Image forming apparatus equipped with a plurality of developing units

An electrophotographic image forming apparatus capable of bringing one developing unit among a plurality of developing units thereof into an operative state. To accomplish the developing unit changeover operation in the least necessary time, control means accepts an image forming operation start signal during the developing unit changeover operation to start the image forming operation immediately after the completion of the developing unit changeover operation and accepts a selection signal selecting an another developing unit during the developing unit changeover operation. Furthermore, when the developing unit changeover operation for bringing a developing unit into an operative state is interrupted and then the same developing unit is assigned again to operation, the developing unit changeover operation subsequent to the interruption of the developing unit changeover operation process is carried out in a time corresponding to the residual time in a time allocated for the interrupted developing unit changeover operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus equipped with a 
plurality of developing units which are used selectively. 
2. Description of Related Art: 
There has been known an electrophotographic image forming apparatus 
equipped with a plurality of developing units for producing color copies 
in addition to monochromic copies, which are used selectively by operating 
selection keys provided on a control panel to operate desired developing 
units. In such an image forming apparatus, operation of a print key to 
command the image forming apparatus to start the copying operation is 
invalid while the developing unit changeover operation is executed in 
response to the operation of a developing unit selecting key. Therefore, 
the print key must be operated to start the copying operation after the 
developing unit changeover operation has been completed. Furthermore, once 
an incorrect developing unit selection signal is entered by operating the 
selection key to select a wrong developing unit, the developing unit 
selecting procedure needs to be repeated again to select a correct 
developing unit after the completion of the developing unit changeover 
operation for selecting the wrong developing unit, because the developing 
unit changeover operation is started upon the entry of a developing unit 
selection signal by operating the selection key. Several seconds necessary 
for the developing unit changeover operation is a significant loss in 
operating time which cannot be ignored, and the unnecessary repetition of 
the developing unit changeover operation entails the useless wear of the 
components. 
Still further, in the conventional image forming apparatus of such a type, 
the changeover of the developing unit, for example, from a developing unit 
for the monochromatic copying operation by use of black developer to a 
developing unit for the color copying operation by use of color developer 
requires a so-called magnetic brush removing operation for recovering 
black developer remaining on the developing sleeve before starting the 
color copying operation. Imperfect removal the residual developer causes 
muddy color copies with black developer and hence it is impossible to 
produce clear color prints. Accordingly, a fixed time on the order of 
several seconds is allocated for the developing unit changeover operation 
so that the developer remaining on the developing sleve is removed 
completely. 
Accordingly, when the change over of the developing unit is required in 
such a conventional image forming apparatus, a time necessary for the 
developing unit changeover operation is spent unavoidably before operating 
the print start key to start the printing operation, which has been an 
impediment to efficient copying operation. 
Furthermore, in the conventional image forming apparatus, the developing 
unit changeover operation can be interrupted by operating an interrupt key 
when the operator becomes aware that the developing unit changeover 
operation is unnecessary after the developing unit changeover operation 
has been started. However, when the developing unit which has been 
operative before the interrupted developing unit changeover operation is 
selected again, the entire developing unit changeover operation requiring 
a predetermined time is repeated. In such a case, the developer remaining 
on the developing sleeve is recovered to some extent during the 
interrupted developing unit changeover operation and hence the subsequent 
developing unit changeover operation can be accomplished in a time shorter 
than the predetermined time for the normal developing unit changeover 
operation. Nevertheless, the developing unit changeover operation 
subsequent to the interruption of the preceding developing unit changeover 
operation is continued for the predetermined time for the normal 
developing unit changeover operation before the operation of the print 
start key becomes valid unnecessarily wasting time. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an image 
forming apparatus incorporating improvements in developing unit changeover 
operation. 
It is another object of the present invention to provide an image forming 
apparatus capable of accepting a further developing unit changeover 
command, print start command and developing unit changeover operation 
interrupting command during the developing unit changeover operation. 
It is a further object of the present invention to provide an image forming 
apparatus capable of properly carrying out the developing unit changeover 
operation according to a developing unit changeover command and a print 
start command. 
These and other objects of the present invention are achieved by an image 
forming apparatus equipped with a plurality of developing units which are 
selectively brought into an operative state one at a time, comprising: 
developing unit selecting signal input means for giving a developing unit 
selecting signal designating one developing unit among the plurality of 
developing units; developing unit changeover means which executes a 
developing unit changeover operation according to the developing unit 
selecting signal to bring the selected developing unit into an operative 
state and to bring the rest of the developing unit into an inoperative 
state; and control means for controlling the developing unit changeover 
means, when another developing unit selecting signal requesting the 
selection of another developing unit is given during the developing unit 
changeover operation, so as to make the developing unit changeover means 
interrupt the developing unit changeover operation in process and execute 
another developing unit changeover operation for bringing the newly 
selected developing unit into an operative state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be described 
hereinafter with reference to the accompanying drawings. 
