Abstract:
The present invention aims at providing a method of controlling a post-processor connected with an image-forming device that utilizes a conventional image-forming device and post-processing unit, and does not necessarily require to consistently form a sag in continuous paper. The inventive method of controlling the post-processor is carried out by the image-forming device that includes a transfer part and a fixing part, and conveys continuous paper. The image-forming device is connected with the post-processor that processed the continuous paper on which an image has been formed. The method comprises the steps of initiating an electrophotographic image-forming operation in the transfer part and the fixing part so that the image-forming device forms an image onto and conveys the continuous paper, initiating an operation of the post-processor so that the post-processor post-processes and conveys the continuous paper, stopping the operation of the post-processor on or before a completion of a transfer by the transfer part onto the continuous paper, conveying to the fixing part the continuous paper onto which the fixing part has finished transferring, and conveying to the transfer part the continuous paper onto which the fixing part finishes a fixation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to electrophotographic image-forming devices, and more particularly to a method of controlling a post-processor that handles continuous paper printed by an electrophotographic image-forming device. 
     The present invention is suitable, for example, for an output device for use with a computer system that needs to print large amounts of data. 
     Hereupon, the “electrophotographic image-forming device” by which we mean is an image-forming device employing the Carlson process described in U.S. Pat. No. 2,297,691, and denotes a nonimpact printer that provides recording by depositing a developer as a recording material on a recordable medium (continuous paper in the present invention). The “continuous paper” includes fanfold paper and paper in rolls, and denotes a recordable medium that permits a user&#39;s discretionary setting of a recording length. The width of the continuous paper is determined by selecting a type of paper to be used, as necessary, from several types of ready-made paper that have a standardized or predetermined width. 
     The “post-processor” is a device that carries out one or more of various operations such as cutting, sorting, stapling, etc. after recording on the continuous paper. Characteristically, the post-processor is connected with an image-forming device, and carries out an operation under the control of the image-forming device as a host processor. 
     In recent years, the electrophotographic image-forming device (continuous paper printer) that uses continuous paper as a recordable medium for printing large amounts of data is typically connected with the post-processor that automatically carries out one or more of various operations such as folding, cutting, sorting, and stapling for the continuous paper. The electrophotographic continuous paper printer, which generally uses a photoconductive insulator (e.g., photosensitive drum, and photosensitive belt), follows the procedural steps of charging, exposure to light, development, and transfer, and the process further includes the fixing step for the continuous paper after the transfer step. 
     The charging step uniformly electrifies the photosensitive drum (e.g., at −700V). The exposure step irradiates a laser beam or the like on the photosensitive drum, and changes the electrical potential at the irradiated area down, for example, to −50V or so, forming an electrostatic latent image. The development step electrically deposits a developer onto the photosensitive drum using, for example, the reversal process, and visualizes the electrostatic latent image. The transfer step forms a toner image corresponding to the electrostatic latent image on a recordable medium using a transfer unit. The fixing step fuses and fixes the toner image on the medium by the application of heat or pressure, or light irradiation by a fixing unit, thereby obtaining a printed matter. 
     Conventionally, one printing operation in the continuous paper printer that forms an image on the continuous paper is defined as a process going on after a toner image is transferred from the photosensitive drum to the continuous paper until the fixing unit fixes the toner image. When a print command for one job has been finished, and a subsequent print command for the next job has not been received, the continuous paper on which the last line is printed is at a standstill on a continuous paper feed path in the continuous paper printer. To be more specific, the paper feed stops immediately after the last line has passed through the fixing unit. However, when the command for the next job is issued in this situation, and the printing starts, a blank area in which no image is formed is produced between the last line of the image printed by the immediately preceding job, and the first line of the image to be printed by the next job on the continuous paper. This would waste the continuous paper with a length corresponding to a distance from the fixing unit to a transfer position, and thus impair the use efficiency of the continuous paper. Therefore, a so-called reverse feed has conventionally been carried out that moves back the continuous paper by a specific length when a print command for one job is completed, and no print command for the next job comes. This would improve the use efficiency of the continuous paper. 
