Image forming apparatus with detection and indication of defective image data

An image forming apparatus including a circuit for sequentially generating bit-image data in units of predetermined portions of image data, a circuit for forming an image on an image recording medium using the bit-image data generated by the generating circuit, a circuit for judging whether or not generation of the bit-image data by the generating circuit is performed within a predetermined period of time, and a circuit for, when the judging circuit determines that the generation of the bit-image data is not performed within the predetermined period of time, adding data indicating a detective image to the bit-image data, so that the image forming circuit produces an image on the image recording medium on the basis of the obtained bit-image data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus, such as a 
laser printer, for receiving image data from, e.g., an external apparatus, 
and printing a recording image on a paper sheet on the basis of received 
data. 
2. Description of the Related Art 
Image formation in an image forming apparatus such as a laser printer is 
attained via, e.g., electrification, exposure, developing, copy, peeling, 
cleaning, and fixing processes. In such an image forming apparatus, an 
electrification unit, an exposure unit, a developing unit, a copy unit, a 
peeling unit, a cleaning unit, and the like are sequentially arranged 
around a photosensitive drum, and the apparatus also comprises a fixing 
unit for receiving a paper sheet from the peeling unit. These units are 
driven upon rotation of the photosensitive drum, thereby executing the 
above-mentioned image formation processes. In this manner, an image can be 
formed on a paper sheet. 
In a laser printer of this type, a control system is divided into an engine 
control section for controlling the above-mentioned units to control 
operations for attaining the image formation processes and a printer 
control section for controlling operations of the engine control section. 
The engine and printer control sections are connected by exchanging 
commands and status data. The printer control section expands image 
information in units of pages supplied from an external equipment onto a 
partial memory to generate bit-image data, and then outputs the bit-image 
data to the engine control section. 
In the laser printer, when the printer control section detects, based on 
the fact that a time required for expanding and generating the bit-image 
data on the partial memory exceeds a predetermined allowable time, that a 
defective image is formed, an alarm message for a user is transmitted 
outside a printer using a display unit incorporated in the printer. 
When a continuous print operation including various print contents in units 
of print pages is performed, the following problem arises. More 
specifically, even when a time required for expanding and generating 
bit-image data of a print content of a given page on the partial memory 
exceeds the allowable time, and a defective image is formed, an alarm 
message transmitted outside the printer merely indicates that a defective 
image is formed on a certain page or pages during the continuous print 
operation, and a page/pages on which a defective image is formed cannot be 
specified. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an image forming 
apparatus which can specify a page or pages on which a defective image is 
formed when a defective image or images is or are formed on a certain page 
or pages. 
The image forming apparatus of the present invention comprises a circuit 
for sequentially generating bit-image data in units of predetermined 
portions of image data; a circuit for forming an image on an image 
recording medium using the bit-image data generated by the generating 
circuit; a circuit for judging whether or not generation of the bit-image 
data by the generating circuit is performed within a predetermined period 
of time; and a circuit for, when the judging circuit determines that the 
generation of the bit-image data is not performed within the predetermined 
period of time, adding data indicating a defective image to the bit-image 
data, so that the image forming circuit produces an image on the image 
recording medium on the basis of the obtained bit-image data. 
According to the present invention, when continuous image formation is 
performed on the pages of the image recording medium, if it is determined 
that generation of bit-image data of a given page is not performed within 
a predetermined period of time, an image indicating an "image defect" is 
formed on the page where the image defect occurred.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described below with 
reference to the accompanying drawings. 
FIG. 1 shows a structure of an image forming apparatus of the present 
invention, e.g., an image forming unit apparatus as a laser printer 
comprising optional equipments. 
More specifically, in the image forming unit apparatus, optional equipments 
such as multi-cassette feeder 2 for feeding paper sheets (normal paper 
sheets) P having a predetermined thickness, e.g., cut sheets as image 
recording media into printer 1, envelope feeder 3 for feeding paper sheets 
(thick sheets) A thicker than the normal sheets such as envelopes as 
recording media, a jogger as a selector device for distributing paper 
sheets P or A after image formation in units of predetermined number of 
sheets, and the like, are connected to laser printer 1. 
Multi-cassette feeder 2, envelope feeder 3, and the jogger are connected to 
a control section (not shown) in the main body of laser printer 1 in an 
on-line manner. Operation panel 100 (FIG. 2) is arranged on the upper 
surface of laser printer 1. 