Referring to FIG. 1, indicated at 1 is an image forming apparatus and at 2 
is a photosensitive drum which is driven by a main motor, not shown, for 
rotation in a counterclockwise direction. Sequentially disposed at 
intervals around the photosensitive drum along the direction of rotation 
of the photosensitive drum 2 are a main charger 3, a blank eraser 4, a 
first developing unit 5, a second developing unit 6, a transfer charger 7, 
a separating charger 8, a cleaning device 9, and a main eraser 10. An 
optical system 0 is disposed above the photosensitive drum 2 and the 
peripheral components. A copying paper feed system P is disposed on the 
left-hand side of the photosensitive drum 2, and a fixing unit 11 is a 
disposed on the right-hand side of the photosensitive drum 2. The image 
forming apparatus may be equipped with three or more developing units. 
The optical system 0 comprises a scanning unit 14 comprising an integral 
assembly of a slit exposure type light source 12 and a first movable 
mirror 13, second and third movable mirrors 16 and 17 held on a holder 15, 
a lens 18, and a fixed mirror 19. An area in the circumference of the 
photosensitive drum 2 corresponding to a document placed on a contact 
glass 20 and held in place with a document holder 21 is charged with 
static electricity of positive polarity by the main charger 3. The static 
charge in an area in the circumference of the photosensitive drum 2 
corresponding to a blank area on the contact glass 20 is erased later by 
the blank eraser 4. Then the light source 12 is turned on and the scanning 
unit 14 is moved to the left, as viewed in FIG. 1, by a scanning motor, 
not shown, to scan the document placed on the contact glass 20 to form an 
electrostatic latent image corresponding to the image of the document on 
the photosensitive drum 2. 
The traveling speed of the scanning unit 14 is regulated so as to meet a 
relation: V=V.sub.0 /m, where V is the traveling speed of the scanning 
unit 14, V.sub.0 is the circumferential speed of the photosensitive drum 
2, and m is a copying magnification. During the scanning operation of the 
scanning unit 14, the second and third movable mirrors 16 and 17 are moved 
to the left, as viewed in FIG. 1, at a speed of V.sub.0 /2m. When located 
at a fixed starting position, the scanning unit 14 actuates a position 
detecting switch 22. When actuated, the position detecting switch 22 gives 
a position detection signal "1" indicating the location of the scanning 
unit 14 at the starting position to a first CPU 200, which will be 
described hereinafter. A timing switch 23 is provided to generate a timing 
signal for synchronizing the operation of the copying paper feed system P 
with the scanning operation of the optical scanning system O. The scanning 
unit 14 actuates the timing switching 23 after moving a predetermined 
distance from the starting position, then the timing switch 23 gives a 
timing signal "1" to the first CPU 200. 
The first developing unit 5 and the second developing unit 6 are employed 
selectively for copying operation. Toner charged in negative polarity is 
supplied by either the first developing unit 5 or the second developing 
unit 6 to the circumference of the photosensitive drum 2 to visualize (to 
develop) the electrostatic latent image and thereby a toner image 
corresponding to the image of the document is formed in the circumference 
of the photosensitive drum 2. Subsequently, the toner image is transferred 
by the transfer charger 7 to a copying sheet, not shown, fed in 
synchronism with the timing signal to the photosensitive drum 2 by a 
timing roller 24, which is disposed at the rear end of the copying paper 
feed system P with respect to the direction of feed of a copying sheet. 
Then, the copying sheet carrying the toner image is separated from the 
circumference of the photosensitive drum 2 by the separating charger 8. 
Then, the copying sheet is conveyed by a conveyor belt 25 to a fixing unit 
11, which heat-fixes the toner image on the copying sheet. Then, the 
copying sheet carrying the fixed toner image is delivered to a tray 26. 
The toner remaining on the circumference of the photosensitive drum 2 is 
removed by the cleaning device 9. The residual static charge on the 
circumference of the photosensitive drum 2 is eliminated by the main 
eraser 10, which is continuously operative while the main motor is 
operating. 
The copying paper feed system P has a hand-feed unit 27, a first paper feed 
unit 28 of a cassette type (hereinafter, referred to simply as "first feed 
unit") an a second paper feed unit 30 of a casette type (hereinafter 
referred to simply as "second feed unit"). A copying sheet fed by the 
hand-feed unit 27 is conveyed through a hand-feed roller 31 and an 
intermediate roller 32 to the timing roller 24. Copying sheets contained 
in cassettes 28a and 30a respectively provided in the first paper feed 
unit 28 and in the second paper feed unit 30 are fed by a first feed 
roller 33 and a second feed roller 34, respectively, through the 
intermediate roller 32 to the timing roller 24. 
The rollers 24, 31, 32, 33 and 34 are connected to or disconnected from a 
driving system driven by the main motor by clutches, respectively. When 
the clutches are engaged, the corresponding rollers are connected to the 
driving system and are driven for rotation by the main motor. Paper size 
detectors 35 and 36, respectively, for detecting the size of copying 
sheets contained in the cassettes 28a and 30a, and paper exhaustion 
detectors 37 and 38, respectively, for detecting the exhaustion of the 
cassettes 28a and 30a are provided near the cassettes 28a and 30a, 
respectively. 
The constitution of the developing units will be described hereinafter. 
Since the first developing unit 5 and the second developing unit are 
substantially the same in constitution, only the first developing unit 5 
will be described herein to avoid duplication. In FIG. 2 showing the 
external appearance of the first developing unit 5, parenthesized 
reference numerals indicate the components of the second developing unit 
6. 
