Image forming apparatus with removable developing means
This application is a continuation of U.S. Ser. No. 07/243,481, filed Sep. 12, 1988, now U.S. Pat. No. 4,956,673, which in turn is a continuation of U.S. Ser. No. 07/029,354, filed Mar. 23, 1987, now U.S. Pat. No. 4,814,823, which in turn is a continuation of U.S. Ser. No. 06/727,859, filed Apr. 26, 1985, and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus capable of developing a copy image on a copy paper sheet, in a single cycle of its copying operation, using a developing agent of any desired color, and more specifically relates to an imaging-forming apparatus, such as a color copying machine, in which developing agents of different colors are used individually in a plurality of cycles of its copying operation, repeated for a single sheet of copy paper, thereby permitting multicolor copying. 
     Development of color versions has recently been promoted in the field of copying machines. For example, two-color copying machines have been developed for practical use, which can produce color images enjoying another color as well as black. In the conventional color copying machines of this type, however, developing agents of different colors are used individually in a plurality of cartridges which each include a developing unit and a photosensitive member in integral relation. A plurality of color copying operations are performed by alternatively replacing the cartridges with one another. Thus, the prior art color copying machines require a troublesome series of operations. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in consideration of these circumstances, and is intended to provide an image-forming apparatus which, in a selected image-forming mode, can form an image with a plurality of colors (including a standard color), without replace cartridges including development units. 
     According to one aspect of the present invention, there is provided an image-forming apparatus which comprises a housing; an image carrier located in the housing and adapted to carry thereon a latent image corresponding to an original image-developing means for developing the latent image formed on the surface of the image carrier, the developing means including first developing means for developing the latent image by means of a first developing agent, and second developing means for developing the latent image by means of a second developing agent-and means for alternatively driving the first and second developing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view schematically showing a color copying machine, as one embodiment of an image-forming apparatus according to the present invention; 
     FIG. 2 is a vertical sectional view schematically showing a developing device; 
     FIG. 3 is a vertical sectional view schematically showing the developing device of FIG. 2 in a different mode; 
     FIGS. 4, 5 and 6 show the arrangements of driving force transmission systems in which FIG. 4 is a front view showing engagement between a group of gears; FIG. 5 is a sectional view showing the group of gears; and FIG. 6 is a sectional view showing the way a driving force is transmitted to a driving gear; 
     FIG. 7 is a plan view, partially in section, showing a magnet roll drive mechanism; 
     FIG. 8 is a side view of the magnet roll drive mechanism; 
     FIGS. 9A and 9B are side views showing the magnet roll drive mechanism in different positions; 
     FIG. 10 is a perspective view schematically showing the developing device; 
     FIGS. 11A and 11B are top views individually showing a pair of developing units; 
     FIG. 12 is a front view showing means for affixing the developing device; 
     FIGS. 13A, 13B and 13C are front views showing a handle and a hook in different relative positions; 
     FIG. 14 is a plan view of a control panel; 
     FIG. 15 is a block diagram schematically showing a general control circuit; 
     FIG. 16 is a diagram for illustrating developing unit presence signal generating mechanisms and discriminating information generating mechanisms; 
     FIGS. 17A to 17F are flow charts for illustrating an operation mode at the start of power supply; 
     FIGS. 18A to 18D are flow charts for illustrating a copy mode; and 
     FIGS. 19A and 19B are flow charts for illustrating an automatic interruption mode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of an image-forming apparatus according to the present invention, applied to an electronic copying machine, will now be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a two-color copying machine, as an example of the image-forming apparatus of the present invention. In FIG. 1, numeral 1 designates a copying machine housing. The housing 1 carries thereon an original table 2 which can reciprocate in the longitudinal direction of the housing 1 as indicated by arrow a. A paper cassette 3, storing stack of copy paper P1 and a receiving tray 4 are removably attached to the right- and left-hand side portions of the housing 1, respectively. A cassette cover 5 is provided on the top of the paper cassette 3. The top surface of the cassette cover 5 serves as a sheet-bypass guide 6 for copy paper sheets P2, to be manually supplied as required. A photoconductive drum 7 is located in the substantially central portion of the housing 1, so as to be rotatable in clockwise direction a as indicated by arrow b. The photoconductive drum 7 is surrounded by a main charger 8, an optical system 9, a two-color developing device 10 (described in detail later), a transfer charger 11, a separation charger 12, a cleaning unit 13, and a discharge lamp 14, which are arranged successively in the rotational direction of the drum 7. At the lower portion of the housing 1 extends a paper conveying path 17, along which a paper sheet P1 (automatically delivered from the paper cassette 3 by a paper-supply roller 15) or a paper sheet P2 manually) supplied through the sheet-bypass guide 6) is fed and discharged into the receiving tray 4, through an image transfer section 16 defined between the photoconductive drum 7 and the transfer charger 11. A pair of aligning rollers 18 are arranged in the paper conveying path 17, on the upper-course side of the image transfer section 16 with respect to the direction of paper feed, and a pair of heat rollers 19, constituting a fixing unit, and a pair of exit rollers 20 are arranged on the lower-course side. 
     The optical system 9 includes an exposure lamp 22 backed by a reflector 21, mirrors 23, 24, 25, and 26, and a lens 27. 
     The photoconductive drum 7 is driven, in the direction indicated by arrow b, by a drive mechanism (not shown), in synchronism with the reciprocation of the original table 2. In operation, the surface of the photoconductive drum 7 is first charged uniformly by the main charger 8. Then, an original is uniformly irradiated by the exposure lamp 22 and the reflected light from the original is projected on the photoconductive drum 7 by the optical system 9, forming an electrostatic latent image thereupon. The electrostatic latent image formed in this manner is developed into a visible image by the developing device 10, and placed opposite the transfer charger 11. 
     The automatically or manually supplied sheet P1 or P2 is fed into the image transfer section 16 by the aligning rollers 18, timed to the formation of the visible image. In the image transfer section 16, the toner image previously formed on the photoconductive drum 7 is transferred to the surface of the sheet P1 or P2, by the transfer charger 11. Then, the sheet P1 (P2) with the transferred toner image thereon is separated from the photoconductive drum 7 by the separation charger 12, and delivered to the heat rollers 19, through the paper conveying path 17. After the transferred image is melted and fixed on the sheet P1 (P2) by the heat rollers 19, the sheet P1 (P2) is discharged into the receiving tray 4 by the exit rollers 20. 
     After the toner image is transferred to the surface of the sheet P1 (P2), the residual developing agent remaining on the photoconductive drum 7 is removed by the cleaning unit 13, and a any residual image on the photoconductive drum 7 is erased by the discharge lamp 14, so as to be ready for the next cycle of the copying operation. 
     Inside the housing 1, upper and lower frames (not shown) are pivotally mounted at one end portion, on a supporting shaft (not shown). With this arrangement, the other end portions of the two frames can be swung apart at a desired angle i.e., up to about 30 degrees from each other. 