In laser printer 1, paper feed cassette 22, feed roller 23, aligning roller 
pair 25, convey belt 36, gate 38, paper discharge roller pairs 39 and 42, 
and the like are arranged in addition to a process system, such as laser 
optical system 12, photosensitive drum 17, electrification unit 18, 
developing unit 19, copy unit 20, discharging unit 21, peeling unit 35, 
fixing unit 37, cleaning unit 45, and the like. 
Laser optical system 12 comprises a semiconductor laser oscillator (not 
shown) for generating a laser beam, a collimator lens (not shown) for 
collimating the laser beam, polygonal mirror (rotary mirror) 13 as a 
rotary member having an octahedral mirror portion for reflecting the laser 
beam from the lens for each scanning line, f.theta. lens 14, mirrors 15 
and 16, mirror motor 60 for rotating (driving) polygonal mirror 13, and 
the like. 
In the image forming operation, a laser beam from laser optical system 12 
corresponding to an image signal from an external piece of equipment (not 
shown) (or operation panel 100) is focused on a surface of photosensitive 
drum 17. Photosensitive drum 17 is rotated in a direction of an arrow in 
FIG. 1. The surface of drum 17 is charged by electrification unit 18, and 
then exposed by laser optical system 12 in correspondence with the image 
signal. More specifically, the laser beam emitted from the semiconductor 
laser oscillator is scanned at a predetermined speed from the left to the 
right of photosensitive drum 17 upon rotation of polygonal mirror 13 by 
mirror motor 60, thereby forming an electrostatic latent image o the 
surface of drum 17. The latent image is developed by a toner by developing 
unit 19, thus forming a visual image. 
On the other hand, paper sheets P as image recording media in paper feed 
cassette 22 are picked up one by one by feed roller 23. Each sheet is 
guided to aligning roller pair 25 via paper guide path 24, and is then fed 
to a copy section by roller pair 25. 
Paper sheet P picked up from paper feed cassette 30 in multi-cassette 
feeder 2 one by one by feed roller 32 and guided to aligning roller pair 
25 via paper guide paths 34 and 29; paper sheet P picked up from paper 
feed cassette 31 one by one by feed roller 33 and guided to aligning 
roller pair 25 via paper guide paths 34 and 29; paper sheet A picked up 
from stacker 26 in envelope feeder 3 one by one by feed roller 27 and 
guided to aligning roller pair 25 via paper guide paths 28 and 29; or 
paper sheet P fed from manual paper feed unit 44 and guided to aligning 
roller pair 25 via paper guide path 29 is fed to the copy section in 
accordance with an instruction from external equipment or operation panel 
100. 
Paper sheet P or A fed to the copy section is brought into tight contact 
with the surface of photosensitive drum 17 on copy unit 20, and the toner 
image on photosensitive drum 17 is copied onto the sheet upon operation of 
copy unit 20. Paper sheet P or A on which the toner image is copied is 
peeled from photosensitive drum 17 upon operation of peeling unit 35, and 
is fed to fixing unit 37 by convey belt 36. The copied image is thermally 
fixed on the paper sheet by heat roller 371 for generating heat for a 
fixing operation. Heat roller 37.sub.1 incorporates heater lamp 37a used 
for heating. 
Paper sheet P or A after the fixing operation is fed onto paper discharge 
tray 40 by paper discharge roller pair 39 via gate 38. Alternatively, 
paper sheet A is fed to upper convey path 41 by gate 38, and is exhausted 
on paper discharge tray 43 movably supported by the jogger by paper 
discharge roller pair 42. 
After the copy operation, any residual toner is removed from photosensitive 
drum 17 by cleaning unit 45, and any after image erased by discharging 
unit 21. In this manner, drum 17 is ready for the next image forming 
operation. 
Note that fixing unit 37 has a unit structure (fuser unit), and is 
independently detachable from printer 1. Aligning switch 48 for detecting 
a paper feed error to the copy section side is arranged in front of 
aligning roller pair 25, and paper discharge switches 49a and 49b, for 
detecting a paper discharge error, are arranged in front of paper 
discharge roller pairs 39 and 42, respectively. 
Paper sensors 50, 51, and 52 for detecting paper sheets P are respectively 
arranged above paper feed cassettes 22, 30, and 31, thereby detecting the 
presence/absence of paper sheets P in paper feed cassettes 22, 30, and 31. 
