Abstract:
A plurality of chambers are adapted to accommodate a plurality of cartridges each containing toner to be used for printing. A mode changer is operable to change an operation mode of the printing apparatus in accordance with a combination of respective colors of toner in the cartridges accommodated in the chambers. A first detector is operable to detect a residual amount of toner in each of the cartridges. A second detector is operable to detect the respective colors of toner in the cartridges when the operation mode is changed. A controller is operable to generate an image data including a plurality of first identifiers each indicative of the residual amount of toner in one of the cartridges and a plurality of second identifiers each indicative of one of the colors of toner and associated with one of the first identifiers. A display is adapted to display the image data.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a printing apparatus. 
   As well known in the art, some of printing apparatuses perform color printing with using color toners supplied from toner cartridges. In many of printing apparatuses of this kind, the amounts of residual toners (hereinafter, referred to as residual toner amounts) in color toner cartridges are displayed in a display of a control panel to inform the operator of a timing when the color reproducibility of a color image is deteriorated or that when a toner cartridge is to be replaced. 
     FIG. 9  shows an example of a control panel  30  which displays residual toner amounts of toner cartridges in a display  31  disposed together with plural buttons on the upper face of the control panel  30 . In the vicinity of the right side of the lower edge of the display  31 , four oval indicators  32  colored respectively by CMYK (cyan, magenta, yellow and black) colors are arrayed. In the display  31  and above the four indicators  32 , bar graphs respectively showing residual amounts of toners of colors indicated by the indicators are displayed. From the indicators  32  and the bar graphs, the operator can immediately recognize the residual toner amounts of the respective colors. 
   Some of printing apparatuses are configured so that color printing and monochrome printing are switched over in accordance with the combination of colors of toners of toner cartridges which are set in the apparatus. When four toner cartridges respectively filled with CMYK toners are set, for example, a printing apparatus of this kind operates in a color printing mode, and, when four toner cartridges filled with toners of K are set, operates in a monochrome printing mode. 
   When the above-mentioned method of displaying residual toner amounts is applied to a printing apparatus of this kind, in the monochrome printing mode, residual toner amounts of four toner cartridges for K are displayed respectively in the form of the bar graphs in the display  31  of the control panel  30 . In this case, the colors of the toners the residual amounts of which are actually indicated by the bar graphs do not coincide with the colors of the indicators  32 , whereby the operator is confused. 
   In order to provide the user with a message indicating that the toner cartridge is to be replaced with a new one, there is installed a residual toner amount detector which detects that there is no amount of residual toner in a toner cartridge. 
   Recently, in order to reduce the production cost of a printing apparatus of such a kind, there is adopted a configuration where a residual toner amount detector which is not highly accurate is installed, or that where a residual toner amount detector is not installed. Instead, an amount of consumed toner is calculated by counting the number of actually printed dots. 
   In both the configurations where a residual toner amount detector which is not highly accurate is installed, and where a residual toner amount detector is not installed, however, the accuracy of detecting the residual toner amount is not so high. Consequently, there is a problem in that a message indicating that the toner cartridge is to be replaced with a new one would be displayed although a considerable amount of toner remains in the toner cartridge. 
   It is well-known a printing apparatus in which a toner cartridge for supplying toner to a photosensitive drum is sequentially changed one by one every time when printing is performed on a unit number of sheets. 
   When a residual toner amount detector is not installed on such a printing apparatus, the printing apparatus cannot judge whether toner remains in a cartridge or not. In a case where a toner cartridge is sequentially changed one by one every time when printing is performed on a unit number of sheets, there is a problem in that a situation may occur where printing must be performed with using a toner cartridge in which toner is consumed up. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to prevent the operator from being confused by displaying of a residual toner amount in a display even when a printing mode is switched in accordance with the combination of colors of toners charged in toner cartridges. 
   It is also an object of the invention to enable the actual residual toner amount to be known even in the case where a residual toner amount detector has accuracy which is not high, or where a residual toner amount detector is not installed. 
   It is also an object of the invention to provide a printing apparatus which enables printing with toner in only a toner cartridge in which toner remains even when a sensor for detecting the residual toner amount of a toner cartridge is not installed. 
   In order to achieve the above objects, according to the invention, there is provided a printing apparatus, comprising: 
   a plurality of chambers, adapted to accommodate a plurality of cartridges each containing toner to be used for printing; 
   a mode changer operable to change an operation mode of the printing apparatus in accordance with a combination of respective colors of toner in the cartridges accommodated in the chambers; 
   a first detector, operable to detect a residual amount of toner in each of the cartridges; 
   a second detector, operable to detect the respective colors of toner in the cartridges when the operation mode is changed; 
   a controller, operable to generate an image data including a plurality of first identifiers each indicative of the residual amount of toner in one of the cartridges, and a plurality of second identifiers each indicative of one of the colors of toner and associated with one of the first identifiers; and 
   a display, adapted to display the image data. 
   With this configuration, a user is prevented from being confused by displaying of the residual toner amount in the display even when the operation mode is switched in accordance with the combination of colors of toner filled in the cartridges. 
   The image data may include a plurality of third identifiers each indicative of a position of one of the chambers and associated with one of the first identifiers. 
   The controller may be operable to calculate an average residual amount of toner in the cartridges when the second detector detects that the same color of toner is contained in all of the cartridges accommodated in the chambers. The image data may include a third identifier indicative of the average residual amount. 
