Patent Publication Number: US-6991389-B2

Title: Foldable keyboard

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a foldable keyboard with superior portability and good operationality in use and, more particularly, to a foldable keyboard including two keyboard units and a cover member which is turned in synchronization with rotation of each keyboard unit and covers end faces and both side portions of each end face of the keyboard units held in a superposed state. 
   2. Description of Related Art 
   Heretofore, there have been proposed various types of foldable keyboards including two separate keyboard units rotatably connected with each other. When the keyboard is not in use, the keyboard units are superposed one on top of the other to bring the keyboard into a folded compact state, thus making it easy to carry anywhere. When in use, on the other hand, the keyboard units are separated to unfold the keyboard into a spread state, thus providing high operationality equivalent to standard keyboards. 
   However, the above keyboard has the following disadvantages. When the keyboard units rotatably connected with each other are superposed, their end faces (the backsides) are directly exposed. This causes the entry of dirt or the like in the inside of each key switch, leading to operational or mechanical troubles, and also the appearance of the keyboard is deteriorated. 
   To avoid the above disadvantages, for example, Japanese patent unexamined publication No. 2000-56904 (pages 3–5 and  FIGS. 1–12 ) discloses a foldable keyboard including two separate keyboards at least one of which is slidably supported on a support plate. The separate keyboards are rotatably connected to both sides of a back plate, directly or via the support plate. In this keyboard, when the two keyboards are folded, the back plate covers end faces of the keyboards, like a spine of a book, to prevent dust and waterdrops from entering the keyboards. 
   Furthermore, U.S. Pat. No. 5,574,481 (second and third columns and FIGS. 2, 7, and 9) discloses a foldable keyboard including two separate keyboard units which are connected to be brought into or out of contact with each other, so that the keyboard units can be folded while they are set apart from each other. In this keyboard, after folding, a cover plate placed slidably to cover both side portions of each end face of the keyboard units. This cover plate is slid and turned to cover the end faces of the keyboard units. 
   In the former keyboard (JP-A-2000/56904), however, the back plate covers the end faces of the two separate keyboards, whereas it cannot cover over the both side portions of each end face of the keyboards when folded, as shown in FIG. 8 of JP-A-2000/56904. 
   Similarly, in the latter keyboard (U.S. Pat. No. 5,574,481), the cover plate covers the end faces of the two keyboard units, whereas it cannot cover over the both side portions of each end face of the keyboard units when folded, as shown FIG. 9 of U.S. Pat. No. 5,574,481. 
   Furthermore, in the former keyboard (JP-A-2000/56904), the two separate keyboards are rotatably connected to both sides of the back plate. Thus, two rotational connecting parts are provided at the sides of the back plate. With this structure, the folding and opening motions of the keyboards tend to be unstable. 
   In the latter keyboard (U.S. Pat. No. 5,574,481), the two keyboard units have to be mutually folded or horizontally opened while they are set apart from each other. Accordingly, the folding and opening motions of the keyboard units tend to be unstable. 
   Meanwhile, compact portable devices such as a personal digital assistant (PDA) have remarkably progressed and come into wide use. The compact portable devices of this type, provided with key switches, are of a small body which needs small-sized key switches and others. Thus, the development of a portable keyboard that can be connected with such portable devices has been demanded. 
   However, the former keyboard (JP-A-2000/56904) in which the two separate keyboards are rotatably connected with the both sides of the back plate so as to be foldable has been proposed without consideration of the connection of the keyboard with the compact portable devices such as a PDA. 
   The latter keyboard (U.S. Pat. No. 5,574,481) also has been proposed without consideration of the connection of the keyboard with the compact portable devices such as a PDA. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a foldable keyboard including two keyboard units and a cover member which is turned in synchronization with rotation of the keyboard units to cover end faces or both side portions of each end face of the keyboard units when superposed in order to prevent dust and waterdrops from entering the keyboard from the end faces and both side portions of each end face of the keyboard units. 
   The second object of the present invention is to provide a foldable keyboard in which two keyboard units are inhibited from rotating to be folded as long as the keyboard units are in a horizontal state, thereby making sure a stable operation of the keyboard in any places as well as on the desk. 
   The third object of the present invention is to provide a foldable keyboard in which two keyboard units are inhibited from horizontally turning while the keyboard units are not in a horizontal state, thereby securely preventing the keyboard units from protruding outside during carrying of the folded keyboard. 
   The fourth object of the present invention is to provide a foldable keyboard in which two keyboard units are rotatably connected by means of a rotation rod and two turnably jointed links, thereby allowing the keyboard units to stably, smoothly rotate with respect to each other about the rotation rod. 
   The fifth object of the present invention is to provide a foldable keyboard adapted to prevent the keyboard units from tilting to partially lift their bottoms from a setting plane due to the weight of a compact portable device such as a PDA placed on and connected with the keyboard. 
   Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
   To achieve the purpose of the invention, there is provided a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the keyboard further includes: first support parts formed in opposite corners in one side of the first keyboard unit; second support parts formed in opposite corners in one side of the second keyboard unit; a cover member including a cover part, a pair of supporting elements formed on both ends of the cover part, first slots formed in the supporting elements respectively to correspond to each first support part, and second slots formed in the supporting elements to correspond to each second support part; first support shafts supported in the first support parts respectively, each first support shaft passing through each first slot with play; second support shafts supported in the second support parts respectively, each second support shaft passing through each second slot with play; when the first and second keyboard units are rotated from the horizontally arranged state to the superposed state, the cover member is synchronously turned by cooperation between the first support shafts and the first slots and between the second support shafts and the second slots, and the cover part of the cover member covers end faces of the first and second keyboard units in the superposed state and the support elements of the cover member cover side portions of each end face of the first and second keyboard units. 
   According to the above mentioned keyboard, which is constructed such that the first and second slots are formed in the pair of supporting elements provided on both ends of the cover part of the cover member, the first support shafts are supported in the first support parts formed in opposite corners in one side of the first keyboard unit, each first support shaft passing through each first slot with play (clearance), while the second support shafts are supported in the second support parts formed in opposite corners in one side of the second keyboard unit, each second support shaft passing through each second slot with play (clearance), so that the cover member is turned in synchronization with rotation of the first and second keyboard units by cooperation between the first support shafts and the first slots and between the second support shafts and the second slots, and the cover part of the cover member covers the end faces of both units held in the folded state and the supporting elements cover the side portions of the lengthwise ends of each end face, it is possible to prevent dust, waterdrops, etc. from entering the keyboard from the end faces and respective side portions of the ends in the lengthwise direction. An appearance of the keyboard can also be enhanced. 
   According to another aspect, the present invention provides a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the first keyboard unit includes a first base plate and a first unit comprising a first support plate provided to be turnable in a horizontal direction on the first base plate and a plurality of key switches arranged on the first support plate; the second keyboard unit includes a second base plate and a second unit comprising a second support plate provided to be turnable in a horizontal direction on the second base plate and a plurality of key switches arranged on the second support plate; the keyboard further includes: first support parts formed in opposite corners in one side of the first keyboard unit; second support parts formed in opposite corners in one side of the second keyboard unit; a cover member including a pair of supporting elements which are turnably connected with the first and second support parts respectively so that the cover member covers end faces of the first and second keyboard units in the superposed state; and a pair of grooves which are formed on both sides of one of the support elements and in which circular arc faces of the first and second support plates are slidably engaged. 
   In the above foldable keyboard, the pair of supporting elements of the cover member are turnably connected with the first support parts formed in the first base plate and with the second support parts formed in the second base plate, so that the cover member is turned in synchronization with rotation of the first and second keyboard units about the rotation rod, the cover member covers the end faces of both units in the folded state, and the supporting elements of the cover member cover the side portions of the lengthwise ends of each end face, it is possible to prevent dust, waterdrops, etc. from entering the keyboard from the end faces and the side portions of the ends in the lengthwise direction. An appearance of the keyboard can also be enhanced. 
