1. Field of the Invention
The present invention relates to a stacking control system for stacking sheets that are discharged from an image forming apparatus to a plurality of sheet stacking units.
2. Description of the Related Art
In recent years, image forming apparatuses that form an image on a sheet and that discharge a large number of sheets at high speed have been developed. Consequently, there is a demand that a sheet stacking apparatus which receives and stacks the sheets discharged from the image forming apparatus main body is capable of stacking a large number of sheets while maintaining stacking alignment of the sheets. Japanese Patent Application Laid-Open No. 2006-124052 discusses a sheet stacking apparatus (hereinafter referred to as a “stacker apparatus”) which responds to such a requirement.
FIG. 22 illustrates a cross-sectional view of such a conventional stacker apparatus 500.
In stacker apparatus 500, an inlet roller 501 receives a sheet which is discharged from the image forming apparatus main body. A conveyance roller pair 502 then delivers the sheet to a gripper 503. The gripper 503 grips and conveys the sheet, so that a leading edge of the sheet abuts on a leading edge stopper 504. When the sheet abuts on the leading edge stopper 504, the gripper 503 releases the sheet to fall onto a sheet stacking tray 505. At this time, the sheet falls between the leading edge stopper 504 and a trailing edge stopper 508, so that the leading edge and the trailing edge of the sheet are aligned. Further, a side edge of the sheet which is perpendicular to a sheet conveyance direction is aligned by a width alignment mechanism (not illustrated) as necessary.
In the above-described conventional stacker apparatus, if a number of sheets that are stacked on the sheet stacking tray 505 reaches the maximum stacking capacity, or a print job ends before reaching the maximum stacking capacity, the sheets that are stacked on the sheet stacking tray 505 become ready for removal.
Conventionally, in a case where a user wants to increase the stacking capacity, the user may use a plurality of stacker apparatuses that are connected to each other.
FIG. 23 illustrates two stacker apparatuses 500a and 500b connected to each other.
For example, it is assumed that the maximum stacking capacity of each stacker apparatus is 5000 sheets respectively in a case where a user connects and uses a plurality of stacker apparatuses 500a and 500b as illustrated in FIG. 23. Further, suppose that a user inputs a print job that prints 1000 copies of a booklet of 10 pages in a group mode. In the group mode, as a print job, M (where M is an integer) copies of each of one to N (where N is an integer) pages of images are printed, and N groups with M sheets respectively are stacked. In a case where a user prints 1000 copies of original images consisting of page one to ten, a stack of 1000 copies of a page on which the same original image is printed is created, and the batches of the 1000 copies are stacked in the order of pages. In such a case, a set number of sheets of each original page of the images is continuously printed, and the process is repeated for each of the pages.
When an image forming apparatus executes the above-described print job, 1000 copies of each of the first through fifth pages of the document are sequentially stacked on a stacker apparatus 505b. As a result, the number of stacked sheets reaches the maximum stacking capacity, i.e., 5000 sheets. Therefore, 1000 copies of each of the sixth to tenth pages of the document are then sequentially stacked on a stacker apparatus 505a. 
At this point, a user may start compiling booklets whose original consists of ten pages; however, the user cannot create the booklets. Even if the user takes out the sheet stacks that are fully stacked on the stacker apparatus 505b to the outside, sheets of sixth through tenth page of the document are still being stacked on the stacker apparatus 505a. Therefore, the user needs to wait until stacking of the sheets of sixth through tenth page is finished in the stacker apparatus 505a, which lowers the productivity.