     To give another example, a continuous paper printer that includes first and second image-forming parts, and sequentially records images on both sides of the continuous paper forms the images in each image-forming part spaced out due to mechanical requirements. To be specific, the two-sided printer forms images by following the steps of a transfer on a first side of paper by a first image-forming part, a transfer on a second side of paper by a second image-forming part, a fixing on the first side of paper by the first image-forming part, and a fixing on the second side of paper by the second image-forming part in this sequence. In order to improve the use efficiency of continuous paper in the printer configured as above, the reverse feed amount of the continuous paper (distance at which the continuous paper is conveyed in a backward direction) is configured to be larger than that in a single-side printer. 
     The post-processor is electrically connected with the continuous paper printer, and carries out operations such as cutting of the continuous paper conveyed. Following is an exemplified post-processor embodied as a burster unit that separates the continuous paper along perforations into individual sheets. The burster unit upon startup of the printing operation simultaneously starts separation of the continuous paper conveyed; thus, the continuous paper has already been separated when the reverse feed is to be carried out in the continuous paper printer, and cannot be conveyed in a reverse direction. Therefore, if the post-processor having no reverse feed capability is connected with the continuous paper printer having a reverse feed capability, the feed of the continuous paper is controlled so that a reverse feed amount of the continuous paper may sag during printing. In addition, the post-processor is manufactured by a maker other than that of the continuous paper printer in many occasions, and thus may be selected among post-processors capable of accepting a specific post-processing command from the continuous paper printer. 
     A further detailed description will be given of the method of controlling the post-processor with reference to FIG.  9 . FIG. 9 is a flowchart showing a conventional method of controlling printing operations for explaining the method of controlling the post-processor, where the reverse feed amount (distance from the transfer part to the fixing part) is 50 inches. First, for example, when a print command for one job is issued from a host device such as a computer (step  2 ), the continuous paper printer receives the command to initiate printing (step  4 ). Subsequently, the image-forming device, which has experimentally worked out the time required to feed continuous paper over a predetermined distance of 50 inches in advance, sets a timer for that time (step  6 ). The image-forming device then initiates the operation, and does printing as intended on the continuous paper (step  8 ). During the printing operation, if the timer exceeds the set time and indicates zero (step  10 ), the image-forming device, construing it to mean that a 50-inch sag has been formed in the continuous paper, transmits a signal to initiate an operation to the post-processor. Until the timer indicates zero (step  10 ), the printing operation continues all the way, and the signal to initiate the operation is not transmitted to the post-processor. The post-processor eventually receives the command to initiate the operation, and initiate the operation (e.g., of cutting the continuous paper) (step  12 ). If the intended printing operation is completed, the paper feed stops immediately after the last line of the image on the continuous paper has passed through the fixing part. At that moment, the image-forming device and post-processor simultaneously stops the operations. Thereafter, a feeder unit in the image-forming device initiates an operation, and carries out a reverse feed for a distance of 50 inches to get ready for the print command for the next job. The intended printing based upon the command from the host device is then completed (step  14 ). As has been described above, the printing operation for the continuous paper repeats a series of these procedural steps. 
     However, since a sag the amount of which corresponds to the reverse feed amount is continuously provided in the continuous paper while the printing operation is carried out according to the conventional method, if the image-forming device of which the reverse feed amount is large is used, or if there is a difference in processing speed between the continuous paper printer and the post-processor, and the former is faster than the latter, then part of the continuous paper comes in contact with the floor between the image-forming device and the post-processor. Consequently, the printed surface of the continuous paper and the floor rubs together, and disadvantageously disturbs an image or smears the printed surface. In order to provide a means for avoiding this problem, a saucer made of sheet metal or the like is placed on the floor so that the continuous paper ejected from the image-forming device may not get in direct contact with the floor. Nonetheless, the printed continuous paper is constantly in contact with and rubbed against the saucer during the printing operation, and thus there exists high possibilities that the image quality would lower, the continuous paper would smear, and the other problems would occur. In addition, the large amount of the sag in the continuous paper would disadvantageously increase the effect of the wind from outside, or the like, and prevent a stable movement of the continuous paper, thereby producing a jam in the post-processor. 
