Patent Publication Number: US-6907206-B2

Title: Assembly structure for image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus such as a printer, a copier, or a facsimile apparatus, and more particularly to attaching positions and attitudes of circuit boards in an image forming apparatus. 
   2. Description of the Related Art 
   JP-A-10-143053 and JP-A-11-341203 disclose an electrophotographic image forming apparatus such as a laser printer or a laser facsimile in which an image forming unit is disposed in an upper side of a body frame and a plurality of circuit boards (control boards) are placed horizontally in a lower face side of the body frame. 
   According to these related arts, as the number of circuit boards increases, the plan view area of the body frame must be enlarged to house the circuit boards. This causes an increase in size of the image forming apparatus. 
   JP-A-10-143053 and JP-A-11-341203 further disclose that a low voltage power board, a high voltage power board, and a main control board are placed horizontally in the lower part of a main frame and a space that is isolated from a sheet path space through which a printing medium passes is defined at the rear of the main frame for placing an NCU board in an upright state. The NCU board is placed so as to become, in a plan view, orthogonal to a drive system unit placed on one side of the main frame. 
   In these related arts, a laser scanner unit as an exposure unit is placed below a process unit as an image forming unit and thus a wiring between the boards and the laser scanner unit can be shortened. However, in the case where the laser scanner unit, the image forming unit, and the main control board are placed from above, a harness for connecting the main control board and the laser scanner unit lengthens and a high-frequency signal is sent to the harness. Thus, noise is easily produced from the harness and noise occurring from actuators of solenoids, motors, etc., is easily picked up by the harness. If noise is thus picked up, a formed image easily becomes irregular. 
   In the case where the main control board is positioned in the lower part of the main frame, the harness between the main control board and a key switch portion positioned in the upper part of the image forming apparatus lengthens. Further, in a complex image forming apparatus provided by adding a facsimile function and an original read scanner function to a printer, an original read section is generally placed on the top of the apparatus and thus the harness for transmitting a signal from a line CCD image pickup device (image sensor), etc., of the original read section to the main control board lengthens and electric noise easily gets mixed in at the middle of the harness. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the above problems, and therefore an object of the invention to provide an image forming apparatus compact in size, in which harnesses can be shortened. 
   According to an aspect of the invention, there is provided an image forming apparatus including: first and second side frames as a pair of left and right frames extending in a substantially vertical direction; an image forming unit disposed between the first and second side frames for developing an electrostatic latent image formed on an electrostatic latent image support, transferring the latent image to a recording medium, and forming an image on the recording medium; an exposure unit disposed above the image forming unit for exposing the electrostatic latent image support to light for forming an electrostatic latent image; a main control board placed substantially vertically on the side of the first side frame for outputting a light exposure signal to the exposure unit in accordance with image data and outputting control signals to the image forming unit and the exposure unit; a power board placed substantially horizontally below the image forming unit on the side of the second side frame; and an engine control board placed substantially horizontally below the image forming unit for outputting a drive signal to drive sources of the image forming unit and the exposure unit in accordance with a control signal from the main control board, the engine control board placed closer to the first side frame than the power board. 