   Each of the second identifiers may be an alphabetical character or a color of the associated one of the first identifiers. 
   According to the invention, there is also provided a printing apparatus, comprising: 
   a plurality of chambers, adapted to accommodate a plurality of cartridges each containing toner; 
   a print engine, adapted to perform printing on a print medium with the toner in the respective cartridges accommodated in the chambers; 
   a judge, determining whether it is necessary to check residual toner amounts of the respective cartridges; and 
   a controller, operable to cause the print engine to print a test image on the print medium when the judge determines that it is necessary to check the residual toner amounts, 
   wherein the test image includes a plurality of patterns each of which is to be located at different positions on the print medium and is associated with one of the chambers. 
   With this configuration, when a pattern a density of which is reduced exists in the test image, a user can judge that the actual residual toner amount in the cartridge which is loaded in the chamber corresponding to the pattern is reduced. 
   Therefore, it is possible to know the actual residual toner amount even in the case where a residual toner amount detector has accuracy which is not high, or where a residual toner amount detector is not installed. 
   The test image may include a plurality of identifiers each indicative of a position of one of the chambers and associated with one of the patterns. 
   The printing apparatus may further comprise a calculator, calculating a residual toner amount in each of the cartridge by counting the number of toner dots printed on the print medium. The judge may determine that it is necessary to check the residual toner amounts, when the calculated residual toner amount in at least one of the cartridges becomes lower than a prescribed value. 
   The printing apparatus may further comprise a detector, detecting a residual toner amount in each of the cartridge. The judge may determine that it is necessary to check the residual toner amounts, when the detected residual toner amount in at least one of the cartridges becomes lower than a prescribed value. 
   The judge may be activated when at least two of the chambers are occupied by the cartridges. 
   According to the invention, there is also provided a printing apparatus, comprising: 
   a plurality of chambers, adapted to accommodate a plurality of cartridges each containing toner; 
   a print engine, including a plurality of developing devices each of which is adapted to visualize an electrostatic latent image on an image carrier with the toner in associated one of the cartridges accommodated in the chambers; 
   a storage, storing a plurality of flags each of which is associated with one of the developing devices, and adapted to be switchable between an enabling state or a disabling state; 
   a controller, operable to drive the print engine so as to operate at least one of the developing devices that is associated with one of the flags which is in the enabling state; and 
   a first switcher, adapted to receive an instruction from a user, and operable to switch each of the flags to one of the enabling state and disabling state. 
   With the above configuration, a user can set a developing device to which a cartridge having a reduced amount of toner is attached, as a developing device which cannot operate. Even when a residual toner amount detector is not installed, therefore, toner in only a cartridge in which toner remains can be used in printing. 
   The printing apparatus may further comprise: a detector, operable to detect that a cartridge replacement is performed at each of the chambers; and a second switcher, operable to switch at least one of the flags which is associated with one of the chambers that the cartridge replacement is detected, to the enabling state. 
   The printing apparatus may further comprising a judge, determining whether it is necessary to check residual toner amounts of the respective cartridges. The controller may be operable to cause the print engine to print a test image on a print medium when the judge determines that it is necessary to check the residual toner amounts. The test image may include a plurality of patterns each of which is to be located at different positions on the print medium and is associated with one of the chambers. 
   The test image may include a plurality of identifiers each indicative of a position of one of the chambers and associated with one of the patterns. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram showing an internal configuration of a printing apparatus according to a first embodiment of the invention; 
       FIG. 2  is a plan view of a control panel of the printing apparatus; 
       FIG. 3  is a schematic view showing the inside of the printing apparatus; 
       FIG. 4  is a plan view of a display in a control panel of the printing apparatus, showing a case that a color mode is selected; 
       FIG. 5  is a plan view of the display in the control panel, showing a case that a 4-cartridge monochrome mode is selected; 
       FIG. 6  is a plan view of a display in the control panel, showing a case that a 1-cartridge monochrome mode is selected; 
       FIG. 7  is a plan view of a test pattern image printed by a printing apparatus according to a second embodiment of the invention, when a 4-cartridge monochrome mode is selected; 
       FIG. 8  is a plan view of the test pattern image, showing a case that a residual toner amount in a toner cartridge loaded in a cartridge chamber for cyan is lower than a prescribed value; and 
       FIG. 9  is a plan view of a display in a control panel of a related-art printing apparatus. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Embodiments of the invention will be described below in detail with reference to the accompanying drawings. 
   A printing apparatus  10  according to a first embodiment receives a print request from a host computer which is not shown or a camera device which is not shown, and operates in response to the request. As shown in  FIG. 1 , the apparatus comprises a control panel  11 , a print engine  12 , and a controller  13 . 
   The control panel  11  is a device which obtains from the operator instructions related to a process to be executed, and which is disposed in the upper face of the printing apparatus  10 . As shown in  FIG. 2 , the upper face of the control panel  11  comprises interface components which are required in operation, such as an LCD  111 , buttons  112 , and LEDs  113 . The interface components  111  to  113  are attached to the casing, and connected to a control circuit on a board in the control panel, via signal lines which are not shown. The board is connected to the controller  13  shown in  FIG. 1  via a cable which is not shown. 
   The print engine  12  is a mechanism which actually performs printing on a sheet. As described later in detail, the print engine  12  can perform color printing and monochrome printing. 
   The controller  13  performs a process of controlling the driving of the print engine  12  on the basis of the print request from the host computer or camera device which is not shown, and various other processes on the basis of instructions obtained through the control panel  11 . 