     There is a method of removing an excess sag in the continuous paper, in which the continuous paper is not brought into contact with the floor due to a mechanical buffer mechanism provided for a sagged portion in the continuous paper. However, the method would disadvantageously result in increased costs for providing the buffer mechanism. Another method of avoiding the sag problem is conceivable, in which the post-processor is also provided with the reverse feed mechanism, but the continuous paper that has already undergone the post-process such as cutting cannot be conveyed in the reverse direction, and therefore the current practice adopts the aforementioned controlling method at the same time to avoid the problem. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an exemplified general object of the present invention to provide a novel and useful post-processor connectible with an image-forming device in which the above disadvantages are eliminated. 
     Another exemplified and more specific object of the present invention is to provide a method of controlling a post processor connectible with an image forming device that utilizes a conventional image-forming device and post-processor, and dispenses with a sag to be formed in continuous paper all the time during printing operation. 
     In order to achieve the above objects, a method of controlling a post-processor as one exemplified embodiment of the present invention is carried out by an image-forming device that includes a transfer part and a fixing part, and conveys continuous paper. The image-forming device is connected with the post-processor that carries out processing on continuous paper on which an image has been formed. This method comprises the steps of initiating an electrophotographic image-forming operation in the transfer part and the fixing part so that the image-forming device forms an image onto and conveys the continuous paper, initiating an operation of the post-processor so that the post-processor post-processes and conveys the continuous paper, stopping the operation of the post-processor on or before completion of a transfer by the transfer part onto the continuous paper, conveying to the fixing part the continuous paper onto which the transfer part has finished transferring, and conveying to the transfer part the continuous paper onto which the fixing part finishes a fixation. According to this control method, the continuous paper is conveyed from the transfer part to the fixing part after the operation of the post-processor has stopped, to provide a sag in the continuous paper between the image-forming device and the post-processor, and thus is prevented from being cut off even if the continuous paper is conveyed in a reverse direction from the fixing part to the transfer part to get ready for a print command for the next job. 
     An image-forming device as one exemplified embodiment of the present invention in order to achieve the above objects is an electrophotographic image-forming device comprises a mechanical controller which controls a printing mechanism that forms an image, and a conveyor mechanism that conveys a continuous paper, and a main controller that controls the mechanical controller. The main controller controls the mechanical controller so as to generate a signal for stopping a conveyance by a post-processor connected with the image-forming device a specified time before the conveyor mechanism stops. This image-forming device may generate the signal for stopping the conveyance of the post-processor prior to stopping a conveyance mechanism of the image-forming device, and thus be connected with and control the post-processor without any changes such as an addition of parts or modification to a conventional post-processor. 
     Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view of principal part as a structural illustration of an image-forming device and a post-processor. 
     FIG. 2 is a block diagram for explaining the principle of the present invention. 
     FIG. 3 is a block diagram for explaining the controlling principle of a burster unit. 
     FIG. 4 is a flowchart for explaining a method of controlling the burster unit. 
     FIG. 5 is a time chart for explaining an operation shown in the flowchart in FIG. 4 with respect to an operating time. 
     FIG. 6 is a schematic sectional view for showing a sag amount in continuous paper during printing operation. 
     FIG. 7 is a schematic sectional view for showing a sag amount in continuous paper that has been conveyed from a transfer position to a fixing unit and is at a standstill. 
     FIG. 8 is a schematic sectional view for showing a sag amount in continuous paper after a reverse feed. 