A bracket 40 is provided at one longitudinal end of the first developing 
unit 5. A toner replenishing bottle 45 for replenishing the first 
developing unit 5 with toner is mounted detachably on the bracket 40 and 
is rotated by a replenishing motor, not shown, for supplying toner to the 
first developing unit 5. A developing motor, not shown, drives a conveying 
device to convey the toner supplied from the toner replenishing bottle 45 
into the first developing unit 5, stirs the toner and rotates a developing 
sleeve. 
A first bottle detector 41 for detecting the toner replenishing bottle 45 
as mounted on the bracket 40, and a first toner exhaustion detector 46 for 
detecting the exhaustion of the toner replenishig bottle 45 are provided 
near the toner replenishing bottle 45. 
Toner identifying magnets 47 and 48 for identifying the color of the toner 
supplied to the first developing unit 5 can be mounted on top of the 
casing of the first developing unit 5. First and second reed switches 49 
and 50 which are actuated by the toner identifying magnets 47 and 48, 
respectively, are disposed opposite to the toner identifying magnets 47 
and 48, respectively, on the frame of the image forming apparatus. For 
example, when black toner is supplied to the first developing unit 5, both 
the magnets 47 and 48 are mounted on the casing to turn both the first and 
second reed switches 49 and 50 on to provide an identification signal 
indicating that black toner is supplied to the first developing unit 5. 
Since four conditions can be indicated with the combination of two magnets 
47 and 48, four kinds of toners of different colors can be identified with 
the two magnets 47 and 48. 
The construction of the developing units 5 and 6 will be described 
hereinafter with reference to FIGS. 3 and 4. The construction of the 
second developing unit 6 will be described first. 
Suppose that second developing unit 6 is charged with black toner and is 
disposed on the down stream side of the first developing unit 5 charged 
with color toner with respect to the direction of rotation of the 
photosensitive drum 2. 
Referring to FIG. 3, the toner supplied into a toner supply channel 71 from 
the toner replenishing bottle 58 is conveyed by a screw conveyor or the 
like, not shown, so that the toner is supplied uniformly over the entire 
width of the photosensitive drum 2. Also, shown in FIG. 3 are a stirring 
bucket 72 for stirring the toner and magnetic carrier, and a shutter 73 
for permitting the flow of the developer, namely, a mixture of the toner 
and the magnetic carrier, to the developing sleeve 76 and for intercepting 
the flow of the developer to the developing sleeve 76. 
FIG. 5 is a perspective view showing the shutter 73 and the associated 
pats. Normally, the shutter 73 is positioned at an open position (FIG. 3) 
by an arm 79 connected to the shutter 73 and pulled up by a spring 78. In 
executing a magnetic brush removing operation to bring the second 
developing unit 6 to an inoperative state, the solenoid 80 is energized to 
pull down the arm 79 to a position where the arm 79 rests on a stopper 81, 
so that the shutter 73 is shifted to a closed position as shown in FIG. 4. 
In FIGS. 3 and 4, indicated at 75 is a cylindrical magnet having seven 
magnetic poles on the circumference thereof. The developing sleeve 76 
receives the magnet 75 therein and is driven for rotation by the 
developing motor, not shown. 
When the second developing unit 6 is in an operative state, the stirring 
bucket 72 and the developing sleeve 76 are rotated in directions indicated 
by arrows a and b, respectively, to supply the developer from the stirring 
bucket 72 over the shutter 73 to the developing sleeve 76 as indicated by 
an arrow P. The developer supplied to the developing sleeve 76 is conveyed 
in the direction of the arrow b as the developing sleeve 76 is rotated. 
Magnetic brushes 77 are formed on the circumference of the developing 
sleeve 76 at positions corresponding to the magnetic poles of the magnet 
75. The magnetic brushes 77 touch and adhere to the electrostatic latent 
image formed in the circumference of the photosensitive drum 2 to develop 
the electrostatic latent image. The residual developer is conveyed further 
as the developing sleeve 76 is rotated, falls off the developing sleeve 76 
at a position C where no magnetic pole is provided on the magnet 75, and 
then flows in the direction of an arrow R into the stirring bucket 72. 
When the second developing unit 6 is brought to an inoperative state, the 
shutter 73 is moved to the closed position (FIG. 4), so that the developer 
is not supplied to the developing sleeve 76 even if the stirring bucket 72 
is rotated, the developer remaining the developing sleeve 76 falls off the 
developing sleeve 76 at the position C where no magnetic pole is provided 
on the magnet 75, flows in the direction of the arrow R, and is recovered 
into the stirring bucket 72. Thus, no developer remains on the developing 
sleeve 76. When the developing sleeve 76 is stopped after all the 
developer has been removed from the developing sleeve 76, no developer is 
supplied to the developing sleeve 76 even if the solenoid 80 is turned off 
to return the shutter 73 to the open position shown in FIG. 3. 
In bringing the second developing unit 6 again to the operative state, the 
magnetic brushes 77 are formed instantly in the circumference of the 
developing sleeve 76 immediately after the start of rotation of the 
developing sleeve 76. 