   Further, the pair of grooves are formed on both sides of one of the supporting elements, one of which slidably receives the circular arc face of the first support plate of the first unit horizontally turnably provided on the first base plate, while the other slidably receives the circular arc face of the second support plate of the second unit horizontally turnably provided on the second base plate. Accordingly, when the first and second keyboard units are horizontally turned, the circular arc faces are engaged in the associated grooves of the supporting element. This makes it possible to inhibit a folding operation of the first and second keyboard units due to an engagement relation between each circular arc face and the corresponding grooves. Thus, even when the first and second keyboard units are horizontally turned, a horizontal position of each keyboard unit can surely be maintained, which allows for a stable operation of the keyboard in any place as well as on a desk. 
   According to another aspect, furthermore, the present invention provides a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the first keyboard unit includes a first base plate and a first unit comprising a first support plate provided to be turnable in a horizontal direction on the first base plate and a plurality of key switches arranged on the first support plate; the second keyboard unit includes a second base plate and a second unit comprising a second support plate provided to be turnable in a horizontal direction on the second base plate and a plurality of key switches arranged on the second support plate; the keyboard further includes: first support parts formed in opposite corners in one side of the first keyboard unit; second support parts formed in opposite corners in one side of the second keyboard unit; a cover member including a pair of supporting elements which are turnably connected to the first and second support parts respectively so that the cover member covers end faces of the first and second keyboard units in the superposed state; and the cover member inhibits turning of the first and second support plates in the superposed state and in course of rotation from the superposed state to the horizontally arranged state. 
   In the above foldable keyboard, the pair of supporting elements of the cover member are turnably connected with the first support parts formed in the first base plate and with the second support parts formed in the second base plate, so that the cover member is turned in synchronization with rotation of the first and second keyboard units about the rotation rod, the cover member covers the end faces of both units in the folded state, and the supporting elements of the cover member cover the side portions of the lengthwise ends of each end face, it is possible to prevent dust, waterdrops, etc. from entering the keyboard from the end faces and the side portions of the ends in the lengthwise direction. An appearance of the keyboard can also be enhanced. 
   Further, the cover member is constructed to inhibit the turning of the first and second support plates as long as the first and second keyboard units are held in the superposed state or they are rotated from the superposed state toward the horizontally arranged state. Accordingly, when the first and second keyboard units are not held in the horizontal state, they are inhibited from turning horizontally. Thus, the keyboard units can be prevented surely from protruding outside during carrying or transport of the folded keyboard. 
   According to another aspect, furthermore, the present invention provides a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the keyboard further includes: first support parts formed in opposite corners in one side of the first keyboard unit; second support parts formed in opposite corners in one side of the second keyboard unit; a first link turnably supported in the first support part through the first support shaft; a second link turnably supported in the second support part through the second support shaft; and a joint rod which pivotally joints the first and second links to allow for turning of the links with respect to each other. 
   In the above foldable keyboard, the first and second keyboard units are rotatably connected through the rotation rod, the first link is turnably supported by the first support shaft and the second link is turnably supported by the second support shaft so that the first link and second links are pivotally jointed by the joint rod. Accordingly, the first and second keyboard units can be rotated mutually stably by a linkage action of the first and second links. 
   According to another aspect, the present invention provides a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the keyboard further includes: first support parts formed in opposite corners in one side of the first keyboard unit; second support parts formed in opposite corners in one side of the second keyboard unit; a cover member including a pair of supporting elements which are turnably connected to the first and second support parts respectively and a cover part formed between the supporting elements, the cover member being adapted to cover end faces of the first and second keyboard units in the superposed state; a device support member which is raised to a standing orientation in conjunction with the rotation of the first and second keyboard units in a direction that they come apart; and a keyboard stabilizing member provided in the cover part of the cover member so that the stabilizing member is pulled in an axial direction of the rotation rod to outside of the first and second keyboard units; the keyboard stabilizing member is in a pulled state outside the first and second keyboard units while the device support member supports a device to prevent the first and second keyboard units from so tilting as to partially lift their bottoms from a setting plane. 
   In the above foldable keyboard, the cover part of the cover member is provided with the keyboard stabilizing member provided to be pulled outside the first and second keyboard units in the axial direction of the rotation rod. The keyboard stabilizing member is pulled outside the first and second keyboard units to support a device in the device support members, thereby preventing both units from tilting to lift their bottom from a setting place. Thus, even when a compact portable device such as a PDA set on the device support members, the keyboard can be operated stably. 
   In addition, the cover member is turned in synchronization with rotation of the first and second keyboard units, the cover part of the cover member covers the end faces of both units in the superposed state and the supporting elements of the cover part cover the side portions of lengthwise ends of each end face. It is therefore possible to prevent dust, waterdrops, etc. from entering the keyboard from the end faces of both units and the side portions of the lengthwise ends of each end face. An appearance of the keyboard can also be enhanced. 
   Furthermore, according to another aspect, the present invention provides a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the first keyboard unit includes a first base plate with a first frame portion and a first unit comprising a first support plate with a first wall member provided to be turnable in a horizontal direction on the first base plate and a plurality of key switches arranged on the first support plate; the second keyboard unit includes a second base plate with a second frame portion and a second unit comprising a second support plate with a second wall member provided to be turnable in a horizontal direction on the second base plate and a plurality of key switches arranged on the second support plate; the keyboard further includes: first support parts formed in opposite corners in one side of the first keyboard unit; second support parts formed in opposite corners in one side of the second keyboard unit; a cover member including a pair of support elements which are turnably connected to the first and second support parts respectively so that the cover member covers end faces of the first and second keyboard units in the superposed state; and a pair of first and second pawl members formed in one of the support elements of the cover member, each pawl member having pawl on a top end thereof; wherein the pawl of the first pawl member contacts with a wall plane of the first wall member and the pawl of the second pawl member contacts with a wall plane of the second wall member when the first and the second keyboard unit are horizontally arranged with each other, and wherein the pawl of the first pawl member engages with a shoulder formed between the first frame portion and the first wall member and the pawl of the second pawl member engages with a shoulder formed between the second frame portion and the second wall member, when the first support plate is turned in the horizontal direction on the first base plate and the second support plate is turned in the horizontal direction on the second base plate, thereby the first and second keyboard units are inhibited from folding thereof. 
   According to another aspect, the present invention provides a foldable keyboard including a first keyboard unit and a second keyboard unit which are rotatably connected by a rotation rod so that the first and second keyboard units are rotated about the rotation rod to come apart from each other into an unfolded, horizontally arranged state for use of the keyboard, whereas the first and second keyboard units are rotated about the rotation rod to come close to each other into a folded, superposed state for nonuse of the keyboard, wherein the first keyboard unit includes a first base plate with a first frame portion and a first unit comprising a first support plate with a first wall member provided to be turnable in a horizontal direction on the first base plate and a plurality of key switches arranged on the first support plate; the second keyboard unit includes a second base plate with a second frame portion and a second unit comprising a second support plate with a second wall member provided to be turnable in a horizontal direction on the second base plate and a plurality of key switches arranged on the second support plate; the keyboard further includes: a first locking groove formed in the first wall member; a first slide groove formed in the first wall member, the first slide groove being connected to the first locking groove and extended toward the rotation rod; a second locking groove formed in the second wall member; a second slide groove formed in the second wall member, the second slide groove being connected to the second locking groove and extended toward the rotation rod; a first locking member having a projection and a shaft connected to the projection, the projection being slidably arranged within the first locking groove and the shaft being slidably arranged in the first slide groove; a second locking member having a projection and a shaft connected to the projection, the projection being slidably arranged within the second locking groove and the shaft being slidably arranged in the second slide groove; a first spring arranged in the first locking groove, the first spring pressing the shaft of the first locking member toward the rotation rod; a second spring arranged in the second locking groove, the second spring pressing the shaft of the second locking member toward the rotation rod; wherein both end planes of the shafts of the first locking member and the second locking member contact with each other when the first and the second keyboard units are horizontally arranged, thereby the first and second keyboard units are made turnable in the horizontal direction, and wherein the projection of the first locking member is locked in the first locking groove and the projection of the second locking member is locked in the second locking groove when the first and second keyboard units are in a state other than a state that the first and second keyboard units are horizontally arranged, thereby the first and second keyboard units are inhibited from turning in the horizontal direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification illustrate an embodiment of the invention and, together with the description, serve to explain the objects, advantages and principles of the invention. 