   According to the configuration, the image forming apparatus can be made compact in size and connection lines to the boards or the machines such as drive sources can be shortened. Particularly, the main control board is placed substantially vertically and thus the connection line between the exposure unit driven at a high frequency and the main control board can be shortened and noise can be prevented from occurring and irregularity in an image caused by noise can be prevented. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a schematic perspective view showing a laser printer as an image forming apparatus according to an embodiment of the invention; 
       FIG. 2  is a sectional view taken along line II—II in  FIG. 1 ; 
       FIG. 3  is a sectional view taken along line III—III in  FIG. 1 ; 
       FIG. 4  is a perspective view showing a process of attaching control boards while a body frame is turned upside down; 
       FIG. 5  is another perspective view showing a process of attaching control boards while a body frame is turned upside down; 
       FIG. 6  is a schematic perspective view showing the laser printer viewed from a different direction; 
       FIG. 7  is a plan view showing a schematic layout of the body frame, boards, etc.; 
       FIG. 8  is a perspective view showing a scanner unit viewed from the bottom thereof; 
       FIG. 9  is a perspective view showing a connection part of a process unit, etc., and a high voltage power board viewed from the same direction as that in  FIG. 1 ; 
       FIG. 10  is a perspective view showing a connection part of a process unit, etc., and a main control board viewed from the same direction as that in  FIG. 1 ; 
       FIG. 11  is a perspective view showing an electronic parts mount side of a low voltage power board; 
       FIG. 12  is a perspective view showing an electronic parts mount side of the high voltage power board; 
       FIG. 13  is a plan view showing an engine control board; and 
       FIG. 14  is a perspective view of a complex machine including an image reader placed on and joined to the top of the image forming apparatus. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the invention will be described with reference to the accompanying drawings.  FIG. 1  is a schematic perspective view showing a laser printer as an image forming apparatus according to an embodiment of the invention. In  FIG. 1 , an insertion side of a sheet feed tray (body front side) is shown as the left front.  FIG. 2  is a sectional view taken along line II—II in FIG.  1 .  FIG. 3  is a sectional view taken along line III—III in FIG.  1 .  FIG. 4  is a perspective view of the laser printer turned upside down, when viewed from one direction.  FIG. 5  is a perspective view of the laser printer turned upside down, when viewed from another direction.  FIG. 6  is another schematic perspective view showing the laser printer. In  FIG. 6 , the rear of the body is shown as the left front. 
     FIG. 1  is a perspective view of a laser printer  1 . In this embodiment, an insertion side of a sheet feed tray  6  described later is called a front side, and the opposite side is called a rear side of the laser printer  1 . Side cover members  4   a  and  4   b  as decorative covers are detachably attached to the right and left outer sides of a body frame  2  made of a synthetic resin by screws (not shown) when viewed from the front of the laser printer  1  (when viewed from the left in FIG.  1 ). A front cover member  4   c  and a rear cover member  4   d  as decorative covers are detachably attached to the front and rear outer sides of the body frame  2  by screws (not shown). Further, a top cover member  4   e  as a decorative cover which has a sheet discharging tray  36 , an operation section, etc., is detachably attached to the top of the body frame  2  by screws (not shown). These cover members constitute a body case K. 
   Each of the side cover members  4   a  and  4   b , the front and rear cover members  4   c  and  4   d , and the top cover member  4   e  is also made of a synthetic resin. 
   As shown in  FIGS. 2 ,  3 ,  4 , and  5 , the body frame  2  includes right and left side frame parts  2   a  and  2   b  to which the right and left side cover members  4   a  and  4   b  are attached and a partition wall frame part  2   c  for partitioning the body frame  2  to upper and lower parts at a vertical intermediate position of the inner faces of the side frame parts  2   a  and  2   b . These frame parts  2   a ,  2   b  and  2   c  are integrally formed in one piece. Control boards  14 ,  15 , and  16  are placed in a lower face side of the partition wall frame part  2   c  as described later, and lower faces of the control boards are covered with a bottom cover member  50  that is a base plate made of a metal plate, etc. An operation panel  4   f  is provided on the top of the left cover member  4   b  as shown in FIG.  1 . 
   A sheet feeder  5  for feeding sheets  3  (cut sheets) as recording mediums is placed below the bottom cover member  50 . Placed above the partition wall frame part  2   c  are a process unit  18  as an image forming unit configured to form a predetermined image on a fed sheet  3 , a scanner unit  17 , a fixer  19  as a fixing unit, and the like. 
   The sheet feeder  5  includes the sheet feed tray  6  detachably installed in the body frame  2 , a sheet pressing plate  8  disposed in the sheet feed tray  6 , a D-shaped sheet feeding roller  9  disposed above one end portion of the sheet feed tray  6  for intermittently making one rotation, and a separating member  10 . 