The first developing unit 5 is provided with a fixed guide plate at a 
position corresponding to the open position of the shutter 73 of the 
second developing unit 6 instead of the shutter 73. The rest of the 
constitution of the first developing unit 5 is the same as that of the 
second developing unit 6. 
Since the first developing unit 5 is charged with color toner, the first 
developing unit 5 is brought into an operative state for developing the 
electrostatic latent image in a color image. When operative, the first 
developing unit 5 operates in a manner similar that of the operation of 
the second developing unit 6. When the first developing unit 5 is brought 
into an operative state, the second developing unit 6 remains in an 
inoperative state and hence black magnetic brushes are not formed on the 
developing sleeve of the second developing unit 6. Therefore, the 
clearness of the color toner image is not affected by the second 
developing unit 6. When the second developing unit 6 is employed for 
developing an electrostatic latent image formed on the photosensitive drum 
2, and the first developing unit 5 remains in an inoperative state, the 
developing sleeve thereof is stopped and the developing bias of the first 
developing unit 5 is changed to make the adhesion of the toner of the 
first developing unit 5 to the electrostatic latent image difficult. 
Consequently, the color toner of the first developing unit 5 will not 
adhere to the electrostatic latent image even if color magnetic brushes 
are formed on the developing sleeve of the first developing unit 5 and the 
electrostatic latent image is developed with the black developer of the 
second developing unit 6 without any practical trouble. 
The control panel 100 of the image forming apparatus 1 will be described 
hereinafter with reference to FIG. 6. 
The control panel is provided in the upper part of the image forming 
apparatus 1. Arranged on the control panel 100 are a print start key 102 
for giving a print start command, an interrupt key 103, a display 104 
comprising a plurality of LEDs for indicating a set number of copies, a 
clear/stop key 105 for interrupting the copying operation of the image 
forming apparatus immediately after the start of copying operation or 
interrupting copying operation of a multicopy mode (a mode in which a 
plurality of copies of a single document are produced successively), for 
clearing the set number of copies indicated on the display 104, and for 
resetting the image forming apparatus 1 for a standard copying mode "1", 
numeric keys 106 to 115 for entering the number of copies to be produced, 
an exposure up key 116 for increasing exposure, an exposure down key 117 
for decreasing exposure, a group A of LEDs (light emitting diodes) which 
are lit up selectively to indicate the current exposure, a paper feed unit 
selecting key 118 for selecting either the first paper feed unit 28 or the 
second paper feed unit 30, LEDs 120 to 123 for indicating the size of 
copying paper to be fed by the selected copying paper feed unit, a first 
developing unit selecting key 124 for selecting the first developing unit 
5, a second developing unit selecting key 125 for selecting the second 
developing unit 6, a LED 126 which is lit up when the first developing 
unit 5 is selected, a LED 127 which is lit up when the second developing 
unit 6 is selected, LEDs 128 to 131 for indicating the color of the toner 
of the selected developing unit, and a LED 132 which is lit up when the 
toner replenishing bottle 45 mounted on the first developing unit 5 or the 
toner replenishing bottle 58 mounted on the second developing unit 6 is 
empty. Although the LEDs 128 to 131 corresponding to four color toners are 
provided on the control panel in this embodiment, only those corresponding 
to the developing units provided in the image forming apparatus function. 
Referring to FIG. 7 showing a control circuit in a block diagram, the 
control circuit has a first CPU (central processing unit) 200 and a second 
CPU 300. In this embodiment, the first CPU 200 and the second CPU 300 are 
microprocessors. A switch matrix S including switches operated by the keys 
provided on the control panel 100, the first bottle detector 41, the 
second bottle detector 53, the first toner exhaustion detector 46, the 
second toner exhaustion detector 60, the first reed switch 49, the second 
reed switch 50, the third reed switch 63 and the fourth reed switch 64 
which are arranged in a matrix are connected to the first CPU 200. The 
first CPU 200 controls the respective operations of the main motor, 
clutches for connecting the rollers to and disconnecting the same from the 
driving system, and the solenoid actuator 80 according to operation of the 
keys and the action of the detectors. The LEDs including those of the 
display 104 are turned on and off by the first CPU 200 through a decoder 
133. 
The position detecting switch 22 and the timing switch 23 are connected to 
the second CPU 300. Mainly, the second CPU 300 controls the operation of 
the optical system 0. The first CPU 200 and the second CPU 300 are 
interconnected for synchronous operation. 
A developing unit changeover operation, in a first embodiment, according to 
the present invention will be described hereinafter. 
First, the manner of operation of the control circuit will be described. 
The developing unit selecting key 124 or 125 is operated to give a 
developing unit selection signal indicating the selection of the 
developing unit charged with toner of a desired color, namely, either the 
first developing unit 5 or the second developing unit 6 to the first CPU 
200. Then, the first CPU 200 provides a signal indicating the color of the 
toner of the selected developing unit upon the reception of the developing 
unit selection signal to light up the corresponding LED among the LEDs 128 
to 131. At this stage, the developing unit changeover operation is not 
started. Signals indicating a selected paper size and the number of copies 
to be produced also are given to the first CPU 200 by operating the 
corresponding keys on the control panel 100. 