   As principal components, the controller  13  comprises on a printed circuit board a network interface  131 , a USB interface  132 , a RAM  133 , a video signal generator  134 , a ROM  135 , a memory controller  136 , an interface  137 , an EEPROM  138 , and a CPU  139 . 
   The network interface  131  is a unit which receives the print request from the host computer which is not shown, and specifically a communication interface port such as a LAN board. The USB interface  132  is a unit which receives the print request from the camera device which is not shown, and specifically a communication interface port which controls data communication in accordance with the USB standard. The camera device is a device which can obtain image data of a still picture, and which is provided with a USB port, and specifically a digital still camera, a digital video camera, a camera-equipped mobile phone, or the like. 
   The RAM  133  is a memory which is used for temporarily storing the print request transmitted from the host computer or camera device which is not shown, and also for, based on the print request, producing print data to be supplied to the print engine  12 . 
   The video signal generator  134  converts the print data to be supplied to the print engine  12 , to an electric signal the form of which can be processed by a scanning unit  123  (see  FIG. 3 ) in the print engine  12 . 
   The ROM  135  is a nonvolatile memory which stores programs for controlling the printing apparatus  10 , and font data that are used when print data are produced from the print request. 
   The memory controller  136  is a circuit for controlling: DMA transfer of the print request which is received from the host computer or camera device which is not shown, via the network interface  131  or the USB interface  132 ; that of the print data in the RAM  133  to the video signal generator  134 ; and writing of the programs and data in the ROM  135  into the RAM  133 . 
   The interface  137  controls transmission and reception of various signals with respect to units in the control panel  11 , and those of various signals with respect to units in the print engine  12 . 
   The EEPROM  138  is a rewritable memory for recording various settings of printing, and various information relating to use results. The various information relating to use results includes the following information for each of toners which will be described later: the residual toner amount in a toner cartridge  21 ; an average of ratios of the number of dots which are actually printed to the total dot number of one page; and an average of the number of dots which are required in consumption of 1% of the toner amount. 
   The CPU  139  is a processor which integrally controls various portions in accordance with the programs installed in the ROM  135 , thereby conducting a print control process which causes the print engine  12  to perform printing in accordance with the print request transmitted from the host computer or camera device which is not shown, and a process in which instructions for a process to be executed are obtained from the user through an operation on the control panel  11 . 
   As shown in  FIG. 3 , the print engine  12  incorporates a photosensitive drum  121 , a charging unit  122 , the scanning unit  123 , a rotary developing unit  124 , a transfer belt unit  125 , a secondary transferring unit  126 , and a fusing unit  127 . 
   The photosensitive drum  121  is a cylindrical drum in which a photoconductive material is deposited on the surface, and which is rotated about the central axis. 
   The charging unit  122  removes (discharges) static electricity possessed by the surface of the photosensitive drum  121 , removes (cleans) toners adhering to the surface by a rubber blade or the like, and gives (charges) static electricity to the surface. 
   The scanning unit  123  scans the surface of the photosensitive drum  121  with a laser beam which is on/off modulated on the basis of the print data, to partially remove static electricity on the surface, thereby forming an electrostatic latent image. A signal for on/off modulating the laser beam is supplied from the video signal generator  134 . 
   The rotary developing unit  124  conveys toner from the toner cartridge  21  to the photosensitive drum  121  in order to cause toner to adhere to the surface of the photosensitive drum  121 . The configuration of the rotary developing unit  124  will be described later in detail. 
   The transfer belt unit  125  rotates a transfer belt which serves as an intermediate transfer medium (intermediate member), and which receives toner from the surface of the photosensitive drum  121 . When color printing using color components of CMYK is to be performed, the transfer belt unit  125  performs a primary transfer process in which a toner image is transferred from the photosensitive drum  121  to the transfer belt in a superposed manner, that is, four times of transfer operations are performed for an image of one page. 
   The secondary transferring unit  126  is a unit which, when a toner image of one page is transferred to the transfer belt of the transfer belt unit  125 , secondarily transfers the toner image to a printing sheet. The secondary transferring unit  126  comprises a clutch roller which cooperates with one roller for rotating the transfer belt to clamp the transfer roller and the sheet. When secondary transfer is not performed, the clutch roller is caused to idly rotate. 
   The fusing unit  127  heats toner attached to the sheet as a result of the secondary transfer, thereby fusing the toner to the sheet. 
   The rotary developing unit  124  incorporates a cartridge rotor  124   rt , a CSIC reader  124   rd , a C-developing roller  124   dc , an M-developing roller  124   dm , a Y-developing roller  124   dy , a K-developing roller  124   dk , and a motor driver  124   md.    
   The cartridge rotor  124   rt  has a framework structure which is substantially cylindrical, and is rotatable about the central axis. Inside the cartridge rotor  124   rt , cartridge chambers  124 C,  124 M,  124 Y,  124 K having a structure for loading the toner cartridge  21  which is substantially columnar are formed respectively at four symmetrical positions which have the central axis of the rotor as an axis of symmetry. 
   Each of the cartridge chambers  124 C,  124 M,  124 Y,  124 K comprises a sensor which, when the toner cartridge  21  is loaded, detects the residual amount of toner in the toner cartridge  21 . Information indicative of the residual toner amount detected by the sensor is transmitted from the sensor to the CPU  139  through the interface  137 . 