     The upper frame is fitted by suitable means, with the photoconductive drum 7 and the other devices surrounding the same, including the main charger 8, the optical system 9, the exposure lamp 22, the developing device 10, the cleaning unit 13, and the discharge lamp 14. The upper frame is further mounted with the original table 2 and the paper-supply roller 15, thus constituting an upper unit 1a. On the other hand, the lower frame is fitted by suitable means with the paper cassette 3, the transfer charger 11, the separation charger 12, the heat rollers 19, the exit rollers 20, the receiving tray 4, and a main motor 28, thus constituting a lower unit 1b. Therefore, the paper conveying path 17 can be substantially fully exposed by removing a front cover (not shown) and then swinging the upper unit 1a upward from the lower unit 1b, by releasing a housing lock device (not shown). 
     The cleaning unit 13 is provided with a blade solenoid 29 for causing a cleaning blade 30 to make contact with and withdraw from the photoconductive drum 7. 
     The construction of the developing device 10 will now be described in detail. 
     As shown in detail in FIG. 2, the developing device 10 includes first and second developing rollers 31a and 31b, which are alternatively driven for regular developing using black as a standard color, or color developing using a color other than black, such as red, yellow, blue, or green. 
     The developing device 10 is vertically divided into two parts, a first developing unit 32a including the first developing roller 31a, and a second developing unit 32b including the second developing roller 31b. Both the first and second developing units 32a and 32b can be removably set in the housing 1. In this embodiment, the first or upper developing unit 32a is constructed so that a user can easily draw it out upward from the housing 1, while the second or lower developing unit 32b is designed for a serviceman&#39;s lateral attachment for detachment for maintenance or inspection. A black developing agent G2, in greater demand, is used in the second developing unit 32b, and a color developing agent G1 in the first developing unit 32a. 
     The first developing unit 32a comprises a developing mechanism section 33a and a developing agent supply section 34a. The developing mechanism section 33a includes the first developing roller 31a; a doctor 37a located on the upper-course side of a developing position 36a or a sliding contact region between the photoconductive drum 7 and a magnetic brush 35a of the color developing agent G1 formed on the surface of the developing roller 31a, whereby the thickness of the magnetic brush 35a is regulated; a scraper 39a located on the lower-course side of the developing position 36a and adapted to scrape off the magnetic brush 35a on the surface of the developing roller 31a and feed it into a developing agent storage portion 38a; a developing agent stirrer 40a in the developing agent storage portion 38a, and a casing 41a containing all these members. A developing agent density detector 42a is attached to that portion of the casing 41a which corresponds to the upper portion of the developing roller 31a. The density detector 42a detects the density of the color developing agent G1, by magnetically sensing a change in the permeability of the developing agent G1. 
     The developing roller 31a is formed of a magnet roll 43a, whose center lies on a straight line L2 which passes through the center of rotation of the photoconductive drum 7, and is inclined at an angle α (about 51 degrees) to a horizontal line L1, and sleeve 44a, fitted on the outer peripheral surface of the magnet roll 43a and rotating in the clockwise direction of FIG. 2. The magnet roll 43a includes first to fifth polar blocks 45a, 46a, 47a, 48a and 49a. The first, third and fifth polar blocks 45a, 47a and 49a are north poles, while the second and fourth polar blocks 46a and 48a are south poles. The angle θ 1  between the first and second polar blocks 45a and 46a is about 50 degrees, angle θ 2  between 46a and 47a is about 71 degrees, angle θ 3  between 47a and 48a is about 60 degrees, and angle θ 4  between 48a and 49b is about 60 degrees. 
     The developing agent supply section 34a includes a hopper 51a with a developing agent supply port 50a, facing the developing agent storage portion 38a of the developing mechanism section 33a; a developing agent supply roller 52a in the hopper 51a located so as to close the developing agent supply port 50a; and a pair of stirring rollers 53a for stirring the developing agent G1 in the hopper 51a so that the developing agent G1 is fed to the developing agent supply roller 52a. 
     The second developing unit 32b has substantially the same basic construction as the first developing unit 32a described above. The former differs from the latter in the shape of a hopper 51b of a developing agent supply section 34b, the arrangement of poles of a magnet roll 43b of the developing roller 31b, the attachment position of a developing agent density detector 42b, and the addition of a narrow scraper 54 (e.g., 50 mm thick) with a tilt of about 20 degrees to cope with the shift of the detector 42b. In the description to follow, like reference numbers, but with a suffix &#34;b&#34; each in place of &#34;a&#34;, are used to designate like members as included in the first developing unit 32a, and a detailed description of these numbers is therefore omitted. 
     The magnet roll 43b of the developing roller 31b includes first to fourth polar blocks 45b, 46b, 47b and 48b. The first and third polar blocks 45b and 47b are north poles, while the second and fourth polar blocks 46b and 48b are south poles. The angle θ 5  between the first and second polar blocks 45b and 46b is about 70 degrees, angle θ 6  between 46b and 47b is about 70 degrees, and angle θ 7  between 47b and 48b is about 80 degrees. The center of the magnet roll 43b lies on a straight line L3 which passes through the center of rotation of the photoconductive drum 7, and is inclined at an angle β (about 1 degree) to the horizontal line L1. 
     The respective magnet rolls 43a and 43b of the first and second developing units 32a and 32b are allowed to rock through about 25 degrees each. As the magnet rolls 43a and 43b rock within this range, magnetic brushes 35a and 35b of the developing agents can be formed on the surfaces of the developing rollers 31a and 31b, respectively, or removed therefrom. When the magnet rolls 43a and 43b (of the first and second developing units 32a and 32b) are shifted to their prescribed positions by a magnet roll drive mechanism (mentioned later), the magnetic brush 35a and 35b is formed on the surface of only one of the developing rollers 31a and 31b (of the first and second developing units 32a and 32b). 
     In operating the first developing unit 32a, the magnet roll 43a (of the first developing unit 32a) is set in a position such that the third polar block 47a faces the developing position 36a, and that the doctor 37a is located substantially halfway between the first and second polar blocks 45a and 46b, while the magnet roll 43b(of the second developing unit 32b) is set so that the first polar block 45b faces a doctor 37b, as shown in FIG. 2. Thus, the magnetic brush 35a is formed only on the surface of the developing roller 31a of the first developing unit 32a. 
     In operating the second developing unit 32b, on the other hand, the magnet roll 43a(of the first developing unit 32a) is rocked clockwise through about 25 degrees from the position of FIG. 2, so that the first polar block 45a faces the doctor 37a, and the magnet roll 43b (of the second developing unit 32b) is rocked counterclockwise through about 25 degrees from the position of FIG. 2, so that the doctor 37b is located substantially halfway between the first and second polar blocks 45b and 46b, as shown in FIG. 3. Thus, the magnetic brush 35b is formed only on the surface of the developing roller 31bof the second developing unit 32b. 
     When the first polar block 45a (45b) of the magnet roll 43a (43b) is opposed to the doctor 37a (37b) (formed from a non-magnetic material) the magnetic brush 35a (35b) ceases to be formed on the surface of the developing roller 31a (31b) for the following reason. The magnetic brush on that portion of the surface of the sleeve 44a (44b), corresponding to the first polar block 45a (45b), is so sparse that it cannot positively attract the developing agent G1 (G2). Therefore, the formation of the magnetic brush 35a (35b) can easily be controlled by the doctor 37a (37b). Thus, by opposing the first polar block 45a (45b) to the doctor 37a (37b), the developing agent G1 (G2) is prevented from passing, by the doctor 37a (37b), even though the sleeve 44a (44b) rotates. 