An engine control circuit board carrying engine controller 70 for 
controlling electrical units arranged in printer main body 1 to control 
operations for executing electrophotographic processes, and a circuit 
board carrying printer controller 71 for controlling operations of engine 
controller 70 are arranged above laser optical system 12. 
A maximum of three circuit boards for printer controller 71 can be loaded 
in accordance with a degree of addition of functions (e.g., kinds of 
fonts, kanji, and the like are expanded). Functions can be further added 
by inserting IC cards 517 for adding functions in three IC card connectors 
72 arranged at the front edge portion of the circuit board for printer 
controller 71. A connector (not shown) to be connected to host apparatus 
409 (to be described later) as an external output unit such as an 
electronic computer, a word processor or the like is arranged at the left 
end face portion of the circuit board for printer controller 71. 
Operation panel 100 comprises liquid crystal display 100a for displaying a 
sheet count, a mode, a guide message, and the like, LED indicator 100b for 
indicating various operating states by turning on LEDs, switch 100c for 
instructing various operations, and on-line key 100d for switching an 
on-line/off-line mode. 
The arrangement of the engine control section will be described below. 
FIG. 2 is a block diagram showing an arrangement of the main part of engine 
control section 300. In FIG. 2, reference numeral 302 denotes a power 
supply unit which outputs power supply voltages of +5 V and +24 V when 
main switch 301 is turned on. The +5 V power supply voltage is supplied to 
engine controller 70, and is supplied to printer controller 71 via engine 
controller 70. The +24 V power supply voltage is supplied to engine 
controller 70 via cover switches 303 and 304 in turn. The power supply 
voltage is then supplied to scanner controller 101, high voltage power 
source 305, and mechanism controller 306 via engine controller 70. Scanner 
controller 101 then supplies the power supply voltage to semiconductor 
laser 90 and mirror motor 92, and mechanism controller 306 supplies the 
power supply voltage to pre-exposure unit 21X, main motor 307, manual feed 
solenoid 308, cassette paper feed solenoid 309, aligning solenoid 310, 
toner supply solenoid 311, cooling fan 500, and the like as their driving 
power source. 
Power supply unit 302 includes a zero-crossing switch type heater lamp 
controller (not shown), comprising, e.g., a photo triac coupler and a 
triac, for driving heater lamp 37a in fixing unit 3. The +24 V voltage is 
used as a driving power source for a light-emitting LED of the photo triac 
coupler. In the heater lamp controller with this arrangement, when the 
light-emitting LED is turned on/off, the light-emitting photo triac is 
turned on/off at a zerocrossing point of an AC power source, thereby 
turning on/off a triac as a main switch element at the next stage. In this 
manner, AC power source S1 is supplied to or cut from heater lamp 37a. 
Heater control signal S2 for turning on/off the light-emitting LED is 
supplied from engine controller 70 to power supply unit 302, and a 
temperature signal detected by thermistor 37b arranged in fixing unit 37 
is supplied to engine controller 70. 
Cover switch 303 is turned off when a top cover (not shown) is pivoted 
upward, and cover switch 304 is turned off when a rear cover (not shown) 
is opened. Therefore, when the top or rear cover is opened, the +24 V 
voltage is cut off by switch 303 or 304, so that operations of 
semiconductor laser 90, mirror motor 60, high voltage power source 305, 
main motor 307, solenoids 308, 309, 310, and 311, cooling fan 500, heater 
lamp 37a, and the like are stopped. As a result, an operator can safely 
touch the interior of printer main body 1. 
FIG. 3 is a block diagram showing the arrangement of engine controller 70. 
In FIG. 3, CPU 350 controls entire engine control section 300, and is 
operated in accordance with a control program stored in ROM 351. RAM 352 
is used as a working buffer for CPU 350. E.sup.2 PROM 353 stores a total 
print count, and the like. Printer interface 354 interfaces exchange of 
interface signal S3 with printer controller 71. 
Laser modulation controller 355 periodically and forcibly turns on 
semiconductor laser 90 in order to generate laser beam detection signal 
S4, and modulates semiconductor laser 90 in accordance with image data 
sent from printer controller 71 in response to interface signal S3. In 
this case, controller 355 outputs laser modulation signal S5 to scanner 
controller 101. 
Output register 356 outputs control signals S6, S7, S8, and S2 for 
respectively controlling mechanism controller 306, high voltage power 
source 305, scanner controller 101, and the heater lamp controller. A/D 
converter 357 receives voltage signals S9 and S10 generated by thermistor 
37b and toner sensor 324, and converts these voltage values into digital 
values. Input register 358 receives state signals S11, S12, S13, S14, S15, 
and S17 from paper empty switch 320, manual feed switch 321, paper 
discharge switch 49a and 49b, mount switch 323, aligning switch 48, and 
paper sensors 50, 51, 52, and 53, and ON/OFF state signal S16 of the +24 V 
voltage. 