     In the drawings, 
       FIG. 1  is an exploded perspective view of a foldable keyboard in an embodiment, seen from the front; 
       FIG. 2  is an exploded perspective view of the foldable keyboard, seen from the back; 
       FIG. 3  is an enlarged view of a linkage system; 
       FIG. 4  is an enlarged view of a cover member; 
       FIG. 5  is a perspective view of the keyboard in use, in which a first and second keyboard units are set in a horizontally arranged state; 
       FIG. 6  is an enlarged partial view showing the cover in  FIG. 5 ; 
       FIG. 7  is an enlarged front view of the linkage system in  FIG. 6 , from which the cover is removed for convenience of explanation; 
       FIG. 8  is a perspective view of the keyboard, in which the first and second keyboard units are turned on a first and second base plates; 
       FIG. 9  is an enlarged partial view of the cover in  FIG. 8 ; 
       FIG. 10  is a perspective view of the keyboard, in which a key supporting plate of the first keyboard unit is slid from the position shown in  FIG. 5 ; 
       FIG. 11  is an enlarged partial view of the cover placed when the second keyboard unit is slightly rotated from the position in  FIG. 5 ; 
       FIG. 12  is an enlarged front view of the linkage system placed when the second keyboard unit is further rotated from the position in  FIG. 11 ; 
       FIG. 13  is a perspective view of the keyboard when the second keyboard unit is further rotated from the position in  FIG. 12 ; 
       FIG. 14  is an enlarged partial view of the cover placed when the second keyboard is further rotated from the position in  FIG. 13 ; 
       FIG. 15  is a front view of the linkage system in  FIG. 14 , from which the cover is removed for convenience of explanation; 
       FIG. 16  is a perspective view of the keyboard in a completely folded state where the first and second keyboard units are superposed one on top of the other; 
       FIG. 17 . is an enlarged partial view of the cover in  FIG. 16 ; 
       FIG. 18  is a front view of the linkage system in  FIG. 17 , from which the cover is removed for convenience of explanation; 
       FIGS. 19A to 19C  are explanatory views to sequentially show motions of the first and second keyboard units from a folded state to a horizontally arranged position; 
       FIG. 20  is a perspective bottom view of the keyboard in a state shown in  FIG. 5 ; 
       FIG. 21  is a perspective bottom view of the keyboard in  FIG. 20 , in which a stabilizing member is pulled out; 
       FIG. 22  is a side view of the keyboard in  FIG. 21 ; 
       FIG. 23  is an enlarged partial view of a cover member shown as another variation; 
       FIG. 24  is an enlarged partial view of the cover in  FIG. 23  in another state; 
       FIG. 25  is an enlarged partial view of the back portion of a keyboard shown as a second variation, in which the first and second keyboard units are horizontally arranged; 
       FIG. 26  is an enlarged plan view of the back portion in  FIG. 25  with a partially cutaway view; 
       FIG. 27  is an enlarged partial view of the back portion of the keyboard shown in  FIG. 25 , in which the first and second keyboard units are turned horizontally; 
       FIG. 28  is an enlarged plan view of the back portion in  FIG. 27  with a partially cutaway view; 
       FIG. 29  is an enlarged partial view of the back portion of the keyboard in a state to inhibit rotation of the first and second keyboard units after slightly folded; and 
       FIG. 30  is a plane view of a part of the first keyboard unit in a position shown in  FIG. 29 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A detailed description of a preferred embodiment of a foldable keyboard embodying the present invention will now be given referring to the accompanying drawings. 
   First, the structure of the keyboard in the present embodiment is explained with reference to  FIGS. 1 and 2 .  FIG. 1  is an exploded perspective view of the keyboard, seen from the front; and  FIG. 2  is an exploded perspective view of the keyboard, seen from the back. 
   In  FIGS. 1 and 2 , a keyboard  1  is mainly constructed of a first keyboard unit  3  and a second keyboard unit  4  which are rotatably connected with each other through a first rotational connecting part  2 A (at the back side in  FIGS. 1 and 2 ) and a second rotational connecting part  2 B (at the front side in  FIGS. 1 and 2 ). The first keyboard unit  3  includes a first surface plate  3 A attached with two rubber support legs  3 B in two outer corners, a first base plate  5 , a first support plate  6  mounted to be horizontally turnable on the first base plate  5 , and a plurality of key switches  7  arranged on the first support plate  6 . The second keyboard unit  4  includes a second surface plate  4 A attached with two rubber support legs  4 B in two outer corners, a second base plate  8 , a second support plate  9  mounted to be horizontally turnable on the second base plate  8 , and a plurality of key switches  10  arranged on the second support plate  9 . 
   The first rotational connecting part  2 A is first explained below. The first base plate  5  is made of a metallic, e.g., aluminum, thin plate and is provided, in a side end (a right end in  FIG. 1 ), in a corner  12 A positioned at the back in  FIG. 1 , with a resinous bearing member  14  constituting a part of the first rotational connecting part  2 A. The bearing member  14  is formed of three bearings  14 B which are spaced apart and each have a bearing hole  14 A. The second base plate  8  is, like the first base plate  5 , made of a metallic, e.g., aluminum, thin plate and is provided, in a side end (a left end in  FIG. 1 ), in a corner  16 A positioned at the back in  FIG. 1 , with a resinous bearing member  18  constituting a part of the first rotational connecting part  2 A. The bearing member  18  is formed of two bearings  18 B which are spaced apart and each have a bearing hole  18 A. 
   Each bearing  18 B is fit between adjacent two of the bearings  14 B. At this time, the bearing holes  18 A of the bearings  18 B and the bearing holes  14 A of the bearings  14 B are aligned with one another and therein a rotation rod  19  is inserted (see  FIG. 2 ). This rod  19  supports the first base plate  5  and the second base plate  8  to be rotatable with respect to each other. 
   The bearing member  14  in the first base plate  5  is integrally formed with a hollow frame portion  23  through which a signal line (not shown) runs to connect each key switch  7  in the first keyboard unit  3  to a connector unit  60  mentioned later. In the frame portion  23 , a connector storage recess  61  for storing the connector unit  60  is formed adjacent to the bearing member  14 . The connector storage recess  61  is formed with screw holes  62  in opposite spaced inner walls  61 A (only one of which is shown in  FIG. 1 ). The screw holes  62  will be mentioned later together with a support system for the connector unit  60 . 
   The connector storage recess  61  is used, in cooperation with another connector storage recess (mentioned later) formed in the second base plate  8  in the second keyboard unit  4 , to store the connector unit  60  in the keyboard  1  when the first and second keyboard units  3  and  4  are folded. 
   Alongside of the connector storage recess  61 , the frame portion  23  is provided with a storage concave portion  63  to store therein a pair of support members  80  in a folded state. Specifically, this storage concave portion  63  is used, in cooperation with a storage concave portion (mentioned later) formed in the second base plate  8 , to store the pair of support members  80  when the support members  80  are folded in synchronization with the folding of the first and second keyboard units  3  and  4 . 
   The bearing member  18  in the second base plate  8  is integrally formed with a hollow frame portion  24  which a signal line (not shown) runs through to connect each key switch  10  arranged in the second keyboard unit  4  to the connector unit  60 . In the frame portion  24 , a connector storage recess  64  for storing the connector unit  60  is formed adjacent to the bearing member  18 . The connector storage recess  64  is formed with screw holes  65  in opposite spaced inner walls  64 A (only one of which is shown in  FIG. 1 ). The screw holes  65  will be mentioned later together with the support system for the connector unit  60 . 
   The connector storage recess  64  is used, in cooperation with the connector storage recess  61  formed in the first base plate  5  in the first keyboard unit  3 , to store the connector unit  60  in the keyboard  1  when the first and second keyboard units  3  and  4  are folded. 
   Furthermore, alongside of the connector storage recess  64 , the frame portion  24  is provided with a storage concave portion  66  to store therein the pair of support members  80 . Specifically, this storage concave portion  66  is used in cooperation with the storage concave portion  63  formed in the first base plate  5  to store the pair of support members  80 , when the support members  80  are folded in synchronization with the folding of the first and second keyboard units  3  and  4 . 