     Driving forces for the moving parts of the first developing unit 32a, including the sleeve 44a of the developing roller 31a, the developing agent stirrer 40a, and the developing agent supply roller 52a, are transmitted by means of a first driving force transmission system 61, which will be described in detail later. Driving forces for the moving parts of the second developing unit 32b, including the sleeve 44b of the developing roller 31b and the developing agent stirrer 40b, are transmitted by means of a second driving force transmission system 62 mentioned later. The first and second driving force transmission systems 61 and 62, as shown in FIGS. 4 to 6, are adapted to alternatively actuate the drive system for one of the first and second developing units 32a and 32b when a reversible motor 63, as a common drive source, is rotated forwardly or reversely. 
     Referring now to FIGS. 4 to 6, the driving force transmission systems 61 and 62 will be described in detail. The first driving force transmission system 61 includes an intermediate gear 65 in mesh with a driving gear 64, which is driven by the reversible motor 63; a first driven gear 66 in mesh with the intermediate gear 65; an intermediate gear 67 in mesh with the driven gear 66; and a second driven gear 68 in mesh with the intermediate gear 67. The second driving force transmission system 62 includes a third driven gear 69 in mesh with the driving gear 64; an intermediate gear 70 in mesh with the driven gear 69; and a fourth driven gear 71 in mesh with the intermediate gear 70. 
     When the driving gear 64 is rotated forwardly or in the counterclockwise direction (indicated by full-line arrow A in FIG. 4) by the reversible motor 63, the individual gears 65, 66, 67 and 68 of the first driving force transmission system 61, and the gears 69, 70, and 71 of the second driving force transmission system 62, rotate in the directions indicated by the individual full-line arrows. On the other hand, when the driving gear 64 is rotated reversely or in a clockwise direction (indicated by broken-line arrow B) by the reversible motor 63, the gears 65 to 68 (of the first driving force transmission system 61) and the gears 69 to 71 (of the second driving force transmission system 62) rotate in the directions indicated by the individual broken-line arrows. 
     The first and second driven gears 66 and 68 are mounted on a driving shaft 72a, integral with the sleeve 44a, and a driving shaft 73a of the developing agent stirrer 40a, respectively, by means of their corresponding one-way clutches 74. The one-way clutches 74 transmit the driving force to the driving shafts 72a and 73a to rotate the same in the direction indicated by the chain-line arrows, only when the gears 66 and 68, in the direction indicated by the full-line arrows, that is, when the driving gear 64 rotates forwardly. 
     The third and fourth driven gears 69 and 71 are mounted on a driving shaft 72b, integral with the sleeve 44b of the second developing roller 31b, and a driving shaft 73b of the developing agent stirrer 40b, respectively, by means of their corresponding one-way clutches 75. The one-way clutches 75 transmit the driving force to the driving shafts 72b and 73b to rotate the same in the direction indicated by the chain-line arrows, only when the gears 69 and 71 rotate in the direction indicated by the broken-line arrows, that is, when the driving gear 64 rotates reversely. 
     As shown in FIG. 6, the driving gear 64 is fixedly mounted on a rotating shaft 79 which is rotatably supported by bearings 76, and linked to a driving shaft 78 of the reversible motor 63, by means of a gear mechanism 77. More specifically, a gear 80 is mounted on the rotating shaft 79. The gear 80 is in mesh with a gear 85 which is rotatably mounted, by means of a pair of bearings 84, on a supporting shaft 83 stretched between a motor mounting frame 81 and a stay 82 integral therewith. The gear 85 is integrally formed with a larger gear 86. The gear 86 is in mesh with a gear 87, which is mounted on the driving shaft 78 of the reversible motor 63. Thus, both the forward and reverse rotations of the driving shaft 78 (of the reversible motor 63) are transmitted to the rotating shaft 79, and hence to the driving gear 64, through the gears 87, 86, 85 and 80, so that the first and second developing units 32a and 32b can be alternatively actuated by only changing the rotating direction of the reversible motor 63. 
     The change of the rotational direction of the reversible motor 63 is accomplished by depressing a color selection key 156 on a control panel, which will be described in detail later. At the same time, the magnet roll 43a or 43b which is not engaged in operation is shifted by the magnet roll drive mechanism (described in detail later), so that its first polar block 45a or 45b faces its corresponding doctor 37a or 37b. 
     The intermediate gear 65, in mesh with the driving gear 64 and the first driven gear 66, is rotatably mounted on a supporting shaft 89, by means of a bearing 88. The supporting shaft 89 is attached to an arm 91, swingable around a housing 90 for holding the bearings 76, which support the rotating shaft 79. Thus, the intermediate gear 65 can shift its position, so that it may securely engage the driving gear 64 and the first driven gear 66. 
     One stirring roller 53a of the first developing unit 32a is intermittently driven by a first actuating mechanism 92a shown in FIG. 4. The other stirring roller 53a and the developing agent supply roller 52a are simultaneously driven in association with the rotation of the one stirring roller 53a. A ratchet wheel 94a is mounted on one end of a shaft 93a, of the one stirring roller 53a, of the first developing unit 32a. The ratchet wheel 94a is intermittently rotated by regular angles, regulated by a pawl 97a attached to a swinging arm 96a, which rocks as a solenoid 95a is energized or deenergized. Sprocket wheels (not shown) are fitted individually on the respective shafts 93a and 98 of the pair of stirring rollers 53a, and on a shaft 99a of the developing agent supply roller 52a. A driving chain (not shown) is stretched between these sprockets, thereby constituting a driving force transmission system (not shown). The stirring rollers 53a and the developing agent supply roller 52a are driven together by this driving force transmission system. 
     A stirring roller 53b and a developing agent supply roller 52b, of the second developing unit 32b, are simultaneously intermittently driven by a second actuating mechanism 92b with the same construction as the first actuating mechanism 92a, as well as by another driving force transmission system constructed in substantially the same manner as the aforesaid one. In FIG. 4, like reference numerals, but with a suffix &#34;b&#34; each in place of &#34;a&#34;, are used to designate like members of the second actuating mechanism 92b, as included in the first actuating mechanism 92a. 
     Referring now to FIGS. 7, 8, 9A and 9B, the construction of a magnet roll drive mechanism 100a will be described, which rotates the magnet roll 43a, to form on or remove the magnetic brush 35a from the surface of the developing roller 31a. 
     One end of a shaft 101a of the magnet roll 43a is supported by a bearing 103a, which is attached to a frame 102. A lever 104a is attached to the extreme end of the shaft 101a. The distal end of the lever 104a is fitted in an engaging groove 107a of an arm 106a, which is rockably mounted on a supporting shaft 105a. A support portion 108a formed at the lower portion of the arm 106a, on its pivotal end side, is coupled to a plunger 110a of a solenoid 109a. One end of a tension spring 112a is coupled to a support portion 111a, which is formed at the upper portion of the arm 106a on the pivotal end side. 