   A transport path  7  for the sheet  3  elongating from the sheet feeding roller  9  to an image forming position (a contact area between a photosensitive drum  23  and a transfer roller  25 , i.e., a transfer position where a toner image on the photosensitive drum  23  is transferred to the sheet  3 ) is defined between an upper face of the partition wall frame part  2   c  and the bottom of a case of the process unit  18 , and between the right and left side frame parts  2   a  and  2   b  in the body frame  2 , as shown in FIG.  2 . On the transport path  7  as the sheet path, a transport roller pair  11  and a registration roller pair  12  placed just before the image forming position are disposed at an adequate interval downstream from the sheet feeding roller  9  in a transport direction. 
   The sheets  3  are stacked on the sheet pressing plate  8 . The sheet pressing plate  8  is swingably supported at an end portion which is remote from the sheet feeding roller  9 , thereby enabling an end portion close to the roller to be vertically movable. The sheet pressing plate is upward urged from a back side by a spring  8   a . The sheet feeding roller  9  and the separating member  10  are placed so as to be opposed to each other. The separating member  10  includes a separating pad (not shown) that is formed by a member having a large coefficient of friction. The separating pad is pressed toward the sheet feeding roller  9  by a spring  10   b  disposed on a back side of a pad support member  10   c  of the separating member  10 . 
   The separating pad and the sheet feeding roller  9  are formed to be shorter in width in a direction perpendicular to the transport direction of the sheet  3 , than the sheet  3 , and placed so as to be in contact only with an approximately central area of the sheet  3  in the width direction during a feeding process. 
   Among the sheets  3  stacked on the sheet pressing plate  8 , the uppermost sheet  3  is pressed against the sheet feeding roller  9  and is separated by the separating member  10  as the sheet feeding roller  9  rotates for feeding one sheet at a time. The fed sheet  3  is delivered to the transport roller pair  11  and the registration roller pair  12  in order and the leading end of the sheet  3  is subjected to registration. The sheet  3  is then delivered to the image forming position. 
   A manual tray  13  for manually supplying the sheet  3  is foldably placed above the transport roller pair  11  (downstream in the transport direction: in the front cover member  4   c  of the body frame  2  at a position above the sheet feeder  5 ). (See  FIGS. 1 and 2 .) 
   The scanner unit  17  as an exposure unit is placed above the image forming unit (described later) on the lower face of the sheet discharging tray  36  of the top cover member  4   e  in the upper part of the body frame  2 , as shown in  FIGS. 2 ,  9 , and  10 . The scanner unit  17  includes a laser diode  17   b  placed in a scanner case  17   a  with the bottom open, a polygon mirror  20  being rotatably driven, a fθ lens  21   a , a lens  21   b , a reflecting mirror  22 , etc., as shown in  FIGS. 8 ,  9 , and  10 . Based on prepared image data, the scanner unit  17  allows a laser beam (light exposure signal) emitted from the laser diode  17   b  at a predetermined timing to pass through or be reflected on the polygon mirror  20 , the fθ lens  21   a , the reflecting mirror  22 , and the lens  21   b  in order and scans the laser beam at high speed over the surface of the photosensitive drum  23  as one example of an electrostatic latent image support in the process unit  18 . 
   The process unit  18  as a part of the image forming unit includes a drum cartridge having the photosensitive drum  23  as an electrostatic latent image support, a scorotron charger  37  as an charging unit, a transfer roller  25  as a transfer section, and a developing cartridge  24  that can be attached to and detached from the drum cartridge. The front cover  4   c  can be opened and closed. When the front cover  4   c  is open, the process unit  18  attached as shown in  FIG. 2  can be taken out through the front side. 
   The developing cartridge  24  includes a toner storage section  26 , a developing roller  27  as a developing section, a layer thickness regulation blade (not shown), a toner supply roller  29 , etc. 
   The toner storage section  26  is filled with nonmagnetic mono component polymerized toner of positive charge property as a developer. The toner is supplied by the toner supply roller  29  to the developing roller  27 . At this time, the toner is positively frictionally charged between the toner supply roller  29  and the developing roller  27 . Further, the toner supplied onto the developing roller  27  is supported onto the developing roller  27  as a thin layer of a constant thickness by friction of the layer thickness regulation blade in accordance with rotation of the developing roller  27 . On the other hand, the rotating photosensitive drum  23  is placed facing the developing roller  27  and has a drum main body grounded and the surface is formed with a positively charged photosensitive layer made of an organic photoconductor material such as polycarbonate. 