   The CSIC reader  124   rd  reads in a non-contact manner information recorded in a CSIC (not shown) which is installed on each of the toner cartridges  21  loaded in the cartridge chambers  124 C,  124 M,  124 Y,  124 K. The CSIC reader is placed in the vicinity of the cartridge rotor  124   rt . The CSIC which is not shown stores information related to the color of toner charged in the toner cartridge  21  on which the CSIC is installed. The information read from the CSIC which is not shown is transmitted as color identifying information from the CSIC reader  124   rd  to the CPU  139  through the interface  137 . 
   The four developing rollers  124   dc ,  124   dm ,  124   dy ,  124   dk  are devices which visualize an electrostatic latent image formed in the surface of the photosensitive drum  121 , and incorporated at symmetrical positions in the four cartridge chambers  124 C,  124 M,  124 Y,  124 K respectively. Specifically, each of the developing rollers  124   c ,  124   dm ,  124   dy ,  124   dk  provides static electricity to toner which is in the toner cartridges  21  loaded in the corresponding one of the cartridge chambers  124 C,  124 M,  124 Y,  124 K, and which is in contact with the surface of the roller, thereby causing the toner to adhere to the surface, and rotates about the central axis to transport the toner on the surface to the photosensitive drum  121 . 
   The motor driver  124   md  rotates the cartridge rotor  124   rt , and controls the rotation. Specifically, the motor driver  124   md  receives information designating one of the four developing rollers  124   dc ,  124   dm ,  124   dy ,  124   dk , from the CPU  139  through the interface  137 , and then rotates the cartridge rotor  124   rt  via a motor and gear mechanism which are not shown, while monitoring the position the cartridge rotor  124   rt  by a position sensor which is not shown, whereby the designated developing roller is located at a position in the vicinity of the photosensitive drum  121 . 
   Although not shown, the thus configured rotary developing unit  124  comprises a replacement port through which the toner cartridges  21  are removed from or inserted into the cartridge chambers  124 C,  124 M,  124 Y,  124 K. The replacement port which is not shown has a size which allows one toner cartridge  21  to be inserted into the chamber. In order to access each of the four cartridge chambers  124 C,  124 M,  124 Y,  124 K, an operation of locating the cartridge chamber at the position of the replacement port which is not shown must be conducted. The motor driver  124   md  which receives instructions from the CPU  139  rotates the cartridge rotor  124   rt  at a 90 degrees increment, thereby conducting this operation. 
   The printing apparatus  10  which is configured as described above operates in one of printing modes including a color mode, a 4-cartridge monochrome mode, and a 1-cartridge monochrome mode. The color mode is a printing mode where color printing is performed with using toners of color components of CMYK, and the 4-cartridge monochrome mode and the 1-cartridge monochrome mode are printing modes where black and white or monochrome printing is performed with using toners of a color component of K. 
   The printing mode in which the printing apparatus  10  operates depends on the color combination of toners of the toner cartridges  21  which are loaded respectively in the four cartridge chambers  124 C,  124 M,  124 Y,  124 K of the rotary developing unit  124 . 
   Specifically, when four toner cartridges  21  respectively having toners of CMYK are loaded in the four cartridge chambers  124 C,  124 M,  124 Y,  124 K, the CPU  139  of the printing apparatus  10  determines that the printing mode is switched to the color mode, sets the printing apparatus  10  to a state where the printing apparatus can operate in the color mode, and initializes information which is recorded in the EEPROM  138  as information relating to use results. 
   When four toner cartridges  21  having toners of K are loaded respectively in the four cartridge chambers  124 C,  124 M,  124 Y,  124 K, the CPU  139  determines that the printing mode is switched to the 4-cartridge monochrome mode, sets the printing apparatus  10  to a state where the printing apparatus can operate in the 4-cartridge monochrome mode, and initializes information which is recorded in the EEPROM  138  as information relating to use results. 
   When one toner cartridge  21  having a toner of K remains to be loaded in the cartridge chamber  124 K for K and the other toner cartridges  21  are removed from the cartridge chambers  124 C,  124 M,  124 Y, the CPU  139  determines that the printing mode is switched to the 1-cartridge monochrome mode, sets the printing apparatus  10  to a state where the printing apparatus can operate in the 1-cartridge monochrome mode, and initializes information which is recorded in the EEPROM  138  as information relating to use results. 
   The ROM  135  stores correspondence information which, for each of the printing modes, defines correspondences between toner color(s) used in the printing mode and the cartridge chambers. On the bases of the correspondence information, the CPU  139  determines whether toner cartridges  21  of adequate colors are loaded in the cartridge chambers  124 K,  124 C,  124 M,  124 Y or not, and switches the printing mode when adequate toner cartridges  21  are loaded. 
   In the case where the printing mode is changed to the color mode as a result of the printing mode switching process, upon receiving a print request, the CPU  139  controls the print engine  12  so as to conduct the operation of primary-transferring the toner image from the photosensitive drum  121  to the transfer belt, four times for each image of one page as described above. Namely, the CPU  139  controls the cartridge rotor  124   rt  so as to make one rotation for one printing sheet. 
   In the case where the printing mode is changed to the 1-cartridge monochrome mode, upon receiving a print request, the CPU  139  controls the print engine  12  so as to use only the cartridge chamber  124 K for K. In the 1 cartridge monochrome mode, the cartridge chambers  124 C,  124 M,  124 Y which are not for K are not used. 