     In the arrangement described above, when the solenoid 109a is off, the arm 106a is caused, by the urging force of the tension spring 112a, to hold the lever 104a in a position indicated by two-dot chain lines in FIG. 8, in which the doctor 37a faces the first polar block 45a, as shown in FIG. 9A. Thus, when the solenoid 109a is off, the magnetic brush 35a cannot be formed on the surface of the developing roller 31a. When the solenoid 109a is on, on the other hand, the arm 106a causes the lever 104a to rock against the urging force of the tension spring 112a, to a position indicated by full lines in FIG. 8, in which that portion of the magnet roll 43a halfway between the first and second polar blocks 45a and 46a, faces the doctor 37a, as shown in FIG. 9B. Thus, when the solenoid 109a is on, the magnetic brush 35a is formed on the surface of the developing roller 31a. 
     A magnet roll drive mechanism 100b for rotating the magnet roll 43b, to form on or remove the magnetic brush 35b from the surface of the developing roller 31b, is constructed in substantially the same manner as the magnet roll drive mechanism 100a described above. In FIGS. 7 to 9B, like reference numbers, but with a suffix &#34;b&#34; each in place of &#34;a&#34;, are used to designate like members of the magnet roll drive mechanism 100b as included in the magnet roll drive mechanism 100a. 
     A solenoid 109b is designed so as to be off and on when the solenoid 109a is on and off, respectively. Thus, the magnetic brush 35a or 35b is alternatively formed on the surface of the first or second developing roller 31a or 31b. 
     As shown in FIGS. 2 and 3, the doctor 37b includes a doctor body 113b formed of nonmagnetic material, a magnetic member 114 formed of a beltlike iron plate extending along the longitudinal direction of the doctor body 113b, and a pair of magnetic members 115b (only one shown) at either end portion of the doctor body 113b. The magnetic members 114 and 115b and the first polar block 45b form lines of magnetic force between them, to positively prevent the developing agent G2 from being carried away from the hopper 51b, when the first polar block 45b is opposed to the doctor 37b, so that the developing agent G2, on the surface of the developing roller 31b, is removed. 
     The doctor 37a includes a doctor body 113a formed of nonmagnetic material, and a pair of magnetic members 115a (only one shown) at either end portion of the doctor body 113a. The doctor 37a serves to prevent the developing agent G1 from being carried away, in the same manner as does the doctor 37b. Unlike the doctor 37b, the doctor 37a is not provided with any magnetic member extending along the longitudinal direction of the doctor body 113a. The developing agent G1 is prevented from being carried away from the hopper 51a, by, taking advantage of lines of magnetic force formed between the first polar block 45a of the developing roller 31a, opposite the doctor 37a and the fourth polar block 48b of the developing roller 31b. 
     As shown in FIGS. 2 and 3, a magnetic plate 116 is interposed between the two developing rollers 31a and 31b. The magnetic plate 116 serves to reduce the influences of the magnetic flux density and polar distribution of the one magnet roll 43a (43b) on the other magnet roll 43b (43a), thereby ensuring satisfactory feed of the developing agents. 
     As shown in FIGS. 2 and 3, moreover, a developing agent shortage detecting unit 117b is located in the hopper 51b of the second developing unit 32b. The developing agent shortage detecting unit 117b detects reduction of the residual quantity of the developing agent G2, in the hopper 51b, to a predetermined level. The developing agent shortage detecting unit 117b includes a detecting lever 118, adapted to rock in accordance with the residual quantity of the developing agent G2 in the hopper 51b, an actuator 119 integral with the detecting lever 118, a permanent magnet 120 attached to the actuator 119, and a reed switch 121 as a sensor outside the hopper 51b. When the quality of developing agent G2, in the hopper 51b, is reduced, the permanent magnet 120 approaches the reed switch 121 to actuate the same. A developing agent shortage detecting unit 117a (not illustrated in FIGS. 2 and 3), similar to the detecting unit 117b, is located in the hopper 51a of the first developing unit 32a. 
     As shown in FIGS. 2, 3 and 10, a hopper cover 131, constituting part of the top face of the housing 1, is swingably located on the top side of the developing device 10. When the hopper cover 131 is removed, a lid 133a covering a top opening 132a for developing agent supply of the first developing unit 32a and a lid 133b covering a top opening 132b for developing agent supply of the second developing unit 32b are exposed at the same time. Thus, with the lids 133a and 133b taken off, the developing agents G1 and G2 can readily be supplied from the top side. The lids 133a and 133b are provided with color indicator 134a and 134b, respectively. The color indicators 134a and 134b facilitate discrimination by color between the developing agents G1 and G2 stored in the hoppers 51a and 51b, preventing the developing agents of different colors from being mixed. 
     In FIGS. 2 and 3, number 135 designates a cover position detecting unit for detecting the state of the hopper cover 131. The cover position detecting unit 135 includes a permanent magnet 136 attached to the front end portion of the hopper cover 131, and a reed switch 137 as a sensor attached to the housing 1, and adapted to be turned on when the hopper cover 131 is shut down, to bring the magnet 136 into contact with the reed switch 137. 
     As shown in FIGS. 10, 11A and 11B, integrating timers 139a and 139b as life indicators, for indicating the remainder of the life of the developing agents, and developing agent absent indicators 140a and 140b, for indicating reduction in the quantity of developing agents in the hoppers 51a and 51b (of the first and second developing units 32a and 32b) to a predetermined level, are provided on top surface 138a and 138b of the hoppers 51a and 51b, on either side of the lids 133a and 133b. The integrating timers 139a and 139b indicate the remainder of the life of their corresponding developing agents by, for example, measuring the amount of rotation (time) of the sleeves 44a and 44b which constitute the developing rollers 31a and 31b, respectively. In this embodiment, for example, FC Timer (trademark of electrolytic integrating timer produced by Fuji Ceramics Co., Ltd.) is used for the integrating timers 139a and 139b. 
     The first developing unit 32a, storing the color developing agent G1, is in the form of a cartridge which can be removably set in the housing 1 from the top side thereof. The first developing unit 32a can easily be removed from the housing 1 by raising and drawing up a handle 141 (FIG. 10). A pair of positioning members 142 (only one of which is shown) are attached individually to the front and rear walls of the developing agent supply section 34a of the first developing unit 32a. Both end support portions 144 of the handle 141 are individually swingably mounted on the positioning members 142 by means of supporting shafts 143, individually. As shown in FIGS. 12, 13A, 13B and 13C, each support portion 144 of the handle 141 is fitted integrally with a hook 145 which can rock around its corresponding supporting shaft 143. The hook 145 engages its corresponding retaining pin 146 on the housing 1, thereby restraining the handle 141 from moving upward. An engaging recess 147 as a first positioning portion, and an end face 148 as a second positioning portion, both formed on the bottom side of each positioning member 142, are adapted to engage and abut against positioning pins 149 and 150 on the housing 1, respectively. Thus, the first developing unit 32a is held in a prescribed position. 
     When the handle 141 is raised, the hook 145 is disengaged from the retaining pin 146, as shown in FIG. 13A. when the handle 141 is brought down to a substantially horizontal position, on the other hand, the hook 145 engages the retaining pin 146, as shown in FIG. 13B. When the handle 141 in the horizontal position is further pressed downward, the hook 145 and the retaining pin 146 engage each other, more securely as shown in FIG. 13C, so that the first developing unit 32a is held down. Thus, when the handle 141 is raised, the first developing unit 32a can be loaded or unloaded from the top side of the housing 1, and when the handle 141 is brought down after the first developing unit 32a is loaded, the first developing unit 32a can be affixed. In conclusion, the setting of the first developing unit 32a is very easy. 