   The scorotron charger  37  as the charging section is disposed above the photosensitive drum  23  with a gap defined therebetween so as not to come in contact with the photosensitive drum  23 . The scorotron charger  37  is a positive charging scorotron charger to generate corona discharge from a charging wire of tungsten, etc., and uniformly positively charges the surface of the photosensitive drum  23 . 
   As the photosensitive drum  23  rotates, the surface of the photosensitive drum  23  is uniformly positively charged by the scorotron charger  37  and then a laser beam from the scanner unit  17  is scanned over the surface of the photosensitive drum  23  at high speed to expose the surface to light and an electrostatic latent image based on predetermined image data is formed on the surface of the photosensitive drum  23 . 
   When the positively charged toner supported on the developing roller  27  is opposed to and comes in contact with the photosensitive drum  23  as the developing roller  27  rotates, the toner is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  23 , namely, the toner is selectively supported on the portion exposed to light by the laser beam and lowered in potential in the uniformly positively charged surface of the photosensitive drum  23 , thereby rendering the electrostatic latent image visible as a toner image. 
   The transfer roller  25  is placed below the photosensitive drum  23  so as to be opposed to the photosensitive drum  23 . The transfer roller  25  has a metal roller shaft covered with a roller made of an ionic conductive rubber material. During the transferring process, a transfer bias (transfer forward bias) is applied from a transfer bias application power supply to the transfer roller  25 . Thus, the toner image supported on the surface of the photosensitive drum  23  is transferred to the sheet  3  while the sheet  3  passes through the nip between the photosensitive drum  23  and the transfer roller  25 . 
   Subsequently, the configuration of the fixer  19  as the fixing unit will be discussed. The fixer  19  is disposed downstream in the transport direction from the process unit  18  and includes one heating roller  30 , a pressurization roller  31  placed so as to press the heating roller  30 , and a pair of transport rollers  32  disposed downstream from the rollers  30  and  31 , as shown in FIG.  2 . The heating roller  30  is made of metal of aluminum, etc., and includes a heater such as a halogen lamp for heating. It thermally fixes the toner transferred onto the sheet  3  in the process unit  18  while the sheet  3  passes through the nip between the heating roller  30  and the pressurization roller  31 . The sheet  3  is then transported by the transport rollers  32  and is transported by a sheet discharge roller  35  in a sheet discharge path inside the rear cover member  4   d  in the body case K and is then discharged onto the sheet discharging tray  36 . 
   In the embodiment, the laser printer includes a return section  40  for forming an image on both sides of the sheet  3 . The sheet discharge roller  35  can be selectively rotated forward or reversely so as to serve as a reversal mechanism in the return section  40 . 
   To conduct double-sided printing, the sheet  3  with an image formed on one side is transported by the transport rollers  32  to the sheet discharge roller  35  and is once stopped with the trailing end of the sheet  3  clamped in the part of the sheet discharge roller  35 . Then, when the sheet discharge roller  35  is rotated reversely, the sheet  3  is delivered to an inner return path  41  in the rear cover member  4   d.    
   The sheet  3  is then transported in the return direction R while a side edge of the sheet  3  is abutted against a reference plate (not shown) by skewed rollers  43   a ,  43   b , etc., in the return path  41  on a return tray  42  placed detachably on the top of the sheet feed tray  6 , on the lower face side of the bottom cover member  50 . The sheet  3  is returned to another transport member  45  through a return guide plate  44 . Accordingly, the sheet  3  is reversed so that the nonprint side of the sheet  3  is up at the part of the registration roller  12 . Consequently, when the sheet  3  is passed through the image forming position in this state, an image can be formed on the other side (back) of the sheet  3 . 