     FIG. 9 is a flowchart for explaining a conventional method of controlling a post-processor by showing a conventional method of controlling the printing operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1 to  6  inclusive, a description will be given of a method of controlling a post-processor  200  connected with an image-forming device  100  according to the present invention. In each figure, those elements designated by the same reference numerals denote the same elements, and a duplicate description thereof will be omitted. FIG. 1 is a schematic sectional view of principal part as a structural illustration of the image-forming device  100  and the post-processor  200 , in which some components provided in the actual image-forming device  100  are partially omitted or diagrammatically illustrated for the purpose of explaining a feeding operation of continuous paper P. The image-forming device  100  includes two sets of image-forming parts, which make it possible to form images on both sides of the continuous paper P. 
     In the present invention, the image-forming device  100  is embodied as a continuous paper (two-sided) printer  100 , and the post-processor as a burster unit  200  for illustration purposes. The continuous paper printer  100  includes photosensitive drums  110 A and  110 B, development units  120 A and  120 B, transfer units  130 A and  130 B, and fixing units  140 A and  140 B. Hereinafter, reference numerals without an alphabet comprehensively indicate the elements designated by the same reference numerals with an alphabet attached thereto, unless otherwise specified. 
     The photosensitive drum  110  structurally has a photosensitive dielectric layer on a rotatable drum-shaped conductive support, and may be uniformly charged by a charger (not shown). For example, the photosensitive drum  110  is an OPC or an aluminum drum to which a separated function organic photosensitive body is applied at a thickness of approximately 20 μm, and the external diameter, for instance, of 20 mm, and rotates at a circumferential velocity of 90 mm/s in an arrow direction. The photosensitive drum  110  is exposed to light by an exposure unit (not shown), and a potential in a charged area on the photosensitive drum  110  is neutralized, whereby a latent image corresponding to image data for an image to be recorded is formed. 
     The development unit  120  supplies toner onto the photosensitive drum  110 , and forms a toner image on the photosensitive drum  110 , visualizing the latent image. The toner is usable no matter whether it is monocomponent toner or dual-component toner (i.e., including a carrier). 
     The fixing unit  130  generates an electric field that serves to electrostatically adsorb toner. The toner image on the photosensitive drum  110  is electrostatically adsorbed to the continuous paper P using a transfer current, deposited thereon, and eventually transferred thereto. As shown in FIG. 1, the transfer unit  130  disposed opposite the photosensitive drum  110  with respect to the continuous paper P. According to this structure, the continuous paper printer  100  that prints on both sides of the continuous paper P has a transfer position A at which the transfer step is carried out with the photosensitive drum  100 A and the transfer unit  130 A, and a transfer position B at which the transfer step is carried out with the photosensitive drum  110 B and the transfer unit  130 B, displaced from each other along a conveyor path of the continuous paper P. 
     The fixing unit  140  is a device that fuses the toner into the continuous paper P. The toner that has been transferred is electrostatically adhered onto the continuous paper P, and thus easily flaked off. Therefore, the toner is fixed utilizing energy such as pressure and heat, and it is necessary to transform solid toner into a liquid state in order to acquire a sufficient fixing capability. Providing the energy makes the solid toner sinter, spread, and penetrate, thereafter putting the fixing process to an end. As described above, the transfer positions A and B are displaced from each other, and accordingly, the fixing units  140 A and  140 B displaced from each other by the same distance. 
     To illustrate an operation of the continuous paper printer  100  shown in FIG. 1, in the first place, the photosensitive drum  110  is uniformly negatively charged (e.g., at −700V) by the charger (not shown). When a light is irradiated on the photosensitive drum  110  from the exposure unit (not shown), the uniform charge at the irradiated area on the photosensitive drum  110  corresponding to an original image is eliminated through the exposure to light, and a latent image is formed. The development unit  120  then develops the latent image. To be specific, the toner as a charged particle (or powder) bearing the electric charge of approximately −50 V is attracted using static electricity onto the area where the uniform charge is eliminated on the photosensitive drum  110 . As a result, the latent image on the photosensitive drum  110  is visualized into a toner image. 
