printer  101  through the serial transmitting port  44   f , and then proceeds to the step S 37  for entering the stand-by state. 
     Among the two sub flows mentioned above, the sub flow C 2  for controlling the ASF sheet feeding operation will be explained in the following, and the sub flow C 3  for controlling the initializing operation will be explained later. 
     FIG. 31 is a flowchart showing the sub flow C 2  for controlling the sheet feeding operation in the ASF  1 . 
     At first in a step S 15 , the ASF control unit  201  reads a driving table T for the sheet feeding motor  27  optimum for the sheet type to be fed, from the ROM  214  to the CPU  213 , based on the sheet type information received together with the sheet feeding command signal from the printer  101 . The driving table T contains information such as the drive speed of the sheet feeding motor  27  composed of a pulse motor, a registration pulse number P 5  for rotating the sheet feeding roller  19  by an optimum amount according to the sheet type for registering operation in a step S 22  to be explained later etc., and plural tables are prepared according to the characteristics of anticipated sheets. 
     After reading the driving table T, the sequence proceeds to a step S 16  in which the ASF control unit  201  sets “0” as the initial value for variables INIT, n and Pc. These variables are stored in the RAM  215 . The variable INIT is a flag indicating whether the rotating phase of the sheet feeding roller  19  is in an initial position; n is a rotation number counter indicating the number of rotations of the sheet feeding roller  19  after the start of the sheet feeding flow C 2 ; and Pc is a pulse number counter indicating the number of pulses given to the sheet feeding motor  27  for driving in the reverse direction. 
     In a next step S 17 , the ASF control unit  201  drives, through the sheet feeding motor driver  216 , the sheet feeding motor  19  by one pulse in the reverse direction. A next step S 18  increases the value of the pulse number counter Pc by 1, and, in a next step S 19 , the ASF control unit  201  compares the value of the pulse number counter Pc with a permitted pulse number Pmax. 
     The permitted pulse number Pmax is the total pulse number from the start of reverse rotation of the sheet feeding motor  27  to the end of rotation of the sheet feeding roller to a position where the toothless portion  19   b  of the sheet feeding roller gear comes opposed to the reverse rotation planet gear  35  as explained in FIG.  27 . Since the condition Pc&lt;Pmax is satisfied immediately after the start of sheet feeding, the sequence proceeds to a step S 20 , in which the ASF control unit  201  confirms the output voltage of the sheet end sensor  108  in the printer  101  through the port  44   h  shown in FIG.  26 . As the sheet  200  does not reach the interior of the printer  101  immediately after the start of sheet feeding, the sheet end sensor  108  provides a LOW output voltage, so that the sequence returns to the step S 17 . Through the repetition of the steps S 17  through S 20 , the reverse rotation planet gear  35  shown in FIG. 27 moves from the broke-lined position to the solid-lined position and meshes with the sheet feeding roller gear  19   a , whereby the sheet feeding roller  19  starts rotation. When the sheet feeding roller  19  starts rotation from the initial phase state, the sheet feeding roller cam  19   c  and the pressure plate cam  26   a  are disengaged whereby the pressure plate  26  is lifted upwards by the pressure plate spring  13  and the sheets  200  stacked on the pressure plate  26  are brought into pressure contact with the sheet feeding rubber member  23 . In this operation,the leading end of the sheets  200 , impinging on the impinging face  36   a  of the bank  36 , is also lifted upwards and is maintained in contact with the approximate center of the bank sheet  37 . 
     The steps S 17  through S 20  are further repeated to continue the reverse rotation of the sheet feeding motor  27 , whereby the sheet feeding roller  19  is rotated to initiate the conveyance of the sheet  200  by the frictional force of the sheet feeding rubber member  23 . The leading end of the sheet  200  is separated from the underlying sheets by a repulsive force generated by bending the elastic bank sheet  37 , whereby only one sheet is advanced. 
     However, the relationship Pc&lt;Pmax no longer stands when the reverse rotation of the sheet feeding motor  27  is continued until the value of the pulse number counter Pc reaches a certain magnitude, whereupon the sequence branches to a step S 24  from step S 19 . In the step S 24 , the ASF control unit  201  drives the sheet feeding motor  27  in the forward direction by a predetermined pulse number P 4 , which is enough for rotating the sheet feeding roller  19  to the initial position by the forward rotating planet gear  31 . Thus, by the execution of the step S 24 , the sheet feeding roller  19  rotates to a phase of one exact rotation from the initial position, wherein the toothless portion  19   b  of the sheet feeding roller gear reaches a position opposed to the forward rotating planet gear  31  so that the sheet feeding roller gear is disengaged and stopped. Then, a step S 25  returns the pulse number counter Pc to “0” and increases the value of the rotation number counter n by “1”. As n=1 in this state in a next step S 26 , the sequence returns to the step S 17  to start the reverse rotation of the sheet feeding motor  27  again. 