   In the case where the printing mode is changed to the 4-cartridge monochrome mode, upon receiving a print request, the CPU  139  controls the print engine  12  so as to use one of the four cartridge chambers  124 K,  124 C,  124 M,  124 Y. 
   Specifically, in accordance with a predetermined program in the ROM  135 , the CPU  139  controls the print engine  12  so as to switch the cartridge chamber used in printing (the cartridge chamber placed in the vicinity of the photosensitive drum  121 ) to another one, every time when printing is performed on 15 sheets. 
   After printing is performed on 15 sheets, the CPU  139  performs a process of rotating the cartridge rotor  124   rt  by 90 degrees to switch the cartridge chamber to the next one. At this time, the CPU  139  judges whether the cartridge chamber to be switched as the next used one is in an enabled state or not, on the basis of flag information which is recorded in the EEPROM  138  as information of the cartridge chamber itself. 
   When the flag information defines that the cartridge chamber is in the enabled state, the CPU  139  causes the cartridge chamber to remain to be placed in front of the photosensitive drum  121 . By contrast, when the flag information defines that the cartridge chamber is in a disabled state, the cartridge rotor  124   rt  is again rotated by 90 degrees, thereby further performing the process of switching the cartridge chamber to the next one. 
   When the process is performed repeatedly as required, the cartridge chamber which is defined to be in the enabled state by the flag information is placed in the vicinity of the photosensitive drum  121 , and the CPU  139  is then prepared for the next print request. 
   Next, the contents displayed in the LCD  111  of the control panel  11  will be described. 
   The CPU  139  outputs information to be displayed on the LCD  111  of the control panel  11  to the control panel  11  through the interface  137 . The board (not shown) in the control panel  11  produces screen data on the basis of the information supplied from the CPU  139 , and causes the LCD  111  to always display the screen based on the screen data. 
     FIG. 4  shows an example of the display screen of the LCD  111  in the color mode. The display screen of the LCD  111  of the control panel  11  is partitioned into upper and lower two areas. The upper area is a status display area where the operation status of the printing apparatus  10  is shown by a letter string. On the other hand, the lower area is an accessory status information display area where the residual toner amounts of the toner cartridges  21 , and the size and residual amount of sheets of a sheet cassette are shown. 
   In the accessory status information display area, four vertical bar graphs are displayed in parallel, and letters “K”, “C”, “M”, and “Y” are displayed above the four bar graphs, respectively. 
   The bar graph below the letter “K” indicates the residual toner amount of the toner cartridge  21  for K which is set in the rotary developing unit  124 . The letter “K” serves as color identifying information for identifying the color component of the toner the residual amount of which is indicated by the bar graph, and also as position identifying information for identifying the cartridge chamber  124 K for K in which the toner cartridge  21  for K is set. 
   Similarly, the bar graphs below the letters “C”, “M”, and “Y” indicate the residual toner amounts of the toner cartridges  21  for C, M, and Y which are set in the rotary developing unit  124 , respectively. Each of the letters “C”, “M”, and “Y” serves as color identifying information for identifying the color component of the toner the residual amount of which is indicated by the corresponding bar graph, and also as position identifying information for identifying the cartridge chamber  124 C,  124 M, or  124 Y In which the toner cartridge  21  for C, M, or Y is set. 
   In this way, the four bar graphs which are displayed on the screen correspondingly with the letters “K”, “C”, “M”, and “Y” enable the operator to easily determine which one of the toner cartridges  21  is to be replaced with a new one. 
   The letters “K”, “C”, “M”, and “Y” are displayed on the basis of the information read by the CSIC reader  124   rd , and the bar graphs are displayed on the basis of the residual amount information read by the sensors disposed in the cartridge chambers  124 C,  124 M,  124 Y,  124 K. 
   Every time when a printing process is performed on one printing sheet, the CPU  139  measures the residual toner amounts in the toner cartridges  21  by the sensors, and updates the sizes of the bar graphs on the screen displayed in the LCD  111  of the control panel  11 . Namely, the residual toner amounts indicated by the bar graphs are immediately updated. 
   Furthermore, the CPU  139  monitors switching of the printing mode in accordance with a predetermined program in the ROM  135 . When it is detected that the printing mode is switched from the color mode or the 1-cartridge monochrome mode to the 4-cartridge monochrome mode, the CPU  139  instructs the control panel  11  to change the color identifying information and position identifying information which are displayed on the screen in the LCD  111  of the control panel  11 . 
     FIG. 5  shows an example of the display screen of the LCD  111  in the 4-cartridge monochrome mode. In the same manner as the color mode, four vertical bar graphs are displayed in the accessory status information display area in the screen displayed in the LCD  111  of the control panel  11 . However, the letter “K” is indicated above each of the four bar graphs, and the letters “K”, “C”, “M”, and “Y” are indicated between the letters “K” and the bar graphs. 
   The bar graph designated by “K” and “K” indicates the residual toner amount of the toner cartridge  21  for K which is set in the cartridge chamber  124 K for K of the rotary developing unit  124 . The upper letter “K” serves as color identifying information for identifying the color component of the toner the residual amount of which is indicated by the bar graph, and the lower letter “K” serves as position identifying information for identifying the cartridge chamber  124 K for K in which the toner cartridge  21  for K is set. 