     Subsequently, the toner image on the photosensitive drum  110 A is transferred first at the transfer position A on a first surface of the continuous paper P that is being conveyed in the transfer unit  130 A. Then, when the toner image on the first surface arrives at the transfer position B, the toner image on the photosensitive drum  110 B is transferred at the transfer position B on a second surface of the continuous paper P in the transfer unit  130 B. In this way, the transfer step is carried out with a specific time lag placed between operations on the first and second surfaces when the two-sided printing is done, and the time lag is controlled so as to make the toner images formed in register on the first and second surfaces of the continuous paper P. The first and second surfaces are herein assigned so that the first surface is a back, and the second surface is a front. 
     Residual toner on the photosensitive drums  110 A and  110 B is respectively collected using a cleaning unit (not shown). Thereafter, the toner on the first surface of the continuous paper P and the toner on the second surface of the continuous paper P are fixed respectively in the fixing unit  140 A, and the fixing unit  140 B, with the same time lag as in the transfer step, and are both fixed permanently. During printing operation, a front edge of the continuous paper P on both sides of which printing has been done is dispensed out of the continuous paper printer  100 , and conveyed to the burster unit  200 . 
     The continuous paper printer  100  continuously carries out the printing operation if a subsequent print command for the next job has been received when a print command for one job has been finished. However, if no print command for the next job has arrived then, the continuous paper printer  100  conveys the continuous paper P from the transfer unit  130 B to the fixing unit  140 B, fixes the toner image thereon, and thereafter, carries out a reverse feed until the last line of the image printed in the previous job reaches a position immediately before the transfer position A, to get ready for the print command for the next job. 
     Referring now to FIG. 1 again, a description will be given of a structure of the burster unit  200 . The burster unit  200  includes a feed roller  210 , burst rollers  220 , a burst cutter  230 , conveyor rollers  240 , and a stacker  250 . 
     The feed roller  210  feeds the continuous paper P to a cutting position, while eliminating a warp and a kink generated by a sag in the continuous paper P. At that moment, sprocket-engaging apertures provided on both edges of the continuous paper P are cut off using a both-end slitter (not shown). Therefore, the continuous paper P is conveyed in the burster unit  200  utilizing a roller, or the like having large frictional resistance. 
     The burst rollers  220  are located on both sides of the continuous paper P, and give a tension when cutting the same. Between the burst rollers  220  is provided the burst cutter  230  that cuts the continuous paper P in a direction orthogonal to a traveling direction of the continuous paper P. The continuous paper P that has been cut into a specific unit length is conveyed to the stacker  250  by the conveyor roller  240 . 
     The continuous paper P includes fanfold paper and paper in rolls, and denotes a recordable medium that permits a user&#39;s discretionary setting of a recording length. At both edges of the continuous paper P in the present embodiment is formed sprocket-engaging apertures, which are engaged with tractor pins for conveying the continuous paper P in the continuous paper printer  100 , and the continuous paper P is thereby conveyed in the continuous paper printer  100 . The width of the continuous paper P to be used is selected, as necessary, among those of ready-made paper that have a standardized or predetermined width. There are two types of the continuous paper P: one in which perforations are formed, and the separation is carried out by pressurizing the perforations; and the other in which the separation is carried out by a cutter equipped in the post-processor  200 . 
     Referring now to FIGS. 2 and 3, a description will be given of the principle of the present invention. FIG. 2 is a block diagram for explaining the principle of the present invention. FIG. 3 is a block diagram for explaining the controlling principle of the burster unit  200 . According to the present invention, as shown in FIG. 2, a host device  50 , a continuous paper printer  100 , and a burster unit  200  constitutes a printer unit. The host device  50  is comprised of a host computer or the like, and serves as a device in which a user may directly issue a command. 