     The developing operation of the developing device 10 will now be described. If the first developing unit 32a is selected for operation by means of the color selection key 156 shown in FIG. 14, the magnet rolls, 43a and 43b will be situated as shown in FIG. 2. Meanwhile, the reversible motor 63 rotates forwardly, and only the sleeve 44a of the first developing roller 31a rotates clockwise from the position of FIG. 2. As a result, the magnetic brush 35a is formed on the surface of the sleeve 44a. Then, the electrostatic latent image, previously formed on the photoconductive drum 7, is developed by the color developing agent G1. When the developing of the electrostatic latent image is accomplished in this manner, the magnet roll 43a rotates clockwise through about 25 degrees so that the polar block 45a faces the doctor 37a. Thus, the magnetic brush 35a ceases to be formed afresh on the surface of the sleeve 44a. In this state, the sleeve 44a further rotates through a predetermined angle, causing the magnetic brush 35a to be removed from the surface of the first developing roller 31a. At this time, the magnetic brush 35b is not formed on the surface of the second developing roller 31b, either. Thus, color mixing or any other trouble cannot be caused if the first or second developing unit 32a or 32b is selected for the next cycle of operation. 
     If the second developing unit 32b is selected for black developing operation, by means of the color selection key 156, the magnet rolls 43a and 43b are situated as shown in FIG. 3. Meanwhile, the reversible motor 63 rotates reversely, and only the sleeve 44b, of the second developing roller 31b, rotates clockwise from the position of FIG. 2. As a result, the magnetic brush 35b is formed on the surface of the sleeve 44b. Then, the electrostatic latent image on the photoconductive drum 7 is developed by the black developing agent G2. Thereafter, the magnetic brush 35b is removed from the surface of the sleeve 44b in the same manner as aforesaid, and the developing operation is thus completed. 
     The developing agent stirrer 40a (40b) and the stirring roller(s) 53a (53b) (of the developing unit 32a (32b)) when engaged in the developing operation, are normally rotating. Meanwhile, the developing agent supply roller 53a (53b) is rotated so that the developing agent G1 (G2) is supplied as required, in accordance with a control signal responsive to an output signal from the developing agent density detector 42a (42b). Thus, satisfactory developing operation can be maintained. 
     FIG. 14 shows the control panel. The control panel bears thereon a copying key 151 for starting the copying operation; an interruption key 152 for designating an interruption mode for interruption copying; an interruption indicator 153 for indicating the interruption mode, a ten-key array 154 for setting the number of copies to be made, a copy number display 155 for indicating the number of copies made; the color selection key 156 for selecting the copy color (e.g., black, red, blue, green, etc.), color indicators 157, 158, 159 and 160 for indicating the selected color; a liquid crystal display section 161 for indicating the operating mode or state; and a density setting section 162 for setting the copy density. 
     The liquid crystal display section 161 includes a ready-to-copy sign 163 for indication that the copying machine is ready for copying operation; a do-not-copy sign 164 for indication that the machine is not ready for copying operation, and a developing agent absent sign 165 for an indication that a hopper or hoppers of the developing device 10 have been emptied of the developing agent(s). 
     Alternative color selection can be accomplished with each successive depression of the color selection key 156. Here let it be supposed that the first and second developing units 32a and 32b store therein the developing agent G1 of, e.g., a red color, and the black developing agent G2, respectively. In this case, when the power supply is turned on, the black copy mode is automatically established, the second developing unit 32b is actuated, and the black color indicator 157 is lighted. If the color selection key 156 is depressed in this state, a red copy mode is established, the first developing unit 32a is actuated, and the red color indicator 158 is lighted. If the color selection key 156 is depressed again in this state, the black copy mode is resumed. Thus, the black and red copy modes can alternately be selected by repeatedly depressing the color selection key 156. 
     FIG. 15 shows a general control circuit of the copying machine. In FIG. 15, number 171 designates a microcomputer as a main control unit for the control of the copying machine as a whole. The input data of the microcomputer 171 is conveyed through an input interface circuit 172 with input switches 173, including the various keys on the control panel; various other switches and sensors 174 required for copying operation; the developing agent density detector 42a and the developing agent shortage detecting unit 117a of the first developing unit 32a, the developing agent density detector 42b and the developing agent shortage detecting unit 117b of the second developing unit 32b; and the cover position detecting unit 135. Also, the microcomputer 171 is connected directly with a memory unit 175. The memory unit 175 is backed up by a stand by power source 176, such as batteries. 
     The output data of the microcomputer 171 is conveyed through an output interface circuit 177, with a display unit 178 which includes the various indicators and display sections on the control panel; the pole position-- - --shifting solenoids 109a and 109b of the first and second developing units 32a and 32b; the reversible motor 63 the integrating timers 139a and 139b of the first and second developing units 32a and 32b; a developing bias power source 179; a charging power source 180 for the main charger 8; a transfer power source 181 for the transfer charger 11; a separation power source 182 for the separation charger 12; the discharge lamp 14; the developing agent absence indicators 140a and 140b; the main motor 28; various other solenoids 183; an original table drive mechanism 184 for driving the original table 2; and an exposure lamp control circuit 185. The exposure lamp control circuit 185 controls the exposure lamp 22, in response to an output signal from an automatic exposure sensor 186 for detecting light from the exposure lamp 22, and a signal from the microcomputer 171. 
     The first and second developing units 32a and 32b are provided, respectively, with developing unit presence signal generating mechanisms 187a and 187b for indicating the presence of the developing units, and discriminating information-- - --generating mechanisms 188a and 188b for producing discriminating codes (indicative of the colors of the developing agents) peculiar to the individual developing units. Signals and discriminating codes from these generating mechanisms 187a, 187b, 188a and 188b are applied to the input of the microcomputer 171, through the input interface circuit 172. The developing unit presence signal-- - --generating mechanisms 187a and 187b and the discriminating information-- - --generating mechanisms 188a and 188b deliver their respective signals and discriminating codes by utilizing developing unit connectors 191a and 191b, as Shown in FIG. 16, for electrically connecting the developing units 32a and 32b, respectively, to the input interface circuit 172. 
     Each connector 191a (191b) includes a jack 192a (192b) and a plug 193a (193b). The jack 192a (192b) is provided on the developing unit side, while the plug 193a (193b) is connected to the input interface circuit 172 of FIG. 15, by means of a cable. A developing unit presence signal is produced by connecting a common terminal 194 1  with a terminal 194 5 , in the jack 192a (192b). On the other hand, a 3-bit discriminating code is produced by connecting the common terminal 194 1  with any of terminals 194 2 , 194 3 , and 194 4 , depending on the developing unit concerned. 
     In the plug 193a (193b), a terminal 195 1  is grounded inside the input interface circuit 172. Thus, when the jack 192a (192b) and the plug 193a (193b) are connected, their respective terminals are connected correspondingly. By this connection, a three-bit discriminating code, indicative of the terminal connection of the jack 192, is obtained from terminals 195 2 , 195 3  and 195 4 , of the plug 193, and a &#34;developing unit present&#34; signal is obtained from a terminal 195 5 . When the jack 192 and the plug 193 are not connected, therefore, the &#34;developing unit present&#34; signal cannot be delivered from the terminal 195 5  of the plug 193, so that the developing unit(s) can be judged absent. 