   Next, the second rotational connecting part  2 B is explained below. The first base plate  5  is provided, in a corner  12 B positioned at the front in  FIG. 1 , opposite to the corner  12 A, with a resinous bearing member  13  constituting a part of the second rotational connecting part  2 B. The bearing member  13  is formed with a bearing hole  13 A. The second base plate  8  is provided, in a corner  16 B positioned at the front in  FIG. 1 , opposite to the corner  16 A, with a resinous bearing member  17  constituting a part of the second rotational connecting part  2 B. The bearing member  17  is formed with a bearing hole  17 A. 
   The above bearing members  13  and  17  are connected with a cover member  100  by means of a linkage system  90  interposed therebetween. 
   This linkage system  90  is explained below with reference to  FIG. 3 .  FIG. 3  is an enlarged view of the linkage system  90 . This system  90  is constructed of a link  93  formed with a support hole  91  at one end (a left end in  FIG. 3 ) and a joint hole  92  at the other end (a right end in  FIG. 3 ) and a link  96  formed with a support hole  94  at one end (a right end in  FIG. 3 ) and a joint hole  95  (a left end in  FIG. 3 ). A joint rod  98  of a joint element  97  is inserted in the aligned joint holes  92  and  95  of the links  93  and  96  disposed in partially overlapping relation. Thus, the links  93  and  96  are jointed to mutually turn about the joint rod  98  of the joint element  97 . It is to be noted that the joint element  97  is also formed with a projection  99  on an opposite side to the joint rod  98 . The effect of this projection  99  will be mentioned later. 
   The cover member  100  is explained below with reference to  FIG. 4 .  FIG. 4  is an enlarged perspective view of the cover member  100 . This cover member  100  is mainly constructed of a cover part  101  and a pair of supporting end plates  102  and  103  provided upright and integrally with the both ends of the cover part  101  in the longitudinal direction. The cover part  101  includes a cover plate  104  which is of a substantial rectangular shape. The width of the cover plate  104  is determined to be slightly wider than the total thickness of the first and second keyboard units  3  and  4  in order to fully cover the end faces of both units  3  and  4  when folded. The cover plate  104  is formed, at an end (a longitudinal back end in  FIG. 4 ), with thick base parts  106  and  107  disposed on both sides of a concave portion  105 . The base parts  106  and  107  are formed respectively with sliding holes  108  and  109  perforated in the longitudinal direction of the cover plate  104 . A stabilizing member  110  shaped like a Japanese syllabary character “ ” in plan view, like a U-shape, is set on the cover plate  104  so that rods  111  and  112  of the stabilizing member  110  are slidably mounted in the sliding holes  108  and  109 . The rods  111  and  112  are formed with stopper grooves  113  and  114  adjacent to respective end portions. In these stopper grooves  113  and  114 , stopper rings  115  and  116  each having a U-shape are fit respectively. The stabilizing member  110  constructed as above can be pulled in the longitudinal direction of the cover plate  104  (corresponding to the axial direction of the rotation rod  19 ) to the outside of the first and second keyboard units  3  and  4 . In use, the stabilizing member  110  is pulled outside from a storage position shown in  FIG. 4  until the stopper rings  115  and  116  come into contact with side faces of the base parts  106  and  107  in which the sliding holes  108  and  109  are opened. In nonuse, on the other hand, the stabilizing member  110  is pushed into the storage position as shown in  FIG. 4 . This stabilizing member  110  is used to securely support the keyboard  1  in the following situation. During use of the first and second keyboard units  3  and  4  arranged in a horizontal state, the support members  80  are set up in a standing orientation to hold a compact portable device such as a personal digital assistance (PDA). At this time, there may be a case where the support members  80  cannot stably hold by itself the compact portable device depending on the weight thereof. In this case, the first and second keyboard units  3  and  4  are caused to tilt toward the back, thus partially lifting the bottoms of the keyboard units  3  and  4  from the setting place. To avoid such unstable state, the stabilizing member  110  is pulled outside (see  FIGS. 21 and 22 ) to stably support the keyboard  1 . 
   In the cover part  101 , the supporting end plate  102  provided upright at one end (a front end in  FIG. 4 ) of the cover plate  104  is formed with a first slot  117  and a second slot  118  which are horizontally aligned. The first slot  117  is positioned corresponding to the bearing hole  13 A of the bearing member  13  formed in the first base plate  5  in the first keyboard unit  3 . The second slot  118  is positioned corresponding to the bearing hole  17 A of the bearing member  17  formed in the second base plate  8  in the second keyboard unit  4 . Further, a third slot  119  is formed in the supporting end plate  102  to extend perpendicular to the longitudinal direction of the slots  117  and  118 . In this third slot  119 , the projection  99  of the joint element  97  is slidably inserted. The supporting end plate  102  is further formed with a groove (notch)  130  in the left side at an almost center in the height direction of the supporting end plate  102  and a groove (notch)  131  in the right side, opposite to the groove  130 . In the groove  130 , a thin plate part  38 A (see  FIG. 9 ) formed in a circular arc face  38  mentioned later of the first support plate  6  in the first keyboard unit  3  will slidably be engaged. In the groove  131 , a thin plate part  48 A (see  FIG. 9 ) formed in a circular arc face  48  mentioned later of the second support plate  9  in the second keyboard unit  4  will slidably be engaged. The effects of the grooves  130  and  131  will be mentioned later. 
   In the cover part  101 , the supporting end plate  103  provided upright at the other end (a back end in  FIG. 4 ) of the cover plate  104  is formed with a first slot  117  which is of the same shape as and positioned opposite to the first slot  117  in the supporting end plate  102  and a second slot  118  which is of the same shape as and positioned opposite to the second slot  118  in the supporting end plate  102 . The first slot  117  in the supporting end plate  103  is positioned corresponding to the support hole  121  (see  FIG. 2 ) formed in the first base plate  5  in the first keyboard unit  3 . The second slot  118  is positioned corresponding to the support hole  120  formed in the second base plate  8  in the second keyboard unit  4 . In addition, the supporting end plate  103 , like the supporting end plate  102 , is formed with a third slot  119  extending perpendicular to the longitudinal direction of the slots  117  and  118 . In this third slot  119  is inserted the end of the rotation rod  19  passing through the bearing holes  14 A and  18 A of the bearings  14 B and  18 B respectively. It is to be noted that a semicircular protruding portion  122  is formed in the supporting end plate  103  at its upper side. This semicircular protruding portion  122  is designed to be semicircular partially corresponding to the circular contour of each bearing  14 B and  18 B protruding upward from each upper surface of the frame portion  23  of the first base plate  5  and the frame portion  24  of the second base plate  8 . Thus, the semicircular protruding portion  122  covers the end face of the bearing  14 B ( 18 B) from the back side of the keyboard  1 , making the bearing  14 B ( 18 B) invisible from the outside the keyboard  1 . 
   Next, explanation is made on the mechanism to support the connection of the above linkage system  90  and the cover member  100  with the first and second keyboard units  3  and  4 . 
   As shown in  FIG. 1 , a support screw  123  passes, with play (clearance), through the first slot  117  in the supporting end plate  102  of the cover member  100  and passes through the support hole  91  in the link  93  of the linkage system  90 . This support screw  123  is then fixedly received in the bearing hole  13 A of the bearing member  13  formed in the first base plate  5  in the first keyboard unit  3 , so that the screw  123  acts as a turning axis. As shown in  FIG. 2 , on the back side of the keyboard  1 , a support screw  124  passes, with play (clearance), through the first slot  117  in the supporting end plate  103  and is fixedly received in the support hole  121  in the first base plate  5  so as to act as a turning axis. With this structure, the cover member  100  is connected with the first keyboard unit  3  on the front side through the linkage system  90  and also connected with the first keyboard unit  3  on the back side. In this state, the cover member  100  is allowed to turn about and move with respect to the support screw  123  through the first slot  117 . The link  93  of the linkage system  90  is turnable about the support screw  123 . As mentioned above, the links  93  and  96  are allowed to mutually turn through the joint rod  98 , and the projection  99  of the joint element  97  is inserted with play in the third slot  119  in the supporting end plate  102 . 