Internal bus 359 is used for exchanging data among CPU 350, ROM 351, RAM 
352, E.sup.2 PROM 353, printer interface 354, laser modulation controller 
355, output register 356, A/D converter 357, and input register 358. 
Mechanism controller 306 includes drivers for driving various motors, 
clutches, and solenoids. These drivers are ON/OFF-controlled in accordance 
with binary control signal S6 output from output register 356. More 
specifically, each driver is turned on when signal S6 is "1", or is turned 
off when it is "0", thereby supplying or cutting off the +24 V voltage to 
or from pre-exposure unit 21X, main motor 307, solenoids 308 to 311, and 
cooling fan 500. 
Scanner controller 101 includes drivers for semiconductor laser 90 and 
mirror motor 60. Semiconductor laser 90 is ON/OFF-controlled by laser 
modulation signal S5 output from laser modulation controller 355. Mirror 
motor 60 is ON/OFF-controlled by control signal S8 output from output 
register 356. 
Laser beam detection sensor 312 comprises a PIN diode. When laser beam a 
passes through laser beam detection sensor 312, a current proportional to 
its light energy flows through the PIN diode. This current signal is sent 
to laser modulation controller 355 as laser beam detection signal S4. 
High voltage power source 305 outputs developing bias, electrification, and 
copy high voltage signals S20, S22, and S24 to a developing bias power 
supply unit (not shown), electrification unit 18, and a wire high voltage 
supply unit (not shown) of copy unit 20. These signals are 
ON/OFF-controlled by "1" or "0" level of control signal S7 output from 
output register 356. 
As described above, in engine control section 300, a power supply voltage 
is supplied to electrical circuits via engine controller 70, and 
respective units are controlled by binary control signals output from 
engine controller 70. Engine control section 300 is coupled to printer 
control section 400 (to be described later) via interface signal S3. 
The arrangement of printer control section 400 will be described below. 
FIG. 4 is a block diagram showing an arrangement of main part of printer 
control section 400. In FIG. 4, CPU 401 controls the entire printer 
control section 400. CPU 401 is connected to timer 401T via internal bus 
412. This timer is used for the timing control of respective processing 
operations to be executed by CPU 401. ROM 402 stores a control program. 
CPU 401 is operated in accordance with the program stored in ROM 02. ROM 
402 also stores an identification number which is verified when data is 
altered, data associated with paper sheet P such as a top margin, a left 
margin, a paper type, and the like, message information to be informed to 
an operator, bit-image data for indicating a defective image, and the 
like. 
RAM 403 is used as a page buffer for temporarily storing image data sent 
from host apparatus 409. Expansion memory 404 is a large-capacity memory 
used when RAM 403 cannot store data for one page when image data sent from 
host apparatus 409 has a large data volume, such as bit map data. 
Video RAM 405 stores image data expanded to bit image, and its output is 
supplied to serial/parallel converter 406. Serial/parallel converter 406 
converts image data expanded to bit-image data in video RAM 405 and sent 
as parallel data into serial data, and supplies the serial data to engine 
controller 70. 
Video RAM 405 is used as a scan buffer (partial memory) for storing 
bit-image data expanded to bit-image data. The scan buffer stores 
bit-image data in units of 64 lines (one band unit). Note that the partial 
memory may be arranged in RAM 403 or 404. 
Host interface 408 is used for exchanging data between host apparatus 409 
comprising an electronic computer or an image reading apparatus (image 
scanner) and printer control section 400. It comprises two kinds of 
transfer lines, i.e., serial transfer line 410a and parallel transfer line 
410b. These lines can be selectively used in accordance with a kind of 
data transferred between host apparatus 409 and printer control section 
400. 
Engine interface the 411 interfaces exchange of interface signal S3 between 
printer controller 71 and engine controller 70. Each of connection 
circuits 413 cuts off a power supply voltage to be supplied to IC card 517 
and signal lines, when IC board 517 is inserted in or removed from a 
connector (not shown), thereby preventing data stored in IC card 517 from 
being destroyed by noise generated when the card is inserted or removed. 