     The ASF control unit  201  repeats the steps S 17  through S 20  as explained in the foregoing whereby the sheet feeding roller  19  starts a twice rotation and the sheet  200  is further conveyed. When the leading end of the sheet  200  reaches the sheet end sensor  108  in the printer  101 , the sheet end sensor  108  generates a HIGH output voltage whereby the sequence proceeds from S 20  to S 21 . In the step S 21 , the ASF control unit  201  compares a sum of the value of the pulse number counter Pc and the registration pulse number P 5  in the read driving table T, with the permitted pulse number Pmax. If Pc+P 5 ≲Pmax, the sequence proceeds to a step S 22  since, in case the sheet feeding motor  27  is further driven in the reverse direction by P 5  pulses, the reverse drive is not released in the course of the drive. 
     On the other hand, if Pc+P 5 &gt;Pmax, the sequence proceeds to a step S 24  since, by further driving the sheet feeding motor  27  in the reverse direction by P 5  pulses, the toothless portion  19   b  of the sheet feeding roller gear comes opposed to the reverse rotation planet gear  35  in the course of such drive whereby the driving transmission to the sheet feeding roller  19  is interrupted. The step S 24  again drives the sheet feeding motor normally by P 4  pulses to return the sheet feeding roller  19  to the initial position. Then a step S 25  sets “0” for Pc and n+1 for n, and the sequence proceeds to a step S 26 . In this state there stands n=2 because the sheet end sensor  108  normally detects the sheet  200  in the twice rotation of the sheet feeding roller, so that the sequence returns to the step S 17 . At this time, as the sheet end sensor  108  has generated a HIGH output voltage and the pulse number counter Pc has just been reset, the sequence proceeds from the step S 17  to S 18 →S 19 →S 20 →S 21  and S 22  because there is now satisfied a relation Pc+P 5 ≲Pmax. 
     The step S 22  executes so-called registering operation. The ASF control unit  201  drives the sheet feeding motor  27  reversely by the pulse number PS in the read driving table T, thereby rotating the sheet feeding roller  19 . In this operation, the leading end of the sheet  200  is further advanced into the printer  101  from the position detected by the sheet end sensor  108 , and is stopped by impinging on the nip formed between the stopped LF roller  109  and the pinch roller  110 , but the trailing portion of the sheet  200  is further advanced by the sheet feeding roller  19 . Consequently, the leading end of the sheet  200  is aligned parallel to the nip formed between the LF roller  109  and the pinch roller  110 . 
     In a next step S 23 , the ASF control unit  201  transmits a signal indicating the completion of sheet feeding to the printer  101  through the serial transmitting port  44   f  shown in FIG. 26, whereupon the sequence is completed. 
     In case no sheet is present on the pressure plate  26 , the sheet end sensor  108  does not generate the HIGH output voltage regardless of the number of rotations of the sheet feeding roller  19 . 
     Therefore, the ASF control unit  201  repeats twice a sequence of executing a loop of S 17 →S 18 →S 19 →S 20 →S 17  by a predetermined number of times and then returning to S 17  through S 19 →S 24 →S 25 →S 26 , and, upon reaching the step S 26  for the third time, the rotation number counter of the sheet feeding roller  19  becomes n=3 and the sequence then proceeds to a step S 27  for transmitting a sheet feeding error signal to the printer  101 , whereupon the sequence is terminated. 
     [Other Operations (Printer and ASF)] 
     FIG. 32 is a flowchart of a sub flow C 3  for controlling the initializing operation of the ASF  1 . Upon receiving an initializing command signal from the printer  101 , the ASF control unit  201  proceeds to a step S 28  for confirming the value of a flag INIT indicating whether the rotational phase of the sheet feeding roller  19  is in an initial position. If INIT=1 indicating that the sheet feeding roller  19  is already in the initial position, the sequence proceeds to a step S 31  for transmitting an initialization completion signal to the printer  101 , whereupon the sequence is terminated. If INIT=0, the sequence proceeds to a step S 29  for driving the sheet feeding motor  27  normally by a predetermined pulse number P 0 , which is selected to be sufficient for rotating the sheet feeding roller gear until the toothless portion  19   b  thereof reaches a position opposed to the forward rotating planet gear  31  thereby rotating the sheet feeding roller  19  to the initial position from any rotational phase. Thus, the step S 29  rotates the sheet feeding roller  19  to the initial position, and the pressure plate  26  and the sheet feeding rubber member  23  are mutually separated to enable smooth setting of the sheets  200 . 