   As shown in  FIG. 1 , a support screw  125  passes, with play (clearance), through the second slot  118  in the supporting end plate  102  of the cover member  100  and passes through the support hole  94  in the link  96  of the linkage system  90 . This support screw  125  is fixedly received in the bearing hole  17 A of the bearing member  17  formed in the first base plate  8  in the second keyboard unit  4 , so that the screw  125  acts as a turning axis. As shown in  FIG. 2 , on the back side of the keyboard  1 , a support screw  126  passes, with play (clearance), through the second slot  118  in the supporting end plate  103  and is fixedly received in the support hole  120  in the second base plate  8  to act as a turning axis. With this structure, the cover member  100  is connected with the second keyboard unit  4  on the front side through the linkage system  90  and also connected with the second keyboard unit  4  on the back side. In this state, the cover member  100  is allowed to turn about and move with respect to the support screw  125  through the second slot  118 . The link  96  of the linkage system  90  is allowed to turn about the support screw  125 . 
   Returning to the  FIG. 1 , the structure of the first keyboard unit  3  is explained again. In the first base plate  5 , a screw seat  27  is formed near the left side in  FIG. 1 , at an almost center. In this screw seat  27  is received a screw  29  having passed with play through a screw hole  28  in the first support plate  6 . Thus, the first support plate  6  is mounted on the first base plate  5  to be horizontally turnable about the screw  29  and the screw seat  27  serving as a turning axis. In the second base plate  8 , similarly, a screw seat  30  is formed near the right side in  FIG. 1 , at an almost center. In this screw seat  30  is received a screw  32  having passed with play through a screw hole  31  in the second support plate  9 . Accordingly, the second support plate  9  is mounted on the second base plate  8  to be horizontally turnable about the screw  32  and the screw seat  30  serving as a turning axis. 
   The first support plate  6  of the first keyboard unit  3  is made of a metallic, e.g., aluminum, thin plate and provided circumferentially, except for the right side, with a peripheral wall member  46  shaped like a reverse Japanese syllabary character “ ” in plan view, like a horizontally oriented U-shape. Sliding grooves  46 A are provided in the lower insides of the peripheral wall member  46  positioned on opposite sides of the first support plate  6  in the longitudinal direction. The sliding grooves  46 A are used to guide a key support plate  25  mentioned later which slides on the first support plate  6 . The peripheral wall member  46  is further formed with a sliding recess  46 B near the left side of the first support plate  6  in  FIG. 1 . This sliding recess  46 B restricts the moving range of a sliding knob  25 A disposed on the key support plate  25 . The peripheral wall member  46  in the first support plate  6  at the left side is centrally formed with a cutout  46 C and through holes  46 D on both sides of the cutout  46 C. A pressing part  44 A centrally formed in an engagement member  44  is placed in the cutout  46 C and engaging pawls  44 B formed on both sides of the pressing part  44 A are inserted through the through holes  46 D. 
   It is to be noted that the pressing part  44 A will be caught in a catching part  56 A formed in a peripheral wall member  56  of the second support plate  9  mentioned later, and the engaging pawls  44 B will be fitted in positioning holes  56 B formed in the peripheral wall member  56 . When folded, or superposed, accordingly, the first and second keyboard units  3  and  4  can be securely held in that folded state. 
   In  FIG. 1 , the first support plate  6  is formed on the right side with the circular arc face  38  coincident with a turning-radius of the plate  6  about the turning axis (i.e., the screw  29  passing through the screw hole  28  with play and the screw seat  27 ). A first gear teeth part  43  is almost centrally formed in the circular arc face  38  and engages with a second gear teeth part  53  formed in the circular arc face  48  of the second support plate  9  mentioned later. Turning one of the first support plate  6  and the second support plate  9  causes the other to turn at the same time into a desired positional (or angle) relation by engagement between the first and second gear teeth parts  43  and  53 . Accordingly, the first and second keyboard units  3  and  4  can be adjusted at a desired angle at which the keyboard  1  is most convenient for each individual user to operate. Thus, it is also seen from the ergonomic standpoint that this keyboard  1  is excellent. 
   The key support plate  25  supporting the plurality of key switches  7  is mounted on the first support plate  6  constructed as above. This key support plate  25  is provided with the sliding knob  25 A near the left side in  FIG. 1 . The sliding knob  25 A is placed in the sliding recess  46 B in the peripheral wall member  46 . This knob  25 A is slidable in the length of the sliding recess  46 B. Sliding the sliding knob  25 A in the sliding recess  46 B causes the key support plate  25  to slide along the sliding grooves  46 A in the peripheral wall member  46 . The key support plate  25  is further provided with a stair-like portion  25 B in the right periphery. This stair-like portion  25 B is of a contour mateable with the stair-like portion  26 B of the key support plate  26 . 
   A switch support plate  45  is mounted on the key support plate  25 . This switch support plate  45  is integrally formed with four supporting parts  33  per one key switch  7  by press working or other techniques. On the switch support plate  45 , there is placed a membrane switch sheet of a three-layer structure (including an upper sheet having a movable electrode, a lower sheet having a fixed electrode, and a spacer sheet placed between the upper and lower sheets and formed with a switching hole to separate the movable electrode and the fixed electrode). Each supporting part  33  is formed protruding upward through holes formed in the membrane switch. The switch support plate  45  is designed to have a right periphery identical in contour to the stair-like portion  25 B of the key support plate  25 . 
   The key switches  7 , which are operated by the user&#39;s left hand, are arranged in a predetermined number on the key support plate  25 . It is to be noted that the number of key switches  7  is determined based on the ISO International Standards (ISO 2126 and ISO 2530). 
   Each key switch  7  is mainly constructed of a key top  34 , a pair of link members  35  for vertically guiding the key top  34 , and a rubber spring  36  which is disposed on the membrane switch at a position corresponding to a switching part defined by the movable electrode and the fixed electrode of the membrane switch and urges the key top  34  upward. Each upper end of the link members  35  is movably connected with the key top  34  at its lower surface and each lower end of the link members  35  is movably engaged in each supporting part  33 . During non-depression, the key top  34  is urged upward by the urging force of the rubber spring  36  and held in a non-depression position. When the key top  34  is pressed down against the urging force of the rubber spring  36 , the rubber spring  36  pushes the movable electrode of the membrane switch to bring the movable electrode into contact with the fixed electrode in the switching hole, thereby performing a switching operation. The above mentioned key switches  7 , the key support plate  25 , and the first support plate  6  constitute a first key unit  37 . The structure of each key switch  7  is well known in the art and the detailed explanation thereof is omitted herein. 
   The structure of each key switch  10  arranged on the second keyboard unit  4  is explained below. The second support plate  9  of the second keyboard unit  4  is made of a metallic, e.g., aluminum, thin plate and provided circumferentially, except for the left side, with a peripheral wall member  56  shaped like a Japanese syllabary character “ ” in plan view, like a horizontally oriented U-shape. The peripheral wall member  56  in the second support plate  9  at the right side is centrally formed with a catching part  56 A and positioning holes  56 B on both sides of the catching part  56 A. The catching part  56 A engages with the pressing part  44 A as mentioned above. The positioning holes  56 B fittingly receive the engaging pawls  44 B individually. Thus, the first and second keyboard units  3  and  4  can be held securely in a folded relation. 
   In  FIG. 1 , the second support plate  9  is formed on the left side with the circular arc face  48  coincident with a turning-radius of the second support plate  9  about the turning axis (i.e., the screw  32  passing through the screw hole  31  with play and the screw seat  30 ). The second gear teeth part  53  is almost centrally formed in the circular arc face  48 . As mentioned above, the second gear teeth part  53  engages with the first gear teeth part  43  formed in the circular arc face  38  of the first support plate  6 . Accordingly, the first and second keyboard units  3  and  4  can be adjusted synchronously at a desired angle at which the keyboard  1  is most convenient to operate. 
   The key support plate  26  supporting the plurality of key switches  10  is mounted on the second support plate  9  constructed as above. This key support plate  26  is provided with the stair-like portion  26 B which is of a contour mateable with the stair-like portion  25 B of the key support plate  25  in the first keyboard unit  3 . 