Operation panel controller 407 performs display control of a guide message 
on liquid crystal display 100a on operation panel 100, ON/OFF/flashing 
control of LED indicator 100b, control for sending data input from switch 
100c to CPU 401, and the like. Internal bus 412 is used for exchanging 
data among CPU 401, ROM 402, RAM 403, expansion memory 404, video RAM 405, 
operation panel controller 407, host interface 408, engine interface 411, 
and connection circuits 413. 
IC card 517 comprises a nonvolatile memory, such as a battery backup static 
RAM, an EPROM, an EPROM, a mask ROM, or the like. Each IC card 517 stores 
character fonts, an emulation program, and the like. 
The operation of printer control section 400 (in particular, CPU 401) shown 
in FIG. 4 will be described below with reference to the flow charts shown 
in FIGS. 5A, 5B, and 5C. 
For example, if the laser printer shown in FIG. 1 is set in an off-line 
state, and CPU 401 detects this off-line state (NO in ST1), it is checked 
if print processing of previous data received from host apparatus 409 is 
completed (ST2). If it is determined that print processing is being 
executed (YES in ST2), the flow branches to step ST12 in FIG. 5B to 
continue the print processing. On the other hand, if it is determined that 
processing of printer control section 400 itself is not completed yet (NO 
in ST35) although the print processing has been completed (NO in ST2), 
steps ST1 and ST2 are repetitively executed to establish an idling state, 
thus waiting for an on-line state of the laser printer. 
If it is determined that the laser printer is set in the on-line state (YES 
in ST1), it is checked if data sent from host apparatus 409 is a command. 
If it is determined that the received data is a command (YES in ST3), the 
received command is executed (ST4); otherwise, the flow skips command 
execution step ST4, and advances to step ST5. For example, the command 
defines attributes of data following the command, and controls the printer 
without exchanging data. 
It is then checked if the page buffer arranged as a buffer for receiving 
data in RAM 403 is full. If it is determined that the buffer is not full 
(NO in ST5), it is checked if data sent from host apparatus 409 is image 
data (VD). If it is determined that the received data is not image data 
(NO in ST6), the flow returns to step ST1, and the series of steps 
described above are repetitively executed to wait for reception of a 
command or image data. If it is detected in this state that image data is 
received (YES in ST6), the received image data is sequentially stored in 
the above-mentioned page buffer (RAM 403) (ST7). 
Subsequently, it is checked if a print request is issued. If it is 
determined that no print request is issued (NO in ST30), it is determined 
that the engine side is not ready for printing. In this case, the flow 
returns to step ST1, and the series of steps described above are executed 
again to wait for a print request. If the print request is detected (YES 
in ST30), it is determined that engine control section 300 is ready for 
printing, and CPU 401 out a print command (ST31). 
It is then checked if the storage of image data for one page is completed. 
If it is determined that the storage operation is not completed (NO in 
ST8), a "data" lamp of LED indicator 100b arranged in operation panel 100 
begins to flash (ST10). In this manner, during the data reception 
operation, the "data" lamp is flashed to inform the current state to an 
operator. The flow returns to step ST1, and the series of steps described 
above are executed to wait until image data for one page is stored in the 
page buffer (403). 
If it is determined that storage of image data of one page is completed 
upon repetitive execution of the series of steps (YES in ST8), the "data" 
lamp is turned off (ST9), and data reception processing is ended. The 
control advances to print processing in step ST12 and subsequent steps. 
If it is determined in step ST5 that the page buffer (403) is full (YES in 
ST5), the data reception operation is interrupted (ST11), and the control 
also advances to print processing in step ST12 and subsequent steps. 
Upon completion of storage of image data for one page in the page buffer 
(403), it is checked if the scan buffer allocated on video RAM 405 is full 
(ST12 in FIG. 5B). If it is determined that the scan buffer is not full 
(NO in ST12), CPU 401 converts image data corresponding to 64 lines of 
image data stored in the page buffer into bit-image data, and stores the 
bit-image data in video RAM 405, serving as the scan buffer (ST13). On the 
other hand, if it is determined that the scan buffer is full (YES in 
ST12), the flow skips step ST13. 
If it is determined that bit image expanding processing is normally 
completed (YES in ST32), it is checked if a VSYNC command has already been 
output (ST20). If it is determined that the VSYNC command is not output 
yet (NO in ST20), it is checked if a VSYNC request is issued from the 
engine side (ST21). If no VSYNC request is detected (NO in ST21), the flow 
returns to step ST1, and the controls wait for the VSYNC request while 
executing the series of steps. If it is determined that a VSYNC request is 
issued (YES in ST21), the VSYNC command is output to the engine side 
(ST22), and the flow returns to step ST1 to wait until horizontal sync 
signal HSYNC0 and video clock VCLK0 are input. 