     The continuous paper printer  100  includes a controller  150 , a mechanical controller  160 , a printing mechanism  170 , and a conveyor mechanism  180 . The controller  150 , which is connected with the host device  50 , receives a print command and printing data from the host device  50 , and transmits a variety of printing process results to the host device  50 . The mechanical controller  160  controls under the control of the controller  150  the printing mechanism  170  of the continuous paper printer  100  including the photosensitive drum  110 , the development unit  120 , the transfer unit  130 , the fixing unit  140 , and the like as shown in FIG.  1 . Although the present embodiment has the control capabilities of the continuous paper printer  100  separated into the controller  150  and the mechanical controller  160  each having an independent MPU, the continuous paper printer  100  may be configured to include only one controller having one MPU. The mechanical controller  160  also controls the burster unit  200 . The printing mechanism  170 , which includes a series of printing operation processes, receives an instruction from the mechanical controller  160 , and carries out the printing operation. The conveyor mechanism  180  receives an instruction from the mechanical controller  160 , and conveys the continuous paper P. 
     The burster unit  200  includes a post-processing mechanism  260 . As shown in FIG. 3, the controller  150  in the continuous paper printer  100  provides a print start command and a print length instruction to the mechanical controller  160 . In response to the instruction, the mechanical controller  160  starts operating a print length storage memory  162  and a ⅙-inch interrupt circuit  164 . The “print length” indicates a size of the image to be printed, and the “print length storage memory” is a memory for storing information of the print length converted from a specified image size. In the present embodiment, the print length corresponding to 1 inch is indicated by 60 in print length, and is measured in units of ⅙ inch (equivalent of 10 in print length); thus 11 inches is represented by 660 in print length. The units for measuring the print length is not limited to ⅙ inch, but may be selected from ½ inch, and ⅓ inch. Accordingly, the “⅙-inch interrupt circuit  164 ” is provided to serve the purpose of checking images formed on the continuous paper P being conveyed during printing operation every ⅙ inch. The ⅙-inch interrupt circuit  164  is used to control standstill time of the burster unit  200  in synchronism with a ⅙-inch clock. 
     The mechanical controller  160  also provides a start and stop instruction to the burster unit  200  or the post-processing mechanism  260 . The burster unit  200  that has received the instruction feeds, cuts, and stores the continuous paper P. The mechanical controller  160  also provides an instruction to the ⅙-inch clock. The “⅙-inch clock” tells necessary time to feed the continuous paper P by ⅙ inch, and carries out a ⅙-inch interrupt service at the time periods. The ⅙-inch interrupt service is used to perform subtractions for the print length stored in the print length storage memory  162 , and can control the standstill time of the burster unit  200 . 
     As above, the method of controlling the post-processor  200  in the present invention may utilize a conventional image-forming device and post-processor as is without any alterations or modifications. The inventive control method may thus be easily introduced to the conventional image-forming device  100 . In addition, the inventive method may measure the image size in a specific unit as a print length, and thus easily accommodate and cope with changes in the reverse feed amount. 
     Hereinafter, a description will be given of a method of controlling the burster unit  200  with reference to FIGS. 4 and 5. FIG. 4 is a flowchart for explaining the method of controlling the burster unit  200 . FIG. 5 is a time chart for explaining an operation shown in the flowchart in FIG. 4 with respect to an operating time. In the following data, the print length corresponding to 1 inch is indicated by 60 in print length, and is measured in units of ⅙ inch (equivalent of 10 in print length). 
     When the host device provides a print command (step  302 ), the controller  150  in the continuous paper printer  100  receives the same, and transmits the print start command and print length data for the first page. The mechanical controller  160  receives the print start command for the first page (step  304 ), and stores the print length data in the print length storage memory  162  (step  306 ). Thereafter, the printing mechanism  170  and conveyor mechanism  180  get started and initiate a printing operation (step  308 ). If the printing has not yet been initiated (step  310 ), the printing operation starts its operation. If the printing has already been initiated (step  310 ), the ⅙-inch interrupt circuit  164  starts its operation, and interrupt service signals every ⅙-inch transport are enabled (step  312 ). The burster unit  200  is then enabled (step  314 ). Subsequently, when a ⅙-inch clock signal is generated from the mechanical controller  160 , and a startup instruction signal for the burster unit  200  is enabled (as shown in FIG.  5 ), the burster  200  starts a conveying operation. 