   The bar graph designated by “K” and “C” indicates the residual toner amount of the toner cartridge  21  for K which is set in the cartridge chamber  124 C for C of the rotary developing unit  124 . The upper letter “K” serves as color identifying information for identifying the color component of the toner the residual amount of which is indicated by the bar graph, and the lower letter “C” serves as position identifying information for identifying the cartridge chamber  124 C for C in which the toner cartridge  21  for K is set. 
   Similarly, the bar graph designated by “K” and “M” and that designated by “K” and “Y” indicate the residual toner amounts of the toner cartridges  21  for K which are set in the cartridge chambers  124 M,  124 Y for M and Y of the rotary developing unit  124 , respectively. The upper letters “K” serve as color identifying information for identifying the color component of the toners the residual amounts of which are indicated by the bar graphs, and the lower letters “M” and “Y” serve as position identifying information for identifying the cartridge chambers  124 M,  124 Y for M and Y in which the toner cartridges  21  for K are set. 
   In this way, the four bar graphs which are displayed on the screen correspondingly with the letters “KK”, “KC”, “KM”, and “KY” enable the operator to easily determine which one of the toner cartridges  21  is to be replaced with a new one. 
   In the 4-cartridge monochrome mode, a further one bar graph is displayed adjacent to the four bar graphs in the accessory status information display area. The bar graph indicates the average of the residual toner amounts of the four toner cartridges  21 , so that it is possible to know the whole residual toner amount of the four toner cartridges  21 . Letters “A” and “V” are displayed above the bar graph indicating the average of the residual toner amounts, and “AV” shows that the bar graph indicates the average of the residual toner amounts. 
   When it is detected that the printing mode is switched from the color mode or the 4-cartridge monochrome mode to the 1-cartridge monochrome mode, the CPU  139  instructs the control panel  11  to change the color identifying information and position identifying information which are displayed on the screen in the LCD  111  of the control panel  11 . 
     FIG. 6  shows an example of the display screen of the LCD  111  in the 1-cartridge monochrome mode. Unlike in the color mode and the 4-cartridge monochrome mode, only one vertical bar graph is displayed in the accessory status information display area in the screen displayed in the LCD  111  of the control panel  11 . The letter “K” is indicated above the bar graph. 
   The one bar graph indicates the residual toner amount of the toner cartridge  21  for K which is set in the rotary developing unit  124 . The letter “K” serves as color identifying information for identifying the color component of the toner the residual amount of which is indicated by the bar graph, and also as position identifying information for identifying the cartridge chamber  124 K for K in which the toner cartridge  21  for K is set. 
   In this way, the one bar graph which is displayed on the screen correspondingly with the letter “K” enables the operator to easily determine whether the toner cartridges  21  is to be replaced with a new one. 
   As described above, according to the printing apparatus  10  of the embodiment, even when the combination of toners charged in the toner cartridges  21  which are set in the rotary developing unit  124  is changed, respective residual toner amounts of the toner cartridges  21  used in the printing mode are displayed in the form of bar graphs on the LCD  111  of the control panel  11 . Moreover, an alphabetical letter shows the residual toner amount of the color which is indicated by each of the bar graphs, and the cartridge chamber to which the toner cartridge  21  the residual toner amount of which is indicated is set. Therefore, the operator is not confused. 
   In the above-described embodiment, the alphabetical characters “K”, “C”, “M”, and “Y” are used as the color identifying information and the position identifying information which are to be displayed on the LCD  111 . However, symbols indicating such colors may be used. Alternatively, the bar graphs themselves are colored in KCMY, respectively, so that the color identifying information and the position identifying information are indicated by the colors of the bar graphs. 
   Next, a second embodiment of the invention will be described. Similar components to those in the first embodiment will be designated by the same reference numerals and repetitive explanations for those will be omitted. 
   In the embodiment, the cartridge chambers  124 K,  124 C,  124 M,  124 Y are not provided with a sensor which directly detects the residual toner amount in the corresponding toner cartridge  21 . 
   As described above in connection with the first embodiment, as information relating to use results, the EEPROM  138  stores: an average of ratios of the number of dots which are actually printed to the total dot number of one page; an average of the number of dots which are required in consumption of 1% of the toner amount; and the residual toner amounts in the toner cartridges  21  which are set in the rotary developing unit  124 . These sets of information are calculated by the CPU  139  in accordance with a predetermined program in the ROM  135 , every time when a printing process is performed on one printing sheet. Particularly, the information related to the residual toner amounts in the toner cartridges  21  is obtained on the basis of the toner use amount calculated from the number of dots which are actually printed, and is not obtained by direct detection of the actual residual toner amounts in the toner cartridges  21  by a sensor or the like. 
   In accordance with another program in the ROM  135  different form the above-mentioned program, the CPU  139  monitors the residual toner amounts obtained from the actually printed dot number. When the residual toner amount of one of the toner cartridges  21  falls below a predetermined threshold, the CPU starts the process described below. 
   From the ROM  135  which stores image data respectively corresponding to the cartridge chambers  124 K,  124 C,  124 M,  124 Y, the CPU  139  reads the image data into the RAM  133 . Thereafter, the CPU causes the cartridge chamber  124 K for K to be placed in front of the photosensitive drum  121 , and the image data corresponding to the cartridge chamber  124 K for K to be supplied from the RAM  133  to the video signal generator  134 . When the toner cartridge  21  is loaded in the cartridge chamber  124 K for K at this time, a toner image is formed on the photosensitive drum, and the toner image is primary-transferred to the transfer belt. 