     A next step S 30  sets “1” as the flag INIT in order to indicate that the sheet feeding roller is in the initial position. Then a step S 31  transmits an initialization completion signal to the printer  101 , and the sequence is terminated. 
     FIG. 33 is a flowchart showing a sub flow C 1  for discriminating, through ports  117   f ,  117   g  shown in FIG. 26, the type of the device externally connected to the printer. At first, in a step S 32 , the main control unit  202  transmits a device type discrimination command signal to the external device through the port  117   g . If a response signal from the external device is not received through the port  117   f  in a step S 33 , the sequence proceeds to a step S 35 , and if a predetermined limit time t 1  has not elapsed, the sequence returns to the step S 33 . If the limit time t 1  has elapsed in the step S 35 , the sequence proceeds to a step S 36  for discriminating that the external device is absent, whereupon the sequence is terminated. 
     If a response signal is received from the external device in the step S 33 , the sequence proceeds to a step S 34 , in which the main control unit  202  reads a partial code ID indicating the type of the mounted device from the received response signal, whereupon the sequence is terminated. 
     [Second Embodiment] 
     FIGS. 34 and 35 show a second embodiment of the control sequence in the printer of the present invention and the external ASF which can be mounted to the printer. Parts or operations equivalent in function or shape to those in the first embodiment will be represented by the same numbers or symbols and will not be explained further. 
     In the first embodiment, as shown in FIG. 31, the ASF control unit  201  drives the sheet feeding motor reversely by P 5  pulses in the step S 22 , and then transmits the sheet feeding completion signal to the printer  101  in the step S 23 . In such case, however, the sheet feeding roller  19  is not returned to the initial position, so that the sheet feeding roller  19  remains in contact with the sheets  200  as shown in FIG.  36 . If the leading end aligning operation or the recording operation is executed in the printer in this state simply by the LF roller  109  alone, the sheet feeding roller  19  generates a backward tension to deteriorate the accuracy of conveyance of the sheet  200 . 
     The second embodiment is to avoid such drawback. 
     As shown in FIG. 35, after the registering operation in the step S 22 , the ASF control unit  201  proceeds to a step S 38  for driving the sheet feeding motor  27  normally (forwardly) by a predetermined pulse number P 6 , which is selected sufficient for rotating the sheet feeding roller  19  to the initial position by the forward rotating planet gear  31 . Simultaneous with the start of forward rotation of the sheet feeding motor  27 , there is activated a counter for measuring a time elapsed from the start of drive, and, after the elapse of a predetermined time t 3 , the sequence proceeds to a step S 39  to transmit a synchronous driving request signal to the printer  101 . The predetermine time t 3  is selected slightly longer than the time from the start of rotation of the sheet feeding motor  27  in the step S 38  to the start of rotation of the sheet feeding roller  19  by the movement of the forward rotating planet gear  31  to the meshing position with the sheet feeding roller gear  19   a.    
     Also, in the step S 38 , the drive speed of the sheet feeding motor  27  is so selected that the peripheral speed of the sheet feeding rubber member  23  mounted on the sheet feeding roller  19  is slightly larger than that of the LF roller  109  rotating in the step S 7  in the printer. 
     Upon completion of the step S 38 , the sheet feeding roller  19  is rotated to a phase the same as in the initial position, and the sequence proceeds to a step S 40 , in which the ASF control unit  201  sets, in the INIT flag, a value “1” indicating that the rotational phase of the sheet feeding roller  19  is in the initial state, and the sequence is terminated. 
     On the other hand, the main control unit  202  of the printer, upon receiving the synchronous driving request signal transmitted by the ASF control unit  201  in the above-mentioned step S 39 , proceeds from the step S 5  in FIG. 34 to a step S 7  for starting the forward (normal) rotation of the LF roller  109 . 
     FIG. 37 is a timing chart outlining the operations of the printer  101  and the ASF  1  of the present embodiment in the course of time. 