If it is determined that the VSYNC command has already been output (YES in 
ST20), it is checked if transfer of image data for one page is completed 
(ST26). If it is determined that transfer of image data is not completed 
yet (NO in ST26), bit-image data for 64 lines stored in the scan buffer 
(403) is output to the engine side (70) in synchronism with horizontal 
sync signal HSYNC0 and video clock VCLK0 (ST25). Thereafter, the flow 
returns to step ST13, and CPU 401 waits for completion of transfer of 
image data for one page while executing the series of steps again. 
In this manner, when transmission of image data for one page is completed, 
the flow returns to step ST1, and printer controller 71, is reset to an 
initial state, so that it is ready for the transfer of image data of the 
next page. 
If it is determined that expanding processing to bit-image data in step 
ST13 is not completed within a predetermined allowable time (i.e., image 
expanding processing cannot be normally executed) (ST32), CPU 401 
interrupts bit-image expanding, generation, and transfer processing 
operations of image data stored in the page buffer (403), and outputs a 
convey command for paper sheet P. In addition, CPU 401 reads out bit-image 
data of NG indication (e.g., black pattern data used as an "NG" mark) from 
ROM 402, and stores it in video RAM 405 (ST33). Judgment in step ST32 is 
made by measuring a time from when image expanding processing is started 
in the scan buffer until the processing is completed using timer 401T. 
When image data for an end portion of paper sheet P is output, CPU 401 
outputs the bit-image data of NG indication to the engine side in 
synchronism with horizontal sync signal HSYNC0 and video clock VCLK0 
(ST34). After the bit-image data of NG indication is output, CPU 401 
causes liquid crystal display 100a of operation panel 100 to set off an 
alarm indicating the information of a defective image. The flow then 
returns to step ST1, and processing for the following page is continued. 
In step ST32 shown in FIG. 5B, once an expanding processing time exceeds 
the allowable time, the corresponding print page is processed as a 
defective page. Alternatively, once the expanding processing is 
unsuccessful, it may be tried once more. The sequence in this case is 
shown in FIG. 5C. 
More specifically, if a time-over occurs due to overflow of the scan buffer 
in image expanding processing in step ST13 before completion of the image 
expanding processing (NO in ST32), CPU 401 sets a storage area (free area) 
of a portion of, e.g., RAM 403 as a second scan buffer area (ST36). 
Bit-image data is expanded again to the scan buffer whose area is 
increased, since the second scan buffer is added to the original scan 
buffer (ST37). 
If the re-expanding processing of bit-image data is normally performed, 
i.e., if the re-expanding processing is completed within a predetermined 
period of time (YES in ST38), this means that a defective image can be 
removed, and the flow advances to step ST20. 
On the other hand, if the re-expanding processing of bit-image data cannot 
be normally performed, i.e., if the re-expanding processing cannot be 
completed within the predetermined period of time (NO in ST38), this means 
that a defective image cannot be removed, and the flow advances to step 
ST33. Thereafter, the same processing as in FIG. 5B is executed. 
A toner image formed on photosensitive drum 17 by developing unit 19 is 
copied by copy unit 20 on paper sheet P. In this case, when bit-image data 
of NG indication is supplied from print control section 400, a bar (black 
portion) of NG indication is copied, i.e., printed as an NG mark on a 
lower portion of paper sheet P, as shown in FIG. 6. In this case, a copy 
count (e.g., "008") extracted from a copy counter (not shown) may be added 
on a portion of an NG indication bar. 
When continuous print operations are performed for a plurality of paper 
sheets upon reception of image data from the host apparatus, if it is 
detected that a defective image is formed, an image corresponding to 
bit-image data for NG indication (black portion) is formed on a paper 
sheet with the defective image. Thus, a page of the defective image during 
the continuous print operations can be specified by a mark (black portion) 
indicating that the defective image is formed on the page. Thereafter, a 
print content of the specified page is changed so as not to generate a 
defective image, and only the page is printed again, thus obtaining a 
necessary print result. As a result, time and paper sheets can be saved. 
As described above, according to the present invention, an image forming 
apparatus which can specify a certain page or pages on which a defective 
image or images is or are formed can be provided. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices, shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.