   Then, the CPU  139  causes the cartridge chamber  124 C for C to be placed in front of the photosensitive drum  121 , and the image data corresponding to the cartridge chamber  124 C for C to be supplied from the RAM  133  to the video signal generator  134 . When the toner cartridge  21  is loaded in the cartridge chamber  124 C for C at this time, a toner image is formed on the photosensitive drum, and the toner image is primary-transferred to the transfer belt. 
   Furthermore, the CPU  139  executes a process similar to the above on the cartridge chamber  124 M for M and the cartridge chamber  124 Y for Y. 
   In the print engine  12 , the toner images transferred to the transfer belt are secondary-transferred to a printing sheet by the secondary transferring unit  126 , and then fusion-bonded to the printing sheet by the fusing unit  127 . As a result, the printing sheet on which the test pattern image is printed is discharged from the printing apparatus  10 . 
     FIG. 7  shows an example of the test pattern image which is printed on a printing sheet by the print engine  12  in the color mode or the 4-cartridge monochrome mode. It is assumed that the vertical direction of this figure coincides with that of the test pattern image. In each of the vicinity of the left edge, the middle portion, and the vicinity of the right edge of the test pattern image, a combination of four linear reference images and letters “K”, “C”, “M”, and “Y” is placed. 
   Among the four linear reference images, the first reference image is an image colored only by the same color as the toner of the toner cartridge  21  loaded in the cartridge chamber  124 K Above the reference image, letter “K” is placed as identifying information for identifying the cartridge chamber  124 K in which the toner cartridge  21  is loaded. The letter “K” and the reference image corresponding to the letter are reproduced on the basis of the image data which are recorded in the ROM  135  as data corresponding to the cartridge chamber  124 K for K. 
   The second reference image is an image colored only by the same color as the toner of the toner cartridge  21  loaded in the cartridge chamber  124 C. Above the reference image, letter “C” is placed as identifying information for identifying the cartridge chamber  124 C in which the toner cartridge  21  is loaded. The letter “C” and the reference image corresponding to the letter are reproduced on the basis of the image data which are recorded in the ROM  135  as data corresponding to the cartridge chamber  124 C for C. 
   Similarly, each of the third and fourth reference images is an image colored only by the same color as the toner of the toner cartridge  21  loaded in the cartridge chamber  124 M or  124 Y Letters “M” and “Y” are placed above the reference images, respectively. The letter “M” and the reference image corresponding to the letter are reproduced on the basis of the image data which are recorded in the ROM  135  as data corresponding to the cartridge chamber  124 M for M. The letter “Y” and the reference image corresponding to the letter are reproduced on the basis of the image data which are recorded in the ROM  135  as data corresponding to the cartridge chamber  124 Y for Y. 
   Since the test pattern image is configured in this way, each of the four linear reference images is printed with using toner of the toner cartridge  21  loaded in the corresponding cartridge chamber. When the printing mode is the color mode, therefore, four lines respectively colored in KCMY are drawn in each of the vicinity of the left edge, the middle portion, and the vicinity of the right edge of the printing sheet. When the printing mode is the 4-cartridge monochrome mode, four lines colored in K are drawn in each of the vicinity of the left edge, the middle portion, and the vicinity of the right edge of the printing sheet. 
   For example, the case where the residual toner amount of the toner cartridge  21  loaded in the cartridge chamber  124 C for C is actually reduced in the 4-cartridge monochrome mode will be considered. As shown in  FIG. 8 , on the printing sheet on which the test pattern image is printed, only the density of the reference image corresponding to letter “C” is lowered, and the reference images corresponding to letters “K”, “M”, and “Y” remain to have the normal density. 
   On the other hand, even in the case where the residual toner amounts recorded in the EEPROM  138  fall below the above-mentioned predetermined threshold, when the actual residual toner amounts are large, all the reference images in the test pattern image printed on the printing sheet remain to have the normal density. 
   The printing apparatus  10  of the embodiment serves as described above. Therefore, the user can refer the test pattern image on the printing sheet which is automatically printed out by the printing apparatus  10  when the residual toner amount falls below the predetermined threshold, whereby the user is enabled to easily determine the presence or absence of a toner cartridge  21  in which the residual toner amount is actually reduced, and the cartridge chamber in the rotary developing unit  124  in which the residual toner amount-reduced toner cartridge  21  is loaded. 
   Namely, when the user finds a reference image in which the density is lowered in the test pattern image on the printing sheet, the user can know that the toner amount of the toner cartridge  21  in the cartridge chamber indicated by the letter corresponding to the reference image in which the density is lowered is reduced. 
   Although it is not specifically shown in the accompanying drawings, in the 1-cartridge monochrome mode, three linear reference images colored in K are printed in the vicinity of the left edge, the middle portion, and the vicinity of the right edge of a printing sheet, respectively. This is produced because of the following reason. Since the toner cartridges  21  are not loaded in the cartridge chambers  124 C,  124 ,  124 Y for C, M, and Y, the reference images and chamber identifying information based on the image data corresponding to the cartridge chambers  124 C,  124 M,  124 Y are not visualized. Also in the 1-cartridge monochrome mode, when a part or whole of the reference images on the printing sheet is blurred or discolored, the user can know that the toner amount of the toner cartridge  21  in the cartridge chamber  124 K for K is reduced. 