     Next, if no command for printing the next and following pages is provided, the mechanical controller  160  does not receive a print start command (step  316 ). Then, if no interrupt service every ⅙-inch transport has been carried out (step  320 ), the continuous paper printer  100  and the burster unit  200  both continue their operations. If the interrupt service every ⅙-inch transport has been carried out (step  320 ), the length corresponding to ⅙ inch is subtracted from the print length stored in the print length storage memory  162  every ⅙-inch clock (step  322 ). 
     Alternatively, if a command for printing the next and following pages is provided, the mechanical controller  160  receives a print start command for the next and following pages (step  316 ). Then, a new print length is added to the print length storage memory  162  (step  318 ). Next, if no interrupt service every ⅙-inch transport has been carried out (step  320 ), the continuous paper printer  100  and the burster unit  200  both continue their operations. If the interrupt service every ⅙-inch transport has been carried out (step  320 ), the length corresponding to ⅙ inch is subtracted from the print length stored in the print length storage memory  162  every ⅙-inch clock (step  322 ). 
     After the step  322 , if the print length stored in the print length storage memory  162  does not become zero (step  324 ), the steps  316  to  322  are repeated until the print length becomes zero. If the print length becomes zero (step  324 ), it means that the printing is to be complete. Therefore, the printing mechanism  170  in the continuous paper printer stops its operation. After step  324 , the interrupt service every ⅙-inch transport is disabled, and thus the burster unit  200  also stops its operation (including conveying operation) in response thereto (step  326 ). From then on as well, the conveyor mechanism  180  continues its operation, and conveys the continuous paper P for a distance from the transfer unit  130 B to the fixing unit  140 B (step  328 ). Accordingly, the toner image on the continuous paper P is fixed, and a sag is generated in the continuous paper P between the continuous paper printer  100  and the burster unit  200 . Then the continuous paper P is conveyed for a distance from the fixing unit  140 B to the transfer unit  130 B in a reverse direction (step  330 ). Thereafter, the continuous paper printer  100  stops its conveying operation (step  332 ). The printing is complete then (step  334 ). 
     A description will be given of the steps shown in FIG. 4 in sequence. First, when the mechanical controller  160  receives a print start command for the first page (step  304 ), a print length for each command is added and stored in the print length storage memory  162  (step  306 ). At that moment, the continuous paper printer  100  and the burster unit  200  are not in operation. Subsequently, the printing mechanism  170  and the conveyor mechanism  180  in the continuous paper printer  100  initiates their operations, and the continuous paper P is conveyed in the device  100 , and printing is done thereon. 
     Immediately after the continuous paper printer  100  starts operation, the burster unit  200  also gets ready for operation, but actually starts the conveying operation in the burster unit  200 , on condition that a startup instruction signal to the burster unit  200  is enabled and that a ⅙-inch clock signal is generated. Thereafter, interrupt service is carried out every ⅙ inch in synchronism with the ⅙-inch clock, and the burster unit  200  starts its conveying operation. The interrupt service subtracts the print length corresponding to ⅙ inch for each ⅙-inch clock from the print length stored in the print length storage memory  162 . 
     When the mechanical controller  160  stops receiving a new print start command, the print length is thereafter not added, but only subtracted repeatedly. If the subtraction is repeated, the print length stored in the print length storage memory  162  eventually becomes zero. Immediately after the print length becomes zero, the burster unit  200  stops its operation, and the ⅙-inch interrupt service is disabled. The conveyor mechanism  180  in the continuous paper printer  100  continues its operation, even if the burster unit  200  stops its operation, during necessary time for forming a sag by the reverse feed in the continuous paper P (a time period necessary for conveying the continuous paper P for a distance from the transfer unit  130 A to the fixing unit  140 B). Thereafter, the continuous paper printer  100  carries out the reverse feed, and stops operating. Thus, the printing operation by a print command for one job is complete. 