   A switch support plate  55  is mounted on the key support plate  26 . This switch support plate  55  is integrally formed with four supporting parts  33  per one key switch  10  by press working or other techniques. On the switch support plate  55 , there is placed a membrane switch sheet of a three-layer structure (including an upper sheet having a movable electrode, a lower sheet having a fixed electrode, and a spacer sheet placed between the upper and lower sheets and formed with a switching hole to separate the movable electrode and the fixed electrode). Each supporting part  33  is formed protruding upward through holes formed in the membrane switch. The switch support plate  55  is designed to have a left periphery identical in contour to the stair-like portion  26 B of the key support plate  26 . 
   The key switches  10 , which are operated by the user&#39;s right hand, are arranged in a predetermined number on the key support plate  26 . It is to be noted that the number of key switches  10  is determined based on the ISO International Standards (ISO 2126 and ISO 2530). 
   Each key switch  10  is mainly constructed of a key top  34 , a pair of link members  35  for vertically guiding the key top  34 , and a rubber spring  36  which is disposed on the membrane switch at a position corresponding to a switching part defined by the movable electrode and the fixed electrode of the membrane switch and urges the key top  34  upward. Each upper end of the link members  35  is movably connected with the key top  34  at its lower surface and each lower end of the link members  35  is movably engaged in each supporting part  33 . During non-depression, the key top  34  is urged upward by the urging force of the rubber spring  36  and held in a non-depression position. When the key top  34  is pressed down against the urging force of the rubber spring  36 , the rubber spring  36  pushes the movable electrode of the membrane switch to bring the movable electrode into contact with the fixed electrode in the switching hole, thereby performing a switching operation. The above mentioned key switches  10 , the key support plate  26 , and the second support plate  9  constitute a second key unit  47 . The structure of each key switch  10  is well known in the art and the detailed explanation thereof is omitted herein. 
   The supporting structures of the connector unit  60  and the support members  80  are explained below. First, the connector unit  60  is mentioned. As shown in  FIG. 2 , two pairs of links  67  and  67  are pivotally attached to the opposite long side surfaces of the connector unit  60 , one pair to one side surface, by means of screws  68 . A screw  69  is attached to a lower end of each link  67  to pass therethrough, when a rod  70  of the screw  69  extends outside. The rods  70  of the screws  69  are pivotally mounted in screw holes  62  in the connector storage recess  61  and screw holes  65  in the connector storage recess  64 , respectively. 
   With the above structure, when the first and second keyboard units  3  and  4  are folded, the connector unit  60  is stored in the keyboard  1  in which the connector storage recesses  61  and  64  are combined. As the first and second keyboard units  3  and  4  are rotated toward an open position, the connector unit  60  is gradually moved up. At the time when the first and second keyboard units  3  and  4  are fully opened into a horizontally arranged state, the center axis (perpendicular to the long sides) of the connector unit  60  is positioned above the first rotational connecting part  2 A. 
   Second, the supporting structure of each support member  80  is explained. As shown in  FIG. 2 , an upper end of one of the support members  80 , namely, a left one in  FIG. 2 , is pivotally attached to a left part  82 A of a joint plate  82  by a locking pin  83 . An upper end of the other support member  80 , namely, a right one in  FIG. 2 , is a pivotally attached to a right part  82 B of the joint plate  82  by a locking pin  84 . The joint plate  82  is so designed to allow the left part  82 A and the right part  82 B to be turned mutually about a rod  81  connecting the parts  82 A and  82 B. A lower end of the left support member  80  is pivotally attached to the inner wall of the storage concave portion  66  in the second keyboard unit  4  by a screw  85 . A lower end of the right support member  80  is pivotally attached to the inner wall of the storage concave portion  63  in the first keyboard unit  3  by a screw  86 . 
   The support members  80  and  80  are stored in a folded state in the keyboard  1  in which the storage concave portions  63  and  66  are combined when the first and second keyboard units  3  and  4  are folded. As the first and second keyboard units  3  and  4  are rotated toward an open position, the support members  80  and  80  are gradually raised while their lower ends are moved away from each other. At the time when the first and second keyboard units  3  and  4  are fully opened into a horizontally arranged position, the support members  80  and  80  come to an upstanding position with the joint plate  82  centrally placed above the connector unit  60  as shown in  FIG. 5 . In this state, the support members  80  and  80  can support a compact portable device such as a PDA. 
   It is to be noted that the supporting structures of the connector unit  60  and the support members  80  and  80  are identical to those disclosed in the specification and drawings of Japanese patent application No. 2002-350329 which was filed by the applicant of this invention. Refer to the specification and drawings of JP-A-2002/350329 for more information about the supporting structures of the connector unit  60  and the support members  80  and  80 . The details thereof are omitted herein. 
   Next explanation is made on the operations of the linkage system  90  and the cover member  100  of the above constructed keyboard  1  during rotation of the first and second keyboard units  3  and  4 . 
   As shown in  FIG. 5 , the first and second keyboard units  3  and  4  are first fully opened into a horizontally arranged position which is a normal usable state. A user may operate the keyboard  1  in this state. 
     FIG. 6  is an enlarged view of a part of the cover member  100  in this state.  FIG. 7  is an enlarged front view of the linkage system  90 , from which the cover member  100  is removed for convenience of explanation. In the cover member  100  shown in  FIG. 6 , the support screw  123  is located in a leftmost position in the first slot  117  and the support screw  125  is located in a rightmost position in the second slot  118 . The projection  99  of the joint element  97  pivotally jointing the links  93  and  96  of the linkage system  90  is located in an uppermost position in the third slot  119 . As shown in  FIG. 7 , the links  93  and  96  of the linkage system  90  are arranged in horizontal alignment with each other. 
   When the first key unit  37  including the key switches  7  and the first support plate  6  and the second key unit  47  including the key switches  10  and the second support plate  9  are turned on the first base plate  5  and the second base plate  8  respectively from the position partially shown in  FIG. 6 , they will be brought into the position shown in  FIG. 8 . The user may also operate the keyboard  1  in this position if desired. 
     FIG. 9  is an enlarged view of a part of the cover member  100  in this state. In  FIG. 9 , the first and second keyboard units  3  and  4  are horizontally arranged as in the case shown in  FIGS. 5 and 6 . Hence, the support screw  123  is located in the leftmost position in the first slot  117  and the support screw  125  is located in the rightmost position in the second slot  118 . The projection  99  of the joint element  97  pivotally jointing the links  93  and  96  of the linkage system  90  is located in the uppermost position in the third slot  119 . The links  93  and  96  are continuously held in horizontal alignment with each other. On the other hand, the first key unit  37  and the second key unit  47  are turned at the same time by engagement between the first gear teeth part  43  and the second gear teeth part  53 . When the units  37  and  47  are turned, the thin plate part  38 A of the circular arc face  38  of the first support plate  6  is caused to slide frontward while remains engaged in the groove  130  formed in the supporting end plate  102  of the cover member  100  and likewise the thin plate part  48 A of the circular arc face  48  of the second support plate  9  is caused to slide frontward while remains engaged in the groove  131  formed in the supporting end plate  102 . 
   As shown in  FIG. 9 , while the first and second key units  37  and  47  are horizontally turned on the first and second base plates  5  and  8  respectively, as mentioned above, the thin plate part  38 A of the circular arc face  38  of the first support plate  6  remains engaged in the groove  130  of the supporting end plate  102  and the thin plate part  48 A of the circular arc face  48  of the second support plate  9  remains engaged in the groove  131 . These engagement relations can prevent the first and second keyboard units  3  and  4  from rotating into a folded state. Consequently, when the first and second keyboard units  3  and  4  are slightly turned horizontally from the state shown in  FIG. 5 , they are inhibited from rotating to be folded. In other words, the keyboard units  3  and  4  are surely held in the horizontal state without rotating, so that the keyboard  1  can be used even when put on a place other than a desk, for example, on operator&#39;s lap. 
   When the slide knob  25 A is grasped and slid by user&#39;s fingers in the sliding recess  46 B, the key support plate  25  is slid rightward in  FIG. 5  together with the key switches  7  mounted thereon. This state is shown in  FIG. 10 . This sliding of the key support plate  25  brings the stair-like portion  25 B into a mated condition with the stair-like portion  26 B of the key support plate  26 , thus closing a gap between the key switches  7  and  10 . Needless to say, the user may also operate the keyboard  1  in this state if desired. 