     For example, the memory unit 175 is formed of a CMOS RAM (random access memory) which serves as a counter for counting copies made (or cycles of copying operation). In this embodiment, the memory unit 175 is provided with a plurality of copy counters (not shown) for counting copies for their corresponding developing units 32a and 32b, and a single total counter for ascertaining the total number of copies. These counters are selected by address designation responsive to the discriminating codes for the developing units, and can store their respective count data. The count data of each copy counter is formed of, for example, four bits by five, and is stored in the form of a BCD code. The count data of the total counter is formed of, for example, four bits by six, and is stored in the form of a BCD code. 
     The operation of the microcomputer 171 of the copying machine constructed in this manner will now be described in detail. 
     Referring first to the flow charts of FIGS. 17A to 17F, the operation of the microcomputer 171, following connection to power supply, will be described. When it is detected in step A0 that the power is on, step A1 is entered. In step A1, the second developing unit 32b, storing the standard-color or black developing agent is actuated, and the automatic exposure mode is selected. Then comes step A2, in which a heater lamp of the heat roller 19 is turned on. The operation then proceeds to step A3, in which the pole position shifting solenoid 109b of the second developing unit 32b is deenergized, and the reversible motor 63 is rotated reversely. Subsequently, in step A4, the reversible motor 63 is stopped. Then step A5 is entered in which the pole position shifting solenoid 109a of the first developing unit 32a is deenergized, and the reversible motor 63 is rotated forwardly. The operation then proceeds to step A6, in which the reversible motor 63 is stopped, whereupon step A7 is entered. Up to step A6, although the second developing unit 32b, for black developing, is in a drivable state after the start of power supply, the magnetic brushes 35a and 35b are prevented from being formed on the surfaces of the first and second developing rollers 31a and 31b. Thus, if an operator selects either developing mode, the copying machine will get ready for the start of developing operations at once. 
     In step A7, the main motor 28 is rotated for a prescribed time, and step A8 is then entered. In step A8, the heat roller 19 is checked to see if its warm-up is completed. If completion of the warm-up is detected, the &#34;ready-to-copy&#34; sign 163 on the control panel is lighted, and the copying machine enters a stand-by mode. Then, in step A9, the copying machine is enabled to accept key operation on the control panel. 
     Thus, the microcomputer 171 automatically executes a series of stand-by control processes after the start of power supply, providing for subsequent copying preparatory control, based on an operator&#39;s operation. When the operator performs key operation in step A9, step A10 is entered, in which the interruption key 152 is checked for activation. If the interruption 152 is off, step A11 is entered. If the key 152 is on, step A12 is entered. In step A12, the copying machine is checked to see if it currently is in the interruption mode. If the machine is not in the interruption mode, the operation proceeds to step A13. In step A13, the copying conditions immediately before the activation of the interruption key 152, including the copy density, copy color, and the set number of copies to be made, are saved, the second developing unit 32b storing the black developing agent, and the automatic exposure mode are selected, the copy number is set to &#34;1&#34;, and the interruption mode is established. Then step A14 is entered. When the interruption mode is established in this manner, the preceding copying conditions are saved, and the machine is switched over to the standard-color or black copy mode. Then, the interruption indicator 153 on the control panel is turned on in step A14, and step A11 is entered. 
     If the copying machine is found to be in the interruption mode in step A12, the operation proceeds to step A15. In step A15, the copying conditions saved in step A13 are recalled, and the developing unit, and the exposure mode used immediately before the start of the interruption mode are reselected, to cancel the interruption mode. Then step A16 is entered, in which the interruption indicator 153 on the control panel is turned off. The operation then proceeds to step A11. 
     In step A11, the second developing unit 32b is checked for presence, in accordance with a developing unit presence signal responsive to the second developing unit 32b. If the second developing unit 32b is absent, step A17 is entered. In step A17, the first developing unit 32a is checked for presence in accordance with a developing unit presence signal responsive to the first developing unit 32a. If the first developing unit 32a is absent, step A18 is entered. In step A18, the color indicators 157 to 160 and the &#34;ready-to-copy&#34; sign 163 on the control panel are turned off to restore the stand-by mode. Thus, neither of the first and second developing units 32a and 32b is set in the machine, and the color indicators 157 to 160 are prohibited from color indication. 
     If the second developing unit 32b is found to be present in step A11, the operation proceeds to step A19. In step A19, the first developing unit 32a is checked for presence. If the first developing unit 32a is present, step A20 is entered, in which the color selection key 156 is checked for activation. If the color selection key 156 is on, step A21 is entered, in which the machine is checked to see if the second developing unit 32b is selected. If the second developing unit 32b is not selected, step A22 is entered. In step A22, the second developing unit 32b is selected, and the operation then proceeds to step 23. If the first developing unit 32a is found to be absent in step A19, steps A20 and A21 are skipped, and step A22 is entered. If the color selection key 156 is found to be off in step A20, steps A21 and A22 are skipped, and step A23 is entered. 
     If the first developing unit 32a is found to be present in step A17, or if the second developing unit 32b is found to be selected in step A21, the operation proceeds to step A24. In step A24, the first developing unit 32a is selected, and step A23 is then entered. Thus, if neither of the first and second developing units 32a and 32b is present, or if one of them is absent, selection of the absent developing unit or units is prohibited. 
     In step A23, the color of the selected developing unit 32a or 32b is indicated. Namely, the color of the developing agent stored in the selected developing unit 32a or 32b is identified by the discriminating code delivered from the developing unit, and the color indicator on the control panel corresponding to the identified color is lighted. If the identified color is black, for example, the black indicator 157 is lighted; if red, then the red indicator 158. 
     After the color indication, the operation proceeds to step A25. In step A25, the first and second developing units 32a and 32b are checked for the presence of developing agent, in accordance with signals from the developing agent shortage detecting units 117a and 117b. If both the first and second developing units 32a and 32b are found to contain their corresponding developing agents, step A26 is entered. In step A26, the respective developing agent absence indicators 140a and 140b of the first and second developing units 32a and 32b are turned off. The operation then proceeds to step A27. 
     If either or both of the developing units 32a and 32b are found to be short of developing agent in step A25, step A28 is entered. In step A28, the developing device 10 is checked, in accordance with a signal from the cover position detecting unit 135, to see if the hopper cover 131 is laid in position. If the hopper cover 131 is in place, step A26 is entered; if not, then step A29. In step A29, the developing agent absence indicators 140a and/or 140b (of the developing unit or units short of developing agent supply) are turned on, and step A27 is then entered. Thus, if the first and second developing units 32a and 32b are short of their corresponding developing agents G1 and G2, the developing agent absence indicators 140a and 140b beside the top openings 132a and 132b are lighted. The developing agent absence indicators 140a and 140b, cannot, however, be lighted when the hopper cover 131 is in place, that is, they can be turned on only when the hopper cover 131 is lifted. 