     When the printer starts the sheet feeding operation, at first a device type discrimination command signal is transmitted to the ASF (S 32 ). The ASF transmits an ID signal, indicating its own device type code to the printer (S 37 ). Then the printer transmits an initializing command signal to the ASF (S 2 ). The ASF, if not in the initialized state, executes initialization by rotating the sheet feeding roller (S 29 ), and transmits an initialization completion signal to the printer (S 31 ). Then the printer transmits a sheet feeding command signal to the ASF (S 4 ). 
     The ASF reads an optimum driving table T based on the sheet type information transmitted together with the sheet feeding command signal (S 15 , omitted in FIG. 37) and drives the sheet feeding motor based on the sheet feeding operation control flow C 2 , thereby rotating the sheet feeding roller (S 18 ). When the sheet end sensor provided in the printer detects a sheet and generates a HIGH output voltage, the ASF further rotates the sheet feeding roller by a rotation amount R 1  based on the aforementioned pulse number P 5 , thereby achieving so-called registering operation (S 22 ). After the registration, the ASF further rotates the sheet feeding roller by a rotation amount R 3  to a position the same as the initial position (S 38 ), and, after the elapse of a time t 3  from the start of driving of the sheet feeding motor, transmits a synchronous driving request signal to the printer (S 39 ). 
     Upon receiving the synchronous driving request signal from the ASF, the printer rotates the LF roller by a rotation amount R 3 , thereby executing so-called leading end feeding operation for feeding a leading end of the sheet to an initial position (S 7 ). 
     In the present embodiments, as will be apparent from the foregoing description, the sheet feeding roller  19  starts rotation in a state after the completion of the step S 22  as shown in FIG. 36, and the LF roller  109  starts rotation slightly later, and the peripheral speed of the sheet feeding rubber member  23  is slightly larger than that of the LF roller  109 . Consequently, when the LF roller  109  starts rotation for the leading end feeding operation in the step S 7 , there is not generated a backward tension on the sheet  200  because the sheet feeding rubber member  23  maintained in contact therewith starts rotation slightly earlier, and the backward tension resulting from the difference in the peripheral speed is also not generated since the peripheral speed of the sheet feeding rubber member  23  is slightly larger than that of the LF roller  109 . Consequently, the accuracy of transportation of the sheet  200  is stabilized in the leading end feeding operation. 
     If the time t 3  is excessively small, the LF roller  109  may start rotation before the driving force of the sheet feeding motor  27  is transmitted to the sheet feeding roller  19 , while, if the time t 3  is excessively large, the sheet feeding roller  19  rotates by a large amount before the LF roller  109  starts rotation whereby the sheet  200  may be deformed on the way or may become not parallel to the nip formed by the LF roller  109  and the pinch roller  110 . Based on experimental results, the optimum range of time t 3  in the present embodiment is 10 ms to 100 ms. Also if the peripheral speed of the sheet feeding rubber member  23  mounted on the sheet feeding roller  19  is not fast enough with respect to the peripheral speed of the LF roller  109 , there may be generated a backward tension when the sheet feeding rubber member  23  causes slippage depending on the type of the sheet  200  or on the environmental conditions, while the sheet  200  may be deformed in case the peripheral speed of the sheet feeding rubber member  23  is too fast. Based on experimental results, the optimum value of the peripheral speed of the sheet feeding rubber member  23  in the step S 38  of the present embodiment is 5 to 50% faster than the peripheral speed of the LF roller  109  in the step S 7 . 
     Also in the present embodiment, a signal corresponding to the “sheet feeding completion signal” in the first embodiment is named as the “synchronous driving request signal” because of the difference in the meaning of operation, but the actual signal may be identical with the “sheet feeding completion signal”. Consequently the sheet feeding control flow is basically same in the first and second embodiments (FIGS.  29  and  34 ). Stated differently, the printer shown in the first embodiment can be used in combination with either of the ASF&#39;s shown in the first and second embodiments. 
     In the following there will be explained, with reference to FIG. 38, the content of the plural driving tables T in the second embodiment. 
     For example, if the sheet type information received by the ASF  1  indicates an ordinary paper, the ASF control unit  201  selects a driving table T 1 . For the ordinary paper, the driving speed is set at a middle speed, because the registering operation in the step S 22  receives a low resistance. Also, as the possibility of skew feed is low during sheet feeding, the amount of pressing to the LF roller  109  need not be large so that the registration pulse number P 5  is selected small. 