   A drive transmission system frame  51  made of a metal plate including a drive gear system for driving the rollers of the sheet feeding roller  9 , the process unit  18 , the developing cartridge  24 , and the fixer  19  is attached in a longitudinal direction (substantially vertical direction) on the outer side of the left side frame part  2   b  (corresponding to a first side frame) in the body frame  2 . The drive transmission system frame  51  extends from a side near to placement of the sheet feeding roller  9  (the front of the laser printer  1 ) to a side part of the fixer  19 . A main control board  52  is attached at the rear of the drive transmission system frame  51 , and a cord connection connector  53  connectable to an external machine and a power cord connector (not shown) are open at the rear end of the laser printer  1 . (See  FIG. 5. ) 
   A cooling fan  54  for discharging heat generated in the fixer  19  to the outside of the body case K is placed on the upper side of the right side frame part  2   a  (corresponding to a second side frame) in the body frame  2 , and a cooling fan  55  for discharging heat generated from a high voltage power board  14 , a low voltage power board  15  and an engine control board  6  (described later) is placed on the lower side of the right side frame part  2   a . The heat is discharged to the outside of the laser printer through exhaust ports  56  and  57  made in the right cover member  4   a  at the positions corresponding to the cooling fans  54  and  55 . (See  FIGS. 1 and 4 .) 
   Subsequently, the layout of the main control board  52 , the high voltage power board  14 , the low voltage power board  15 , and the engine control board  16  as the circuit boards for supplying power to the image forming apparatus and controlling the apparatus will be described in detail with reference to  FIGS. 1  to  5 . 
   The main control board  52  outputs a light exposure signal to the exposure unit in accordance with image data and also outputs control signals to the image forming unit and the exposure unit. To provide a printer function, the main control board  52  includes a control circuit for controlling the image forming apparatus so as to receive print data, a print command, etc., from an external apparatus such as a computer, and execute a print job based on the print command. The low voltage power board  15  drops commercial voltage (100 volts to 220 volts) supplied via a power cord (not shown) to a predetermined low voltage and supplies the low voltage to control circuit sections of the main control board  52 , the high voltage power board  14 , and the engine control board  16 . The high voltage power board  14  generates a high voltage although it is a low current, and applies the high voltage to the charger  37 , the photosensitive drum  23 , etc., in the process unit  18 . The engine control board  16  outputs a drive signal to the drive sources of the image forming unit and the exposure unit, for example, actuators (not shown) of a drive motor in the sheet feeder  5 , or a solenoid of a one-rotation clutch in the sheet feeder  5 , in accordance with a control signal from the main control board  52 . 
   The main control board  52  is placed substantially vertically along the outer side of the left side frame part  2   b  (corresponding to the first side frame). (See  FIGS. 7 ,  9 , and  10 .) The main control board  52  is placed substantially vertically on the side near to the side where the operation panel  4   f  is provided as shown in FIG.  6 . In the case where the operation panel  4   f  and the main control board  52  are connected by a connection line (not shown), the connection line can be shortened. The main control board  52  and a connector  17   d  in a drive circuit board  17   c  of the laser diode  17   b  in the scanner unit  17  are connected by a connection line  65  such as a flat cable. (See  FIGS. 8 and 10 .) Since a high clock signal (high frequency signal) is sent from the main control board  52  to the connection line  65  accompanying a light exposure signal, noise easily occurs and the connection line  65  easily picks up noise. If noise is picked up, light emission of the laser diode  17   b  is adversely affected, leading to irregularity in an image. Therefore, by shortening the connection line  65 ; noise is prevented from occurring and irregularity in an image caused by noise is prevented from occurring. 
   The high voltage power board  14  and the low voltage power board  15  are, as shown in  FIGS. 1  to  12 , placed substantially horizontally with the board faces at the same height at a position near to the right side frame part  2   a  (second side frame) on the lower face side of the partition wall frame part  2   c  in the body frame  2 . The high voltage power board  14  and the low voltage power board  15  are placed upward so that a plurality of electronic parts  58   a  to  58   d ,  58   h  to  58   j , etc., mounted on the low voltage power board  15  and a plurality of electronic parts mounted on the high voltage power board  14  (a transistor  59   a  attached to a heat sink, transformers  59   b  to  59   d , a connector  59   e , etc.,) are directed in the direction of the partition wall frame part  2   c  (to the upper side of the body case K). 