Then, upon the reception of a print start command signal provided by 
operating the print start key 102, the first CPU 200 examines the 
developing unit selection signal previously given thereto. When the 
developing unit selection signal indicates the developing unit which has 
been employed in the preceding copying operation and is positioned at the 
operative position, the first CPU 200 provides a signal to start the 
copying operation. Since control procedures for controlling the copying 
operation are the same as those executed by the known copying machine, the 
description thereof will be omitted. When the developing unit selection 
signal indicates the developing unit other than the developing unit which 
is at the operative position, the first CPU 200 starts control procedures 
for changing over the developing unit from the developing unit at the 
operative position to the selected developing unit, namely, control 
procedures for bringing the selected developing unit into an operative 
state and bringing the developing unit at the operative position into and 
inoperative state. 
Since it is permitted to enter a further developing unit selection signal 
by operating the developing unit selecting key of the control panel during 
the developing unit changeover operation, the first CPU 200 monitors 
whether or not a further developing unit selection signal is entered 
during the developing unit changeover operation. If a further developing 
unit selection signal is entered, the first CPU decides whether or not the 
developing unit designated by the new developing unit selection signal is 
the same as the developing unit which has been designated by the preceding 
developing unit selection signal. If the selection signal is the same, the 
current developing unit changeover operation is continued and, if it is 
not the same, the developing unit changeover operation is interrupted and 
another developing unit changeover operation for bringing the developing 
unit designated by the new developing unit selection signal into an 
operative state is started. 
Upon the reception of a signal indicating the completion of the developing 
unit changeover operation, the first CPU 200 controls the copying machine 
to start the copying operation. 
Routines for controlling the image forming apparatus, particularly, 
routines for controlling the developing unit changeover operation, to be 
executed by the first CPU 200 will be described hereinafter with reference 
to FIGS. 8 and 9. 
Referring to FIG. 8 showing the outline of the main routine for controlling 
the image forming operation, registers, timers and flags are initialized 
in step S1. An internal timer which defines a main routine executing time 
starts a timing operation in step S2. The developing unit changeover 
operation, which will be described in detail hereinafter, is executed in 
step S3. Signals received from the control panel and the detectors are 
processed in step S4. Control signals for controlling the copying 
operations of the image forming apparatus and signals for indicating the 
operating conditions of the image forming apparatus are provided in step 
S5. A decision whether or not the timing operation of the internal timer 
has ended is made in step S6. Upon the completion of the internal timer, 
the routine returns to step S2 for the next control cycle. 
A developing unit changeover routine to be executed in step S3 of FIG. 8 
will be described hereinafter with reference to FIG. 9. 
First, a decision is made in step S11 whether or not the apparatus is in a 
copying operation. In case of "YES", the routine returns to the main 
routine without executing the developing unit changeover control procedure 
and, in case of "NO", a decision is made in step S12 whether or not the 
print start key has on and a trigger signal is issued. When the trigger 
signal is issued, a decision is made in step S13 whether or not the 
selected developing unit is the same as that which has been employed in 
the preceding copying operation and is in an operative state. In case of 
"YES", the routine goes to step S27 to start the copying operation, and 
then returns to the main routine and, in case of "NO", a decision is made 
in step S14 whether or not the first developing unit is selected. When the 
first developing unit is selected, the developing unit changeover 
operation for bringing the first developing unit into an operative state 
is started in step S15. When the first developing unit is not selected, a 
decision is made in step S16 whether or not the second developing unit is 
selected. When the second developing unit is selected, the developing unit 
changeover operation for bringing the second developing unit into an 
operative state is started in step S17 and, when it is not selected, the 
routine returns to the main routine. 
When the decision in step S12 is "No", the routine goes to step S18 to make 
a decision whether or not the developing unit changeover operation is in 
process. In case of "YES", a decision is made in step S19 whether or not 
the developing unit changeover operation is for bringing the first 
developing unit into an operative state. In case of "Yes", a decision is 
made in step S20 whether or not the developing unit being brought to an 
operative state is the first developing unit selected by operating the 
control panel. In case of "YES", the routine returns to the main routine 
without executing further steps and, in case of "NO", the developing unit 
changeover operation for bringing the first developing unit into an 
operative state is interrupted in step S21, then the developing unit 
changeover operation for bringing the second developing unit into an 
operative state is started in step S22, and then the routine returns to 
the main routine. 
When the decision in step S19 is "No", namely, when the developing unit 
changeover operation for bringing the second developing unit into an 
operative state is in process, a decision is made in step S23 whether or 
not the control panel is operated to select the second developing unit. In 
case of "YES", the routine returns to the main routine without executing 
further steps and, in case of "NO", the developing unit changeover 
operation for bringing the second developing unit into an operative state 
is interrupted in step S24, then the developing unit changeover operation 
for bringing the first developing unit into an operative state is started 
in step S25, and then the routine returns to the main routine. 
When the decision in step S18 is "No", a decision is made in step S26 
whether or not a trigger signal indicating the completion of the 
developing unit changeover operation is provided. In case of "YES", the 
developing unit changeover operation is stopped and the copying operation 
is started in step S27 and, in case of "NO", the routine returns to the 
main routine to wait for the completion of the developing unit changeover 
operation. 