     In step A27, the selected developing unit is checked for the presence of developing agent. If the developing agent is present, step A30 is entered in which the developing agent absence sign 165 on the control panel is turned off. The operation then proceeds to step A31. If the selected developing unit is found to be short of the developing agent, in step A27, step A32 is entered, in which the developing agent absence sign 165 on the control panel is lighted. The operation then proceeds to step A31. Thus, the developing agent absence sign 165 is turned on only when the selected developing unit is short of its corresponding developing agent. 
     In step A31, the copying machine is checked for trouble. If the machine is found to be malfunctioning, it is troubleshooted; if not, step A33 is entered. 
     In step A33, the copying machine is checked to see if it is in a manual paper supply mode using the sheet-bypass guide 6. If the machine is found not to be in the manual supply mode, that is, if it is found to be in an automatic paper supply mode using the paper cassette 3, then step A34 is entered. In step A34, the paper cassette 3 is checked for the presence of sheets P1. If the sheets P1 are absent, the stand-by mode is restored; if present, step A35 is entered. If the machine is found to be in the manual paper supply mode in step A33, step A34 is skipped, and step A35 is entered. 
     In step A35, the developing device 10 is checked, in accordance with developing unit presence signals from the first and second developing units 32a and 32b, to see if either of the developing units 32a and 32b has been detached once. If either of the developing units is found to have been detached once, step A36 is entered. In step A36, the developing unit concerned is checked for secureness in attachment, in accordance with a signal from the cover position detecting unit 135. If the cover position detecting unit 135 detects that the hopper cover 131 is in place, the developing unit is judged to be securely attached. 
     When the secure attachment of the developing unit is ascertained, step A37 is entered. In step A37, the developing agent is removed from the surface of the sleeve of the developing unit detached by the processes in steps A3 to A6, and the stand-by mode is restored. Thus, if the developing unit is detached with the power on, the developing agent on the sleeve of the detached developing unit is removed after the developing unit is securely attached. 
     If neither of the developing units 32a and 32b is found to have been detached in step A35, step A38 is entered. In step A38, the copy key 151 on the control panel is checked for activation. If the copy key 151 is on, a copy mode as described later in detail is entered; if not, step A39. In step A39, the copying machine is checked to see if no key operation on the control panel has been performed for a prescribed time. If any key operation is found to have been performed during the prescribed time, the stand-by mode is restored; if not, step A40 is entered. In step A40, the machine is checked to see if it is currently in the interruption mode. If the machine is not found to be in the interruption mode, step A41 is entered. In step A41, the second developing unit 32b storing the black developing agent, and the automatic exposure mode are selected, the copy number is set to &#34;1&#34;, and the stand-by mode is restored. Thus, if the copying machine is not operated for a prescribed time after a cycle of normal copying operation is ended, it is switched over to the standard-color or black-copy mode, and the automatic exposure mode. 
     If the copying machine is found to be in the interruption mode in step A40, step A42 is entered. In step A42, the copying conditions saved at the time of establishing the interruption mode are recalled, the interruption indicator 153 on the control panel is turned off, and the stand-by mode is restored. Thus, if the copying machine is not operated for the prescribed time after a copying operation is performed in the interruption mode, the mode enjoyed immediately before the start of the interruption mode is restored. 
     Referring now to the flow charts of FIGS. 18A to 18D, a copy mode will be described in detail. When it is detected in step B0 that the copy key 151 on the control panel is on, step B1 is entered. In step B1, the pole position shifting solenoid 109a or 109b of the selected developing unit is energized. Then comes step B2, in which the reversible motor 63 is rotated in a direction corresponding to the selected developing unit. The operation then proceeds to step B3, in which the blade solenoid 29 of the cleaning unit 13, discharge lamp 14, main motor 28, transfer power source 181 (transfer charger 11), separation power source 182 (separation charger 12), and developing bias power source 179 are energized or turned on. Then step B4 is entered, in which automatic paper feed from the paper cassette 3 is started. Subsequently, in step B5, the original table 2 is checked for position. If the original table 2 is not in the prescribed position, it is returned thereto in step B6. When the table 2 is thus restored, step B7 is entered. 
     In step B7, the exposure lamp 22 is turned on. Then step B8 is entered, in which the charging power source 180 (main charger 8) is turned on to start charging the photoconductive drum 7, and the original table 2 is advanced to start original scanning. The operation then proceeds to step B9, in which the aligning rollers 18 are rotated to feed the sheet P1 to the image transfer section 16. Then step B10 is entered. 
     In step B10, &#34;1&#34; is subtracted from the number currently indicated by the copy number display 155 on the control panel, and &#34;1&#34; is added to the current number in each of the total counter in the memory unit 175, and the copy counter, in the same memory unit 175, for the selected developing unit. The operation then proceeds to step B11, in which the charging power source 180 is turned off, to stop charging the photoconductive drum 7. Then step B12 is entered, in which the exposure lamp 22 is turned off. Subsequently, in step B13, the original table 2 is returned. Then step B14 is entered. 
     In step B14, the original table 2 is checked for position. If the original table 2 is found to have returned to the prescribed position, step B15 is entered. In step B15, the aligning rollers 18 are stopped from rotating, and the original table 2 is stopped from moving rearward. The operation then proceeds to step B16, in which the copying machine is checked to see if the prescribed number of copies is reached. If the prescribed number is reached, step B17 is entered. 
     In step B17, the selected developing unit 32a or 32b is checked, in accordance with a signal from its developing agent density detector 42a or 42b, to see if the developing agent density of the selected developing unit 32a or 32b is higher than the lower limit. If the developing agent density is higher than the lower limit, step B18 is entered; if not, then step B19. In step B19, the machine is checked to see if the developing agent has continuously been supplied for a prescribed time. If the continuous replenishment is not detected, step B17 is resumed for the repetition of the aforementioned operation. If the continuous developing agent supply for the prescribed time is detected, step B18 is entered; namely, in the automatic developing agent density control, there are provided a proper density level and the lowest density level that would not result in a change of the fluidity of the developing agent, and at the end of the copying operation, the developing unit drive and developing agent supply are continued until the density of the developing agent reaches the lowest level. 
     In step B18, the pole position-- - --shifting solenoid 109a or 109b, of the selected developing unit 32a or 32b, is deenergized. Then step B20 is entered in which the reversible motor 63 is stopped. The operation then proceeds to step B21, in which the blade solenoid 29, main motor 28, discharge lamp 14, transfer power source 181, separation power source 182, and developing bias power source 179 are deenergized or turned off, and the stand-by mode is restored. 
     If the set number of copies is not found to have been reached in step B16, step B22 is entered. In step B22, the developing agent is checked for life by checking the copy counter in the memory unit 175 for the selected developing unit to see if the current number in the copy counter has not reached the prescribed number. If the life of the developing agent is not ended, step B23 is entered. 
     In step B23, a color change request flag (mentioned later) is checked to see if there is a request for a change of color. If there is no such request (flag=0), step B4 is resumed for the repetition of the aforementioned operation. If there is a request for the a color change (flag=1), step B24 is entered. In step B24, the selected developing unit is changed. Then step B25 is entered, in which the pole position-- - --shifting solenoid of the previously selected developing unit is deenergized. The operation then proceeds to step B26, in which the reversible motor 63 is stopped. Then step B1 is resumed for the repetition of the aforementioned operation. Thus, if the developing is required to be changed during the copying operation, it is replaced with the other, after a current cycle of developing operation is ended, at the earliest. 