     Referring now to FIGS. 6 through 8, a description will be given of a relationship between the printing operation and a sag amount in the continuous paper P. FIG. 6 is a schematic sectional view for showing a sag amount in the continuous paper P during printing operation. FIG. 7 is a schematic sectional view for showing a sag amount in the continuous paper P conveyed from the transfer position A to the fixing unit  140 B while the printing operation is at a standstill. FIG. 8 is a schematic sectional view for showing a sag amount in the continuous paper P after a reverse feed. 
     As described above, if the printing operation is initiated, the burster unit  200  starts operation at substantially the same time as the continuous paper printer  100 . Assume that the conveying speeds of the continuous paper printer  100  and the burster unit  200  are the same. Accordingly, as shown in FIG. 6, the sag amount in the continuous paper between the continuous paper printer  100  and the burster unit  200  keeps the same level all the while as was before operation started. 
     As described above, when a print command for one job is complete, if no print command for the next job is provided, the burster unit  200  stops the conveyance and cutting operation, and the continuous paper printer  100  only carries out the operation of conveying the continuous paper P. The continuous paper printer  100  conveys the continuous paper P from the transfer position A to the fixing unit  140 B to fix a toner image on the continuous paper P. On the other hand, the burster unit  200  does not convey the continuous paper P, and thus a sag amount in the continuous paper P increases between the continuous paper printer  100  and the burster unit  200 , as shown in FIG.  7 . In FIG. 7, the sag amount in the continuous paper P reaches a maximum. 
     Subsequently, the continuous paper printer  100  conveys the continuous paper P from the fixing unit  140 A to the transfer position A in a reverse direction, that is, carries out a reverse feed to get ready for the next job. This reduces the sag amount in the continuous paper P. Therefore, after the reverse feed, the sag amount in the continuous paper as shown in FIG. 8 decreases to the same level as that during printing operation as shown in FIG.  6 . As readily understood from FIGS. 6 through 8, it is only immediately before the reverse feed step that the sag amount in the continuous paper P exhibits the maximum value. As a result, the state that the sag amount in the continuous paper P exhibits an excessively increased value does not last, and thus the time during which the printed image formed on the continuous paper P is in contact with a floor or a saucer can be shortened. 
     According to the method of controlling the post-processor  200 , a sag in the continuous paper P for a reverse feed is provided before the reverse feed step, and thus during printing operation, the continuous paper P is kept from a contact with a floor or a saucer, thereby preventing image quality deterioration or dirty continuous paper P from resulting. Moreover, reduced influences of a wind from outside, or the like on the continuous paper P would provide stable transport of the continuous paper P, and high quality image formation. Further, the inventive control method can find out a reverse feed amount in advance from the structure of the image-forming device  100  in use, to determine a sag amount in the continuous paper P considering the reverse feed amount, and is thus suitable for a variety of the image-forming devices  100  each having varied reverse feed amounts. 
     Although the preferred embodiments of the present invention have been described above, various modifications and changes may be made in the present invention without departing from the spirit and scope thereof. For instance, the present invention is also applicable to an integrated image-forming unit in which the image-forming device and the post-processing unit are combined. 
     As described above, the method of controlling the post-processor connected with the image-forming device according to the present invention is configured to initiate conveying operations in the image-forming device and the post-processor approximately simultaneously, and then to stop the conveying operation in the post-processor prior to that in the image-forming device, thereby provide a specified sag in the continuous paper. Accordingly, concern for unstable sag formation in the continuous paper that has always caused a distress is no longer required, and contact of the continuous paper with a floor or a saucer may be prevented invariably. Consequently, the present invention makes it possible to form a high-quality image. 
     The control method according to the present invention may employ a conventional image-forming device and post-processor without any addition or modification, and thus be introduced easily. Furthermore, the inventive control method can accommodate wide-ranging alterations and modifications such as of the reverse feed amount, and the size of the continuous paper.