As stated hereinbefore, in the first embodiment, the first CPU 200 does not 
start the developing unit changeover operation immediately after the 
reception of a developing unit selection signal requesting the setting of 
a desired developing unit among a plurality of developing units, the first 
CPU 200 starts the developing unit changing operation upon the reception 
of a copy start signal given thereto by operating the print start key, and 
then starts the copying operation upon the completion of the developing 
unit changeover operation. Accordingly, the print start key can be 
operated immediately after the operation of the developing unit selecting 
key without waiting the completion of the developing unit changeover 
operation. 
Furthermore, since a correct developing unit selection signal can be 
entered immediately after the perception of entering a wrong developing 
unit selection signal, the developing unit changeover operation for 
bringing the correct developing unit into an operative state can be 
started immediately without entering the correct developing unit selection 
signal after the completion of the developing unit changeover operation 
for bringing the wrong developing unit into an operative state. Thus, loss 
in time is eliminated and useless wear of the mechanical components is 
prevented. 
A developing unit changeover operation, in a second embodiment, according 
to the present invention will be described hereinafter with reference to 
FIG. 10. 
In the second embodiment, a signal provided by operating the print start 
key is accepted by the first CPU 200 even during the developing unit 
changeover operation, and then the copying operation is started after the 
completion of the developing unit changeover operation. 
First, the general control operation of a control circuit will be described 
roughly. A developing unit selection signal is given to the first CPU 200 
by operating the developing unit selecting key 124 or 125 of the control 
panel 100 of the image forming apparatus to select a developing unit 
charged with toner of a desired color, namely, either the first developing 
unit 5 of the second developing unit 6. Signals representing a paper size 
and the required number of copies also are given to the first CPU 200 by 
operating the corresponding keys of the control panel 100. 
Then, the first CPU 200 provides a signal indicating the color of the toner 
of the developing unit designated by the developing unit selection signal 
to light the corresponding LED among the LEDs 128 to 131, and then starts 
the developing unit changeover operation. First, a decision is made 
whether the selected developing unit is the same as the developing unit 
currently set in an operative state. If so, the developing unit changeover 
operation is not executed and the copying operation is started when the 
print start key is turned on. Procedures for controlling the copying 
operation and the same as those for controlling the known copying machine 
and hence the description thereof is omitted. 
When the selected developing unit is not that which is currently in an 
operative state, an internal timer of the first CPU is started for timing 
a time necessary for the developing unit changeover operation, the 
selected developing unit is brought into an operative state, and then an 
operation for bringing the currently operative developing unit into an 
state is started. 
When the print start key 102 is pressed to give a print start command 
signal to the first CPU 200 during the developing unit changeover 
operation, the first CPU sets a print flag to latch the print start 
command signal. Upon the completion of the timing operation of the 
internal timer defining the duration of the developing unit changeover 
operation, a decision is made whether or not a print flag is set, namely, 
whether or not the print start key is pressed. When the print flag is set, 
namely, when the print start key is pressed during the developing unit 
changeover operation, the first CPU 200 starts the copying operation upon 
the completion of the timing operation of the internal timer. 
A developing unit changeover routine to be executed by the first CPU 200 
for controlling the developing unit changeover operation will be described 
hereinafter with reference to FIG. 10. A main routine for controlling the 
image forming apparatus is the same as that described with reference to 
the first embodiment and hence the description thereof is omitted. The 
developing unit changeover routine is executed in step S3 of the main 
routine shown in FIG. 8. 
In step S31, a decision is made whether or not the apparatus is in the 
copying operation. In the case of "YES", the developing unit changeover 
operation is not executed and the routine returns to the main routine and, 
in case of "NO", a decision is made in step S32 whether or not a trigger 
signal is provided by operating the key for selecting the first developing 
unit 5 or the key for selecting the second developing unit 6. In case of 
"YES", a decision is made in step S33 whether or not the selected 
developing unit is the same as that which has been used in the preceding 
copying operation and is in an operative state. In case of "YES", since 
the developing unit changeover operation need not be executed, the routine 
returns to the main routine and, in case of "NO", a decision is made in 
step S34 whether or not the first developing unit 5 is selected. When the 
first developing unit 5 is selected, the internal timer for time counting 
of the developing unit changeover operation is started in step S35, then 
the developing unit changeover operation for bringing the first developing 
unit operative is started in step S36, and then the routine returns to the 
main routine. When the first developing unit 5 is not selected, namely, 
when the second developing unit 6 is selected, the internal timer for 
timing the developing unit changeover operation is started in step S37, 
then the developing unit changeover operation for bringing the second 
developing unit 6 into an operative state is started in step S38, and then 
the routine returns to the main routine. 
When the decision in step S32 is "NO", namely, when any developing unit 
selecting key is not operated and no trigger signal is given to the first 
CPU 200, a decision is made in step S40 whether or not the timing 
operation of the internal timer is in process. In case of "YES", a 
decision is made in step S41 whether or not the print start key is 
operated to give a trigger signal. When the trigger signal is given to the 
first CPU 200, a print flag F is set for "1" in step S42, and then the 
routine returns to the main routine. When the decision in step S41 is 
"NO", the routine returns directly to the main routine. 