     If the life of the developing agent is found to be ended in step B22, step B27 is entered. In step B27, the copying machine is checked for the presence of any other developing unit of the same color. If there is no such developing unit, step B4 is resumed for the repetition of the aforementioned operation. If there is any other developing unit of the same color, step B28 is entered. In step B28, the developing unit of the same color is selected. The operation then proceeds to step B25. Thus, if the life of the developing agent in the developing unit being used is ended, the developing unit is replaced preferentially with another developing unit storing developing agent of the same color, if any. 
     Referring now to the flow charts of FIGS. 19A and 19B, the automatic interruption mode will be described in detail. The copying machine is set so that the automatic interruption is repeated at regular time intervals. 
     When the automatic interruption is started, step C1 is entered, in which the reversible motor 63 is checked for rotation. If the reversible motor 63 is found to be rotating, step C2 is entered, in which the integrating timer 139a or 139b, of the operating developing unit, is energized. The operation then proceeds to step C3, in which a flag A is checked for value. If the flag A is not &#34;1&#34;, step C4 is entered, in which a timer A starts counting. Then, in step C5, the timer A is checked to see if it has counted for 5 seconds. If 5-second counting is detected, step C6 is entered, in which &#34;1&#34; is set in the flag A. The operation then proceeds to step C7. If the 5-second count is not detected in step C5, step C5 is maintained for 5 seconds. 
     If the flag A is found to be &#34;1&#34; in step C3, step C8 is entered. In step C8, the selected developing unit is checked for developing agent density, in accordance with a signal from the developing agent density detector 42a or 42b of the selected developing unit. If the developing agent density is lower than a prescribed level, step C9 is entered. In step C9, the developing agent supply roller 52a or 52b, of the selected developing unit, is driven for developing agent supply. The operation then proceeds to step C7. If the developing agent density is found to be higher than the prescribed level in step C8, step C9 is skipped, and step C7 is entered directly from step C8. 
     Thus, in the density check, the flag A is &#34;0&#34; immediately after the start of power supply, so that it is judged to be not &#34;1&#34; in step C3, and step C4 is entered. In steps C4 to C6 following step C3, &#34;1&#34; is set in the flag A within 5 seconds after the start of counting of the timer A. Thus, in the copying operation, the density check is achieved on the basis of the output of the developing agent density detector 42a or 42b, after the passage of a predetermined time (e.g., about 5 seconds) required for the flow of the developing agent to the detector 42a or 42b to stabilize, after the start of developing agent supply to the magnet roll 43a or 43b. In other words, the density check is performed after the developing agent flow is stabilized, and is therefore improved in reliability. 
     In step C7, the color selection key 156 on the control panel is checked for activation. If the color selection key 156 is on, step C10 is entered, in which &#34;1&#34; is set in the color change request flag. The operation then proceeds to step C11. If the color selection key 156 is found not to be on in step C7, step C10 is skipped, and step C11 is entered directly after step C7. 
     If the reversible motor 63 is found not to be rotating in step C1, step C12 is entered. In step C12, &#34;0&#34; is set in the flag A, the timer A is reset, and the integrating timer 139a or 139b is deenergized. Then step C11 is entered. 
     In step C11, the copying machine is checked to see if there is any request for the indication of the total number of copies (inputted in cipher through the ten-key array 154 on the control panel). If there is a request for the indication, step C13 is entered. In step C13, the current number in the total counter in the memory unit 175 is indicated by the copy number display 155 on the control panel. The operation then proceeds to step C14. 
     If no request for the indication of the total copy number is detected in step C11, step C15 is entered. In step C15, the copying machine is checked to see if there is any request for the indication of the number of copies made, by the use of the first developing unit 32a (inputted in cipher through the ten-key array 154). If there is a request for the indication, step C16 is entered. In step C16, the current number in the copy counter in the memory unit 175, corresponding to the first developing unit 32a, is indicated by the copy number display 155. The operation then proceeds to step C14. 
     If no request for the indication is detected in step C15, step C17 is entered. In step C17, the copying machine is checked to see if there is any request for the indication of the number of copies made by the use of the second developing unit 32b (input in cipher through the ten-key array 154). If there is a request for the indication, step C18 is entered. In step C18, the current number in the copy counter in the memory unit 175, corresponding to the second developing unit 32b, is indicated by the copy member display 155. The operation then proceeds to step C14. 
     If no request for the indication is detected in step C17, step 19 is entered. In step C19, the normal number of copies is indicated by the copy number display 155. The operation then proceeds to step C14, in which the copying machine is checked to see if there is any request for clearing of the number of copies for either of the developing units (input in cipher for the developing unit concerned through the ten-key array 154). If no request for clearing is detected, the automatic interruption mode is ended. If there is a request for clearing, step C20 is entered. In step C20, the current number in the copy counter in the memory unit 175, corresponding to the designated developing unit, is cleared, and the automatic interruption mode is ended. 
     In the present embodiment, as described in detail herein, the developing device is provided with two developing units, and a switch for alternatively driving the two developing units is provided on a control panel. Thus, there may be provided an image-forming apparatus which can form an image in an image-forming mode selected among a plurality of image-forming modes, without replacing cartridges including developing units. 
     In this image-forming apparatus, a copy mode for black, which is commonly used as a standard color, is automatically established when the power is turned on or if no operation is performed for a prescribed time after a copying operation, in a mode for any other color than black, is ended. Thus, the apparatus of this embodiment is very serviceable and easy to operate, requiring no key operation for restoring the black or standard-color copy mode. 
     If one or either of the first and second developing units is not set in the copying machine housing, it is prevented from being selected. Accordingly, there will not be caused such an operation error, where a copying operation will be started without a developing unit in the copying machine, and the operating efficiency will thus be improved. 
     If an interruption mode is established in the middle of a copying operation in a mode for a certain color other than black, the black or standard-color copy mode is automatically selected. Thus, a user, as an interrupter, need not perform any key operation for restoring the black copy mode. Moreover, the selection of the developing unit (or color selection) can be achieved even during the interruption mode, and that mode which has been enjoyed before the start of the interruption mode can automatically be restored when the interruption mode is canceled. Thus, the apparatus or copying machine is further improved in its convenience. 
     The copying machine is checked to see if either of the developing units has been detached with the power on. If a detachment is detected, the developing unit concerned is checked for secureness in attachment. If it is ascertained that the developing unit is securely attached, it is operated so as to remove any developing agent on its developing roller (sleeve). Accordingly, the developing agent cannot remain on the developing roller of the previously detached developing unit, after the developing unit is set in place, so that color mixing will never be caused in developing, through use of the other developing unit. 
     Furthermore, a developing agent storage portion of black developing means is made larger in capacity than that of color developing means. Thus, the selection between black and color developing processes can be achieved very easily, and the black developing process, which should be executed more frequently, can be accomplished without requiring very frequent developing agent supply. 
     Although the standard color has been described as being black in the above embodiment, it may be any other color than black. 
     In the embodiment described above, moreover, the present invention is applied to a two-color copying machine. It is to be understood, however, that the invention is not limited to this embodiment, and may be applied to any image-- - --forming apparatuses that can form images by means of developing units, such as facsimiles or so-called multicolor copying machines using three or more copy colors.