   The engine control board  16  is, as shown in  FIGS. 1  to  10  and  13 , placed so that a plurality of short electronic parts  60  mounted on the board  16  (a gate array  60   a , a comparator  60   b , terminals  60   c  to  60   h  for connecting connection lines of harness, etc., and the like) are directed downward in the body case K. To assemble, the boards  14 ,  15 , and  16  are placed from the bottom side of the body frame  2  (the upper side in  FIGS. 4 and 5 ) with the body frame  2  turned upside down, as shown in  FIGS. 4 and 5 . Electronic parts  58  are mounted on the low voltage power control board. The electronic parts mounted on the low voltage power board  15  include a bulky, large electrolytic capacitor  58   a , a transformer  58   b  and a choke coil  58   c . Also mounted on the board  15  are an FET  58   h , a low voltage IC  58   i  and a triac  58   j , which are attached to tall heat sinks (cooling palates)  58   e  to  58   g . These parts are large parts taller than the electronic parts  60   a  to  60   h  mounted on the engine control board  16 . The electronic part  58   a  to  58   d  and  58   h  to  58   j  and the heat sinks  58   e  to  58   g  for dissipating heat generated from the electronic parts are placed in a space  62  defined by an upward projection of the partition wall frame part  2   c  (see FIGS.  2  and  3 ). 
   As shown in  FIGS. 7 and 9 , connection is made from the low voltage power board  15  to the heater (not shown) in the fixer  19 , the high voltage power board  14 , the main control board  52 , and the engine control board  16  via connection lines  67 ,  68 , and  70  for supplying power. 
   The high voltage power board  14  and the engine control board  16  are connected via a connection line  66 . Further, connection is made from the high voltage power board  14  to the developing roller  27  in the developing cartridge  24  via a connection line  71  for giving a developing bias, and connection is made from the high voltage power board  14  to a grid electrode and the charging wire in the charger  37  of the process unit  18  via wire-like connection lines  73  and  74  for giving a grid bias and a charging bias. (See  FIG. 9. ) The high voltage power board  14  and a cleaning brush of the process unit  18  are also connected via a wire-like connection line  75  for giving a cleaning bias. 
   That is, cartridge side contacts  73   a ,  74   a  and  75   a  to which biases are applied are provided on a right side frame part side of the process cartridge, and body side contacts  73   b ,  74   b  and  75   b  connected to the cartridge side contacts  73   a ,  74   a  and  75   a  and connected to the high voltage power board  14  are provided on a body side. 
   The high voltage power board  14  is positioned close to the right side frame part  2   a  and close to the depth of the body frame  2  and is positioned adjacent to other boards  52 ,  15 , and  16  and therefore the connection lines  65  to  70  are shortened. Since the high voltage power board  14  is placed below the process unit  18 , when the connection lines  73  to  75  from the high voltage power board  14  are connected to the charger  37 , the photosensitive drum  23 , etc., the lengths of the lines can be shortened. 
   Connection is made from the engine control board  16  to the main control board  52  via a connection line  76 , and connection is made from the engine control board  16  via a connection line  72  for supplying power to the drive motor and the solenoid. Since the engine control board  16  and the main control board  52  are placed close to the left side frame part  2   b  and near to placement of the drive transmission system frame  51 , the connection lines  76  and  72  can also be shortened. 
   Thus, the high voltage power board  14 , the low voltage power board  15 , and the engine control board  16  are placed horizontally on the lower face side of the partition wall frame part  2   c  of the body frame  2 . The high voltage power board  14  is placed on the side near to the right side frame part  2   a . The main control board  52  and the drive transmission system frame  51  are placed in the longitudinal direction so that they are contiguous to each other along the outer face of the left side frame part  2   b . The engine control board  16  is placed on the side near to the left side frame part  2   b  and on the side near to the drive transmission system frame  51 . Therefore, the four boards  52 ,  14 ,  15 , and  16  can be brought close to each other in their placement for making the body frame  2  compact in size. 