When the decision in step S40 is "No", namely, when the internal timer is 
not in the timing operation, a decision is made in step S43 whether or not 
a trigger signal indicating the completion of the time counting of the 
internal timer for time counting of the changeover operation is provided. 
When no trigger signal is provided, the routine returns to the main 
routine; when the trigger signal is provided, the developing unit 
changeover operation is stopped in step S44, and then a decision is made 
in step S45 whether or not the print flag F is "1". When the print flag 
F=1, the print flag F is reset for "0" in step S46, then the copying 
operation is started in step S47, and then the routine returns to the main 
routine. When the print flag F .noteq.1, the routine returns directly to 
the main routine. 
As stated above, in the second embodiment, upon the reception of a 
developing unit selection signal for selecting a desired developing unit 
among a plurality of developing units, the first CPU 200 starts the 
developing unit changeover operation. When a print start signal is given 
to the first CPU 200 during the developing unit changeover operation, the 
print start signal is latched until the completion of the developing unit 
changeover operation and, upon the completion of the developing unit 
changeover operation, the latched print start signal is brought valid to 
start the copying operation. Accordingly, the print start key may be 
operated immediately after the developing unit selection key has been 
operation without waiting until the completion of the developing unit 
changeover operation. 
A developing unit changeover operation, in a third embodiment, according to 
the present invention will be described hereinafter with reference to FIG. 
11. 
The third embodiment avoids the repetition of steps of the developing unit 
changeover operation for bringing a developing unit into an operative 
state which have been executed before the same developing unit changeover 
operation is interrupted, when the developing unit changeover operation is 
interrupted, and the same developing unit is selected. 
First, the manner of operation of the control circuit will be described. 
A developing unit selection signal is given to the first CPU 200 by 
operating the developing unit selecting key 124 for selecting the first 
developing unit 5 or the developing unit selecting key 125 for selecting 
the second developing unit 6 to select a developing unit charged with 
toner of a desired color among the first developing unit 5 and the second 
developing unit 6. Signals representing a paper size and the required 
number of copies also are given to the first CPU 200. 
Upon the reception of the developing unit selection signal and the print 
start command signal, the first CPU 200 decides whether or not the 
developing unit designated by the developing unit selection signal is the 
same as that which is currently in an operative state and, if it is the 
same, the first CPU 200 starts the copying operation immediately. 
Procedures for controlling the copying operation are the same as those for 
controlling a known copying machine, and hence the description thereof 
will be omitted. When the selected developing unit is not the currently 
operative developing unit, a decision is made whether the first developing 
unit is selected or the second developing unit is selected. Then, an 
internal timer is set for a predetermined developing unit changeover time 
specific to the selected developing unit, namely, the first developing 
unit or the second developing unit, and is started for timing operation. 
At the same time, a signal representing the color of toner of the selected 
developing unit is provided to light up a corresponding LED among the LEDs 
128 to 131 and the developing unit changeover operation for bringing the 
selected developing unit operative is started. 
Upon the perception of selecting a wrong developing unit during the timing 
operation of the internal timer, the clear/stop key 105 is pressed to give 
a developing unit changeover operation interruption signal to the first 
CPU 200. Upon the reception of the developing unit changeover operation 
interruption signal, the first CPU stops the timing operation of the 
interval timer, stores the current count value of the internal timer 
representing a time elapsed from the start to the interruption of the 
developing unit changeover operation in a memory B, and then interrupts 
the developing unit changeover operation. 
When a developing unit selection signal designating the selection of the 
same developing unit as that which has been being brought into an 
inoperative state through the interrupted developing unit changeover 
operation is given to the first CPU 200 to select the same developing unit 
again and the print start key is operated to give a print start signal to 
the first CPU 200, the first CPU 200 reads the time elapsed from the start 
to the interruption of the preceding developing unit changeover operation 
from the memory B, sets the internal timer for a time obtained by 
subtracting the product of the time elapsed and a predetermined safety 
factor from a predetermined standard developing unit changeover time, 
starts the internal timer for timing operation, and starts the developing 
unit changeover operation. Thus, the time for the subsequent developing 
unit changeover operation is reduced by a time spent for carrying out some 
of the developing unit changeover procedures including a procedure for 
recovering the toner before a moment when the preceding developing unit 
changeover operation was interrupted. 
Upon the completion of the time counting of the internal timer, the first 
CPU 200 starts the copying operation. 
A control routine to be executed by the first CPU 200 for controlling the 
developing unit changeover operation will be described hereinafter with 
reference to FIG. 11. A main routine for controlling the image forming 
apparatus to be executed in the third embodiment is the same as that 
executed by the first embodiment, and hence the description thereof will 
be omitted. The developing unit changeover routine which will be described 
hereinafter is executed in step S3 of the control routine shown in FIG. 8. 