   It is to be noted that the first and second keyboard units  3  and  4  in the state shown in  FIG. 10  are held in horizontal alignment with each other as in the case shown in  FIG. 5 . Accordingly, the cover member  100  and the linkage system  90  are held in respective states shown in  FIG. 6 . 
   The folding operation of the first and second keyboard units  3  and  4  from the horizontally arranged state to the mutually superposed state is explained below. When the second keyboard unit  4  is slightly rotated counterclockwise from the position shown in  FIG. 5  while the first keyboard unit  3  remains held in a horizontal state, the cover member  100  is simultaneously turned in the same direction as shown in  FIG. 11 . At this time the support screw  123  is in a slightly rightward position from the leftmost position in the fist slot  117  and the support screw  125  is in a slightly leftward position from the rightmost position in the second slot  118 . The projection  99  of the joint element  97  pivotally jointing the links  93  and  96  of the linkage system  90  is in a slightly downward position than the uppermost position in the third slot  119 . As shown in  FIG. 12 , the links  93  and  96  are slightly turned counterclockwise as substantially maintaining a straight relation with each other. 
   When the second keyboard unit  4  is further rotated counterclockwise to a position at angle of about 90° to the first keyboard unit  3 , as shown in  FIG. 13 , the cover member  100  is turned in the same direction. At this time, the support screw  123  is slid more rightward in the first slot  117  from the position shown in  FIG. 11  and the support screw  125  is slid more leftward in the second slot  118  from the position shown in  FIG. 11  respectively. The projection  99  of the joint element  97  is slid more downward in the third slot  119  from the position shown in  FIG. 11 . Simultaneously, the links  93  and  96  are turned, or folded, about the joint element  97  from the straight state, reducing the angle between the links  93  and  96 . 
   When the second keyboard unit  4  is still further rotated counterclockwise, it is brought into a state shown in  FIGS. 14 and 15 . The cover member  100  is simultaneously turned in the same direction as shown in  FIG. 14 . At this time, the support screw  123  is slid close to the rightmost position in the first slot  117  and the support screw  125  is slid close to the leftmost position in the second slot  118 . The projection  99  of the joint element  97  is slid close to the lowermost position in the third slot  119 . The links  93  and  96  of the linkage system  90  are further turned, or folded, about the joint element  97  as shown in  FIG. 15 , further reducing the angle between the links  93  and  96 . 
   Finally, the second keyboard unit  4  is superposed on the first keyboard unit  3  into a completely folded state as shown in  FIG. 16 . Simultaneously, the cover member  100  is turned to a position shown in  FIG. 17  at an angle of 90° with respect to the position shown in  FIG. 6 . At this time, the support screw  123  is slid to the rightmost position in the first slot  117 , the support screw  125  is slid to the leftmost position in the second slot  118 , and the projection  99  of the joint element  97  is slid to the lowermost position in the third slot  119 , respectively. The links  93  and  96  of the linkage system  90  are further turned, or folded, about the joint element  97  into the state shown in  FIG. 18 . 
   As explained above, in the keyboard  1  in the present embodiment, the cover member  100  includes the cover part  101  constructed of the cover plate  104  and the pair of supporting end plates  102  and  103  formed on both ends of the cover plate  104 . Each supporting end plate  102  and  103  is formed with the first and second slots  117  and  118 . The support screw  123  having passed, with play, or clearance, through the first slot  117  is received in the bearing seat  13 A of the bearing member  13  formed in the corner  12 B of the first keyboard unit  3 . The support screw  124  having passed, with play, or clearance, through the other first slot  117  is fixedly received as a turning axis in the support hole  121  in the corner  12 A of the first base plate  5 . Similarly, the support screw  125  having passed, with play, or clearance, through the second slot  118  is received in the bearing hole  17 A of the bearing member  17  formed in the corner  16 B of the second keyboard unit  4 . The support screw  126  having passed, with play, or clearance, through the other second slot  118  is fixedly received as a turning axis in the support hole  120  in the corner  16 A of the second base plate  8 . By cooperation between the support screws  123  and  124  and the first slots  117  and between the support screws  125  and  126  and the second slots  118 , the cover member  100  is turned in synchronization with the rotation of the first and second keyboard units  3  and  4 . In the superposed state of the units  3  and  4 , the cover part  101  of the cover member  100  covers the end faces of the units  3  and  4  and the supporting end plates  102  and  103  of the cover member  100  cover the side portions of lengthwise ends of each end face of the units  3  and  4  (namely, the front and back side portions of each unit). Accordingly, it is possible to prevent dust and waterdrops from entering the keyboard  1  through the end faces of the units  3  and  4  and the side portions of the lengthwise ends of each end face and also to provide a good appearance of the keyboard  1 . 
   While the first and second keyboard units  3  and  4  are rotated from the horizontal state to the superposed state, the cover part  101  of the cover member  100  covers the end faces of the units  3  and  4  and the supporting end plates  102  and  103  cover the side portions of lengthwise ends of each end face of the units  3  and  4 . This makes it possible to reliably prevent dust and waterdrops from entering the keyboard  1  from the end faces of the units  3  and  4  and the side portions of lengthwise ends of each end face in any state of the keyboard  1 ; that is, in a usable state, in the course of folding, or in a folded state for carrying. 
   Furthermore, the link  93  is turned about the support screw  123  passing with play through the first slot  117  in the supporting end plate  102  of the cover member  100 . The link  96  is turned about the screw  125  passing with play through the second slot  118  in the supporting end plate  102 . And the links  93  and  96  are jointed to each other to be turnable about the joint rod  98  of the joint element  97 . Based on the cooperation between the support screw  123  and the first slot  117  and between the support screw  125  and the second slot  118 , the cover member  100  can smoothly be turned through the linkage action of the links  93  and  96 . Thus, the first and second keyboard units  3  and  4  can stably be rotated with respect to each other. 
   The projection  99  in the joint rod  98  of the joint element  97  pivotally jointing the links  93  and  96  is slidably mounted in the third slot  119  in the supporting end plate  102  perpendicular to the first and second slots  117  and  118 . Therefore, the projection  99  is caused to slide in the third slot  119  even during the rotation of the first and second keyboard units  3  and  4 . Accordingly, the keyboard units  3  and  4  are rotated symmetrically with respect to the third slot  119 . This makes it possible to ensure the stable rotation of the keyboard units  3  and  4 . 
   Furthermore, as mentioned above, the linkage system  90  has the following structure. When the first and second keyboard units  3  and  4  are opened in the horizontally arranged state, the joint rod  98  of the joint element  97  jointing the links  93  and  96  is positioned below the rotation rod  19 . When the keyboard units  3  and  4  are folded, on the other hand, the joint rod  98  is substantially aligned with the rotation rod  19  as shown in  FIG. 18 . Thus, it is unnecessary to provide an upward protrusion at the front of the keyboard  1  like the bearings  14 B and  18 B for the rotation rod  19 . The operationality of the keyboard  1  can be enhanced accordingly. 
   As explained with reference to  FIG. 8 , the first and second keyboard units  3  and  4  in the horizontally arranged state are allowed to be turned horizontally (about the screws  29  and  32 ) on the first and second base plates  5  and  8  respectively, whereas they are prevented from turning except in the horizontally arranged state. This is explained with reference to  FIGS. 19A–19C . 
   When the first and second keyboard units  3  and  4  are folded into the superposed state, the thin plate part  38 A formed in the circular arc face  38  of the first support plate  6  and the thin plate part  48 A formed in the circular arc face  48  of the second support plate  9  are in contact with the supporting end plate  102  of the cover member  100  as shown in  FIG. 19A . In this state, the keyboard units  3  and  4  are inhibited from turning. While the keyboard units  3  and  4  are in the course of opening from the folded position to the horizontally arranged state, the thin plate parts  38 A and  48 A are in contact with the supporting end plate  102  of the cover member  100  as shown in  FIG. 19B . Also, parts of the keyboard units  3  and  4  are in contact with the supporting end plate  102 . In this state, similarly, the keyboard units  3  and  4  are inhibited from turning. When the keyboard units  3  and  4  are fully opened into the horizontally arranged state, the thin plate parts  38 A and  48 A become engaged in the grooves  130  and  131  respectively in the supporting end plate  102  as shown in  FIG. 19C . In this state, the keyboard units  3  and  4  are permitted to turn on the first and second base plates  5  and  8  respectively. 