   The engine control board  16  is placed with the electronic parts mount side thereof directed downward (directed in the bottom direction of the body frame  2 ), so that a worker can see the electronic parts  60   a  to  60   h  and the connection line  72  to the drive motor and the solenoid. (See  FIG. 4. ) When placing the bottom cover member  50  as described later in this state, the worker can check the arrangement of the connection line  72  and then place the bottom cover member  50  so that the connection line  72  does not get caught in the gap between the bottom cover member  50  and the electronic parts  60   a  to  60   h.    
   The high voltage power board  14  and the low voltage power board  15  on which the electronic parts  58   a  to  58   d ,  58   h  to  58   j , and  59   a  to  59   e  are mounted so as to be directed in the upper direction of the body frame  2  are placed to one side of the body frame  2 , and the engine control board  16  with the electronic parts  60   a  to  60   h  directed in the bottom direction of the body frame  2  is placed to an opposite side of the body frame  2 . The boards  14  and  15  and the engine control board  16  are placed at different levels. The height positions of the boards  14  and  15  are near to the bottom of the body frame  2 . The height position of the engine control board  16  is higher than the height positions of the boards  14  and  15 , but still near to the bottom of the body frame  2 . 
   The board height position of the engine control board  16  with the electronic parts  60   a  to  60   h  directed in the bottom direction of the body frame  2  is set to a higher position, whereby when placing the bottom cover member  50 , the worker can bring the bottom cover member  50  close to the board height position of the engine control board  16  so that the connection line  71  does not get caught in the gap between the bottom cover member  50  and the electronic parts  60   a  to  60   h . Therefore, it becomes unnecessary to define a useless vertical space between the partition wall frame part  2   c  and the bottom cover member  50 , and it is made possible to decrease the height of the body frame  2  and the whole height of the laser printer  1 . 
   With all boards  14  to  16  placed horizontally, an electric insulating film (not shown) is put from the bottom of the body frame  2  and the bottom cover body  50  is then put and fastened to a boss part projecting downward (directed to the bottom) from the lower face of the partition wall frame part  2   c  of the body frame  2  by a plurality of screws  63 . The bottom cover member  50  and the boards  14  to  16  are fastened together by the screws  63 , whereby the assembly work can be simplified as compared with the case where the boards are once fixed by different screws and then the bottom cover member is screwed. 
   Since the high voltage power board  14  and the low voltage power board  15  positioned to one side and the engine control board  16  positioned to an opposite side are placed at different levels, the bottom cover member  50  is formed into a step shape having the different levels. (See  FIG. 4. ) 
   A high different-level part  50   a  of the bottom cover member  50  (corresponding to the place of the engine control board  16 ) can provide a large vertical space with the return tray  42  placed on the upper face of the sheet feed tray  6 . The skewed rollers  43   a ,  43   b  may be placed in th different-level part, to thereby decrease the height of the body frame  2  and the whole height of the body frame  2  and the whole height of the laser printer  1 . 
   The space between the bottom cover member  50  and the partition wall frame part  2   c  is made to communicate with a substantially horizontal air suction path of the cooling fan  55 , so that it is made possible to smoothly discharge heat generated from the boards  14  to  16  placed horizontally. 
   As shown in  FIG. 14 , an image reader  80  including an original placement section shaped like a flat glass plate, press cover member  81  and an original auto-feeder  82  for a copy function, a line CCD image sensor (not shown) for a scanner function, and a circuit board (not shown) and an operation panel section  83  for a facsimile function may be placed on and joined to the top of the body case K of the image forming apparatus  1  having a printer function via foot parts  84  and  84  to constitute a complex image forming apparatus having complex functions. In this case, if the line CCD image sensor is placed in an upper part on the side near to the main control board  52  and a flat connection line therefrom is connected to the main control board  52 , the flat connection line can also be shortened and a feeble read signal can be prevented from being disturbed by the electromagnetic wave from any other connection line or the board  14 ,  15 ,  16  or picking up noise is prevented. 