     In case the sheet type information received by the ASF  1  indicates an envelope, the ASF control unit  201  selects a driving table T 3 . As the envelope shows a high resistance in feeding, particularly in the registering operation in the step S 22 , the drive speed is selected lower in comparison with the case of ordinary paper, thereby securing a large torque, in order to prevent the sheet feeding motor  27  from stepping out of the synchronization. On the other hand, as the envelope tends to cause a skew feed in the course of feeding in comparison with other sheet types, the registration pulse number P 5  in the step S 22  is selected at a middle value, which is larger than in the table T 1  for the ordinary paper. Thus, the leading end of the envelope is pressed by a larger amount to the LF roller  109 , and the leading end of the envelope can be more securely registered. 
     Also, in case the sheet type information indicates glossy paper, the ASF control unit  201  selects a driving table T 4 . The glossy paper shows a large resistance in the registering operation, but tends to generate less skewing. For this reason, in the driving table T 4 , the driving speed is selected low while the registration pulse number P 5  is selected small as in the ordinary paper. 
     In case the sheet type information indicates a postcard, the ASF control unit  201  selects a driving table T 2 . Since the postcard does not show a large resistance in the registering operation, the driving speed in the registering operation is selected at a middle value as in the ordinary paper. 
     On the other hand, if the LF roller  109  of the printer and the sheet feeding roller  19  of the ASF rotate simultaneously in the state shown in FIG. 37, a rigid sheet such as a postcard is not easily deformed in the course of conveyance, so that the sheet feeding roller  19  of the larger peripheral speed may forcedly press in the postcard against the frictional force of the LF roller  109 , whereby the leading end of the postcard may be conveyed in excess of the rotation amount R 3  of the LF roller and the obtained print may become improper. In order to avoid such situation, in the driving table T 2 , the registration pulse number P 5  in the step S 22  is selected as large as possible. More specifically, it is set as a variable, represented by P 5 =Pmax−Pc and determined by the driving pulse number for the reverse rotation of the sheet feeding motor  27  required until the detection of the sheet  200  by the sheet end sensor  108 . Thus, regardless of when the sheet  200  is detected by the sheet end sensor  108 , the total pulse number of the reverse rotation of the sheet feeding motor  27  becomes Pmax at the end of execution of the step S 22 . Stated differently, the toothless portion  19   b  of the sheet feeding roller gear  19   a  securely rotates to a position of disengagement opposed the reverse rotating planet gear  35 . Therefore, after the end of the step S 22 , the rotational phase of the sheet feeding roller  19  is significantly advanced from the initial position, and, if the sheet feeding roller  19  rotates in the step S 40 , the phase thereof promptly returns to the initial position. Consequently the postcards stacked on the pressure plate  26  are promptly separated from the sheet feeding rubber member  23  immediately after the start of the synchronous drive of the LF roller  109  and the sheet feeding roller  19 , so that the sheet feeding roller  19  no longer presses in the postcard against the frictional force of the LF roller  109 . 
     Also in case the sheet type information received by the ASF  1  from the printer  101  indicates a sheet type for which the ASF  1  is not prepared or does not indicate the sheet type, the ASF control unit  201  selects a driving table T 5 . The driving table T 5  of the present embodiment has values the same as those in the driving table T 2  for the postcard, but it is naturally possible, depending on the contemplated conditions, to provide the table T 5  with values the same as those of the table for other sheet types or with values completely different from those of other tables. 
     According to the present invention, as explained in the foregoing, the second guide member of the sheet feeding apparatus is disposed and displaced toward an inner side of the sheet, with respect to the first guide member of the recording apparatus, so that the automatically fed sheet can be prevented from interfering with the first guide member, and there can therefore be avoided a skew feed of the sheet, damage to the sheet end or sheet jamming resulting from such interference. 
     Also, as there is not required a strict relative positional relationship between the first and second guide members, it is not necessary to employ highly precise parts and it is rendered possible to avoid an increase in the cost. 
     Furthermore, even if the sheet generates skewing of a certain extent, it can be prevented from interfering with the first guide member, and there can therefore be avoided the skew feed of the sheet, damage to the sheet end or sheet jamming resulting from such interference. 
     Also, by displacing the image recording position in the transverse direction of the sheet in the case of automatic sheet feeding toward the inner side of the sheet, in comparison with that in the case of manual insert sheet feeding, by an amount approximately equal to the displacement amount between the first and second guide members, it is rendered possible to record the image in a same position regardless of whether the sheet feeding is executing automatically or in manual insert, thereby avoiding a drawback resulting from the difference in the recording position (for example difference in the recording position on the preprinted sheet).