BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of an image forming apparatus; FIGS. 2A and 2B are a block diagram of the image forming apparatus; FIGS. 3 and 4 are block diagrams of a multioutput unit; FIG. 5 is a timing chart; FIGS. 6A to 6C are flow charts of control sequences of control units 19, 1403 and 1304 in FIGS. 2A and 2B respectively; FIGS. 7A and 7B are a blo