   Such a configuration is applied to a laser printer, an ink jet printer, or a multifunctional image forming apparatus including a facsimile function, whereby the height dimension of the image forming apparatus can be decreased in size. 
   As was described above, in an image forming apparatus according to a first aspect of the invention, a image forming unit is placed between a first side frame and a second side frame as a pair of left and right frames extending in a substantially vertical direction; an exposure unit is placed above the image forming unit; a main control board for outputting control signals to the image forming unit and the exposure unit is placed substantially vertically on the side of the first side frame; a power board is placed substantially horizontally below the image forming unit on the side of the second side frame; and an engine control board for outputting a drive signal to drive sources of the image forming unit and the exposure unit in accordance with a control signal from the main control board is placed substantially horizontally below the image forming unit closer to the first side frame than the power board. Thus, the body frame can be made compact in size and the connection lines to the boards or the machines of the drive sources, etc., can also be shortened. Particularly, the main control board is placed substantially vertically and thus the connection line between the exposure unit driven at a high frequency and the main control board can be shortened and noise can be prevented from occurring and irregularity in an image caused by noise can be prevented. 
   In the image forming apparatus according to a second aspect of the invention, an operation panel is placed above the image forming unit and is connected to the main control board placed substantially vertically. Thus, the connection line connecting the operation panel and the main control board can be more shortened and the apparatus body can be made compact in size. 
   In the image forming apparatus according to a third aspect of the invention, the power board includes a low voltage power board being connectable to a commercial power supply for outputting a predetermined voltage and a high voltage power board for applying various biases to the image forming unit. Thus, the layout of the boards when the apparatus is made compact in size can be facilitated. 
   In the image forming apparatus according to a fourth aspect of the invention, a body side contact for connecting to a detachable process cartridge is provided on a second side frame side of an apparatus body and is connected to the high voltage power board placed on the side of the second side frame, so that the connection line between the body side contact and the high voltage power board can be shortened. 
   In the image forming apparatus according to a fifth aspect of the invention, a fixing unit for thermally fixing an image transferred to a recording medium is placed downstream in a recording medium transport direction from the image forming unit and the high voltage power board is placed upstream in the recording medium transport direction from the low voltage power board. Thus, the image forming unit and the high voltage power board are placed together upstream in the recording medium transport direction and the connection line connecting the image forming unit and the high voltage power board can be shortened. 
   In the image forming apparatus according to a sixth aspect of the invention, the low voltage power board supplies power to a heater of the fixing unit placed downstream in the recording medium transport direction from the high voltage power board, so that the connection line connecting the low voltage power board and the heater of the fixing unit can be shortened. 
   In the image forming apparatus according to a seventh aspect of the invention, the high voltage power board is placed at a front side in the image forming apparatus, the low voltage power board is placed at a rear side in the image forming apparatus, and the process cartridge is attached to and detached from the image forming apparatus through a front side of the image forming apparatus. Thus, the process cartridge can be easily attached and detached and the connection line between the boards and the connection line to the image forming unit can also be shortened. 
   In the image forming apparatus according to an eighth aspect of the invention, a space is defined behind the low voltage power board and a part of a return path for guiding a recording medium with an image formed on one side from the upper side of the low voltage power board to the lower side and returning the recording medium to the image forming unit is formed in the space. Thus, a compact apparatus capable of performing double-sided printing can be manufactured. 
   In the image forming apparatus according to a ninth aspect of the invention, a sheet feed tray on which recording mediums are stacked is placed detachably through the front side of the image forming apparatus, below the power board and the main control board. Thus, the sheet feed tray can be easily attached and detached and the apparatus body can be made compact in size. 
   In the image forming apparatus according to a tenth aspect of the invention, a reader is placed on the top of the image forming apparatus and is connected to the main control board. Thus, the connection line connecting the reader and the main control board can be more shortened, assembling is facilitated, and the apparatus becomes compact in size. A feeble read signal in the reader can be prevented from picking up noise generated by the electromagnetic wave from any other connection line or any board placed in the lower part of the body, thereby irregularity in a read signal can be prevented. 
   The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.