   With this structure, the turning of the keyboard units  3  and  4  on the first and second base plates  5  and  8  are inhibited as long as the keyboard units  3  and  4  are in the superposed, folded state and also in the course of opening from the folded state to the horizontal state. Accordingly, during carrying or transport, for example, the keyboard units  3  and  4  in the folded state can be prevented from unexpectedly turning to protrude outside. 
   The engagement between the first gear teeth part  43  in the circular arc face of the first support plate  6  and the second gear teeth part  53  in the circular arc face of the second support plate  9  is released when the keyboard  1  is folded. However, in this folded keyboard  1 , the turning of the first and second keyboard units  3  and  4  is prevented as mentioned above. Accordingly, a proper engagement relation between the first and second gear teeth parts  43  and  53  can be reestablished when the keyboard units  3  and  4  are opened into the horizontally arranged state. 
   Next, the operation of the stabilizing member  110  provided in the cover member  100  is explained with reference to  FIGS. 20 through 22 .  FIG. 20  is a perspective bottom view of the keyboard  1  in which the first and second keyboard units  3  and  4  are in a horizontally arranged state and the stabilizing member  110  is retracted in the cover part  101  of the cover member  100 . 
   In use, the stabilizing member  110  is pulled outside the keyboard  1  as shown in  FIGS. 21 and 22 . At this time, the rods  111  and  112  are pulled outside, sliding through the sliding holes  108  and  109  in the base parts  106  and  107  of the cover plate  104 , until the stopper rings  115  and  116  abut come into contact with the side ends of the base parts  106  and  107 . 
   As mentioned above, the stabilizing member  110  is retracted in the cover part  101  of the cover member  100  in nonuse and is pulled outside in use. The use of such stabilizing member  110  makes it possible to prevent the first and second keyboard units  3  and  4  from so tilting as to partially lift their bottoms from the setting plane due to the weight of the compact portable device (e.g., a PDA) set on the support members  80 ,  80  in the standing orientation when the first and second keyboard units  3  and  4  are opened in the horizontally arranged state. Thus, the keyboard  1  can be stably held. 
   The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For instance, the following modifications may be adopted. 
   For instance, in the above embodiment, when the first and second key units  37  and  47  are horizontally turned with respect to the first and second base plates  5  and  8  respectively, the thin plate parts  38 A in the circular arc face  38  of the first support plate  6  is engaged in the groove  130  of the supporting end plate  102  and the thin plate part  48 A in the circular arc face  48  of the second support plate  9  is engaged in the groove  131 . In this state, the first and second keyboard units  3  and  4  are inhibited from rotating, or folding, into the superposed state. Thus, the keyboard units  3  and  4  can surely be held in the horizontally open state. Instead of using the above engagement structure, for example, a pair of elastic locking pawls  150  may be integrally formed with the end plate  103  of the cover member  100  as shown in  FIGS. 23 and 24 . As long as the keyboard units  3  and  4  are in the horizontally open state (as shown in  FIG. 23 ), pawl ends  151  of the pawls  150  are held in contact with the back walls of the peripheral member  46  of the first support plate  6  and the peripheral member  56  of the second support plate  9 . When the keyboard units  3  and  4  are turned on the first and second base plates  5  and  8  (as shown in  FIG. 24 ), shoulders appear between the frame portion  23  and the peripheral wall  46  in the first base plate  5  and between the frame portion  24  and the peripheral wall  56  in the second base plate  8 . At this time, pawl ends  151  of the elastic locking pawls  150  catch the shoulders. With this structure, the keyboard units  3  and  4  when turned on the base plates  5  and  8  are prevented from rotating. Thus, the keyboard units  3  and  4  can be surely held in the horizontally open state. 
   In the above embodiment, when the keyboard units  3  and  4  are not in the horizontally open state, for example, when they are in the folded state or in the course of rotation from the folded state to the open state, the thin plate part  38 A of the circular arc face  38  of the first support plate  6  and the thin plate part  48 A of the circular arc face  48  of the second support plate  9  are held in contact with the supporting end plate  102  of the cover member  100 , thereby preventing the keyboard units  3  and  4  from turning on the first and second base plates  5  and  8  respectively. Instead of using the above structure, an alternative design shown in  FIGS. 25 through 30  may be adopted. 
   In this example, as shown in  FIGS. 25 and 26 , a recess  161  having an opening  160  is formed in each of the back face of the peripheral wall member  46  in the first support plate  6  in the first keyboard unit  3  and the back face of the peripheral wall member  56  in the second support plate  9  in the second keyboard unit  4 . Adjacent to the recess  161 , a slide locking groove  162  is formed extending longer than the recess  161  toward the rotation rod  19 . Under the slide locking groove  162  and integrally therewith, a sliding groove  163  is formed extending to the end face of each keyboard unit  3  and  4  (namely, the end face close to the rotation rod  19 ). 
   In the slide locking groove  162  and the sliding groove  163 , a slide locking member  164  is slidably mounted with play. That is, the slide locking member  164  is integrally constructed of a projection  165 A slidably positioned in the slide locking groove  162  and a sliding shaft  165 B formed continuously with the locking projection  165 A. This shaft  165 B is slidable in the sliding groove  163 . In the first keyboard unit  3 , a press spring  166  is placed between the right end of the slide locking groove  162  and the right end of the locking projection  165 A. In the second keyboard unit  4 , similarly, a press spring  166  is placed between the left end of the slide locking groove  162  and the left end of the locking projection  165 A. 
   When the first and second keyboard units  3  and  4  are in the horizontally arranged state, as shown in  FIGS. 25 and 26 , the end faces of the sliding shafts  165 B of the slide locking members  164  abut on each other at the end faces of the keyboard units  3  and  4 . In this state, the locking projections  165 A are urged in the direction to make the end faces of the sliding shafts  165 B abut on each other through the press springs  166 . At this time, each locking projection  165 A is positioned in the groove  162  at a position corresponding to the opening  160  of the recess  161 . 
   In the horizontally arranged state of the first and second keyboard units  3  and  4 , when the first key unit  37  of the first keyboard unit  3  is turned on the first base plate  5  and the second key unit  47  of the second keyboard unit  4  is turned on the second base plate  8 , the locking projections  165 A of the slide locking members  164  become exposed outside the opening  160  of the recess  161 . Thus, when the first and second keyboard units  3  and  4  are in the horizontally arranged state, the first and second key units  37  and  47  can be turned. 
   On the other hand, when the folding of the first and second keyboard units  3  and  4  is started, as shown in  FIGS. 29 and 30 , the projections  165 A of the slide locking members  164  being pressed by the press springs  166  toward the rotation rod  19  are caused to slide in the slide locking grooves  162  toward the rotation rod  19 . Simultaneously, the sliding shafts  165 B are caused to slide in the sliding grooves  163  toward the rotation rod  19 , so that the end portions of the sliding shafts  165 B are exposed outside from the end faces of the first and second keyboard units  3  and  4 . 
   When each slide locking member  164  slides toward the rotation rod  19  as above, one of the locking projections  165 A comes into contact with the left end of the slide locking groove  162  in the first keyboard unit  3  and the other locking projection  165 A comes into contact with the right end of the slide locking groove  162  in the second keyboard unit  4 . Thus, the locking projections  165 A are locked at the above respective positions. With this structure, except when the first and second keyboard units  3  and  4  are in the horizontally arranged state, the first and second key units  37  and  47  are inhibited from turning on the first and second base plates  5  and  8  respectively. 
   Accordingly, when the first and second keyboard units  3  and  4  are in the folded, superposed state, and in the course of opening from the folded state to the horizontally arranged state, the first and second key units  37  and  47  are inhibited from turning on the first and second base plates  5  and  8  respectively. Consequently, during carrying, for example, the keyboard units  3  and  4  in the folded state can be surely prevented from unexpectedly turning to protrude outside. 
   While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.