   In this embodiment, when, in the residual toner amounts recorded in the EEPROM  138 , the residual toner amount of one of the toner cartridges  21  falls below the predetermined threshold, the test pattern image is printed. This is because the toner is wastefully used if the test pattern image is printed while a sufficient amount of toner remains. 
   In the embodiment, when one of the residual toner amounts recorded in the EEPROM  138  falls below the predetermined threshold, the test pattern image is printed. However, a sensor for detecting the residual toner amount in the toner cartridge  21  may be attached to each of the cartridge chambers  124 K,  124 C,  124 M,  124 Y, and the test pattern image may be printed when one of the residual toner amounts detected by the sensors falls below a predetermined threshold. In this case, even when the accuracies of the residual toner amount detectors are somewhat poor, the user can easily determine whether the toner cartridge  21  in which the amount of toner is actually reduced, based on the test pattern image on a printing sheet. 
   In the embodiment, as shown in  FIGS. 7 and 8 , the combination of reference images and letters is printed in each of the three places or the vicinity of the left edge, the middle portion, and the vicinity of the right edge of the printing sheet, because it can be determined whether the distribution of toner in the toner cartridge  21  is even or not. Namely, when the density of the reference image(s) of the same cartridge chamber in one or two of the three places is lowered and that of the reference image(s) of the other place(s) remains unchanged, the user can determine that the distribution of toner in the toner cartridge  21  which is loaded in the cartridge chamber is uneven. 
   In the embodiment, alphabetical letters “K”, “C”, “M”, and “Y” are correspondent to the reference images as the chamber identifying information for identifying the cartridge chambers  124 K,  124 C,  124 M,  124 Y. However, these letters are not necessary because of the following reasons. In the color mode, the colors themselves which color the reference images function as the chamber identifying information for identifying the cartridge chambers, In the 4-cartridge monochrome mode, when the arrangement order of the reference images and the order of the cartridge chambers are once defined, the arrangement order of the reference images functions as the chamber identifying information for identifying the cartridge chambers. 
   The user recognizes the cartridge chamber where the toner cartridge  21  in which the residual toner amount is actually reduced is loaded, as a result of the printing of the test pattern image. In order to prevent the toner cartridge  21  in the cartridge chamber from being used in printing, thereafter, the user sets the cartridge chamber to be disabled from being used. 
   Specifically, in the example of  FIG. 8 , the user operates the control panel  11  to change the flag information of the cartridge chamber  1240  for C from the enabled state to the disabled state. The changing operation causes the flag information which is recorded in the EEPROM  138  as information of the cartridge chamber  124 C for C, to be overwritten from “1” indicative of the enabled state to “0” indicative of the disabled state. 
   As a result, in the 4-cartridge monochrome mode, the group of cartridge chambers which can be switched every time when printing is performed on 15 sheets is configured by the three cartridge chambers  124 K,  124 M,  124 Y for K, M, and Y. The cartridge chamber  124 C for C is eliminated from the group of cartridge chambers. 
   In this embodiment, three cartridge chambers can be set at the maximum to the disabled state. When three cartridge chambers are set to the disabled state, only the toner cartridge  21  loaded in the remaining single cartridge chamber supplies toner of K In this case, namely, the embodiment operates in the same manner as in the 1-cartridge monochrome mode. 
   As described above, according to the printing apparatus  10  of the embodiment, in the 4-cartridge monochrome mode in which toner of the same color is loaded in the cartridge chambers  124 K,  124 C,  124 M,  124 Y, the enabled cartridge chamber(s) and the disabled cartridge chamber(s) are defined by the flag information in the EEPROM  138 , and the flag information can be changed by the user. Therefore, the user can cause the test pattern image to be printed, check a cartridge chamber where a toner cartridge in which the residual toner amount is reduced is loaded, and thereafter set the cartridge chamber to the disabled state. Even when a residual toner amount detector is not installed, therefore, toner in only a toner cartridge in which toner remains can be used in printing. 
   The above-described printing apparatus may be configured so that, when replacement of the toner cartridge  21  is conducted on the cartridge chamber which is defined to be in the disabled state by the flag information in the EEPROM  138 , the contents of the flag information corresponding to the cartridge chamber is reset to the enabled state. 
   Specifically, with using a sensor (not shown) for detecting opening and closing of an outer cover which covers the replacement port (not shown), and the CSIC reader  124   rd , the CPU  139  of the printing apparatus  10  monitors whether replacement of the toner cartridge  21  is performed on one of the cartridge chambers. When replacement of the toner cartridge  21  is performed on one of the cartridge chambers, the CPU determines whether the flag information which is recorded in the EEPROM  138  as information corresponding to the cartridge chamber defines the disabled state or not. If the flag information defines the disabled state, the CPU  139  resets the flag information by overwriting the contents to those indicative of the enabled state. 
   With this configuration, when the toner cartridge  21  is replaced because the residual toner amount is reduced, it is possible to save the trouble of manually (on the control panel  11 ) resetting to the enabled state the contents of the flag information relating to the cartridge chamber on which the replacement of the toner cartridge  21  is performed. 
   In the above embodiments, the developing sections are incorporated in a rotor so that one of the developing sections is selectively confronted with the photosensitive drum. However, the printing apparatus of the invention may be configured such that a plurality of developing sections may be arranged around a single photosensitive drum so that developed toner images are superposed on the photosensitive drum. Alternatively, the printing apparatus may be configured such that a plurality of developing sections are disposed on respectively corresponding photosensitive drums to configure a so-called tandem print engine. 
   Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.