First, a decision is made in step S51 whether or not the copying operation 
is in process. In case of "YES", the routine returns to the main routine 
without executing any control procedure for the developing unit changeover 
operation and, in case of "NO", a decision is made in step S52 whether or 
not the print start key is on to give a trigger signal to the first CPU 
200. In case of "YES", a decision is made in step S53 whether or not a 
flag F1, which will be described later, is "1". When F1 .noteq.1, the 
routine goes to step S54 and, when F1=1, the routine goes to step S56. In 
step S54 a decision is made whether or not a flag F2, which will be 
described later, is "1". When F2.noteq.1, the routine gives to step S55 
and, when F2=1, the routine goes to step S56. In step S55, a decision is 
made whether or not the selected developing unit is the same as that which 
has been used for the preceding copying operation and is currently in an 
operative state. In case of "YES", routine goes to step S81 and starts the 
copying operation. When the decision is step S55 is "No", a decision is 
made in step S56 whether or not the first developing unit is selected. In 
case of "YES", a decision is made in step S57 whether or not the flag 
F1=1. When F1.noteq.1, the count value N of the internal timer is set for 
a predetermined count value N.sub.0 in step S58 and, when F1=1, the count 
value stored in the memory B, which will be described later, is 
transferred to a register A in step S59, and then the contents (A) of the 
register A are multiplied by a constant k, for example, k=0.8, and the 
product k.multidot.(1) is subtracted from the predetermined count N.sub.0 
to set the internal timer for a count value N=(N.sub.0 -k.multidot.(A)) in 
step S60. Then, the internal timer starts the time count operation 
corresponding to the count value N in step S61, then the developing unit 
changeover operation for bringing the first developing unit operative is 
started in step S62, then the flags F1 and F2 are reset for "0" in step 
S63, and then the routine returns to the main routine. 
When the decision in step S56 is "No", namely, when the first developing 
unit is not selected, a decision is made in step S64 whether or not the 
second development unit is selected. In case of "NO" the, routine goes to 
step S63 and resets the flag F1 and F2 and the routine returns to the main 
routine and, in case of "YES", a decision is made in step S65 whether or 
not the flag F2=1. When F2.noteq.1, the internal timer is set for a 
predetermined count M.sub.0 in step S66 as a count M to be counted by the 
internal timer and, when F2=1, the count value stored in the memory B is 
transferred to the register A in step S67, and then the contents (A) of 
the register A is multiplied by a constant k, for example, k=0.8, and the 
product k.multidot.(A) is subtracted from the predetermined count M.sub.0 
to set the internal timer for a count M=M.sub.0 -k.multidot.(A) in step 
S68. The internal timer starts the time count operation corresponding to 
the count value M in step S69, then the developing unit changeover 
operation for bringing the second developing unit into an operative state 
is started in step S70, and then the routine goes to step S63. 
When the decision is step S52 is "No", namely, when no trigger signal is 
given to the first CPU 200, a decision is made in step S71 whether or not 
the time count operation of the internal timer is in process. In case of 
"YES", a decision is made in step S72, whether or not the clear/stop key 
is operated for interrupting the developing unit changeover operation 
which has already been started. When the clear/stop key is not operated, 
namely, when any command to interrupt the developing unit changeover 
operation is not provided, the routine returns to the main routine. When 
the clear/stop key is operated, the time count operation of the internal 
timer is stopped and the current count value of the internal timer is 
stored in the memory B in step S73, and then the developing unit 
changeover operation is interrupted in step S74. Then, a decision is made 
in step S75 whether or not the developing unit which has been subjected to 
the interrupted developing unit changeover operation is the first 
developing unit. In case of "YES", the flag F1 is set for "1" in step S76 
and, in case of "NO", namely, when the developing unit is the second 
developing unit, the flag F2 is set for "1" in step S77. Preparation for 
reducing the predetermined developing unit changeover time for bringing 
the same developing unit as that which has been subjected to the 
interrupted developing unit changeover operation by a time spent for 
executing steps S57 to S60 and steps S65 to S68 before the preceding 
developing unit changeover operation was interrupted is accomplished 
through steps S72 to S77, when the same developing unit is selected after 
the interruption of the developing unit changeover operation. 
When the decision in step S71 is "No", namely, when the internal timer is 
not in the timing operation, a decision is made in step S78 whether or not 
a trigger signal which is to be provided upon the end of the timing 
operation of the internal timer is provided. In case of "NO", the routine 
returns to the main routine and, in case of "YES", the developing unit 
changeover operation is interrupted in step S79, then the flags F1 and F2 
are reset for "0" in step S80, and then the copying operation is started 
in step S81. 
Thus, according to the third embodiment of the present invention, when a 
developing unit selection signal for selecting a desired developing unit 
among a plurality of developing units is given to the first CPU 200 and 
the print start key is turned on, the internal timer is set for a 
predetermined developing unit changeover time, and then the developing 
unit changeover operation is started. However, when the developing unit 
changeover operation is interrupted and the same developing unit as that 
which has been subjected to the interrupted developing unit changeover 
operation is selected again, the predetermined developing unit into an 
changeover time for bringing the same developing unit operative state is 
reduced by a time spent in the interrupted developing unit changeover 
operation for procedures including a procedure for recovering the 
developer. Accordingly, time is not used uselessly in carrying out the 
developing unit changeover operation to bring the developing unit into an 
operative state after the interruption of the preceding developing unit 
changeover operation for bringing the same developing unit operative. 
Although only preferred embodiments are specifically illustrated and 
described herein, it will be appreciated that many modifications and 
variations of the present invention are possible in light of the above 
teachings and within the purview of the appended claims without departing 
from the spirit and intended scope of the invention.