In the prior art, an image scanner mounted on a copier, a scanner, or a multifunction device having the functions of the former is known to have a document feeder (ADF), feeding a document from a document input tray to a document discharge tray through a feed path. In addition, a document feeder, which reverses the leading end and the trailing end of a document by inverting the document during feeding the document so as to scan the document on both of a first side and a second side of which images are printed and then feeding the document so as to scan both sides of the document, has also been known (for example, see JP-A-8-85649).
FIG. 49 shows a route of a document feeder of the prior art. As shown, a document P placed on a document input tray 100 with a first side (of a first page) is being directed upward is fed by a pickup roller 101 to a transfer path 102. In the transfer path 102, the document P is fed by suitably disposed feed rollers 103 so that its first side is scanned by an image scanning unit while passing through a scanning position X by image sensor such as CCD or CIS. When the document p having first side read is detected at its trailing end by a sensor, reversible rollers 104 are stopped while nipping the trailing end of the document.
As shown in FIG. 50, the document P is fed to a return path 105 by allowing the discharge rollers 104 to rotate reversely. The document P enters again the upstream side about the scanning position X of the feed path 102 through the return path 105. Then, when the document P is fed by the feed rollers 103 and passes through the scanning position X, a second side of the document P is scanned by the image scanner. When the sensor detects the trailing end of the document P of which the second side has been scanned, the discharge rollers 104 are stopped again in a state where they nip the trailing end of the document and then the document P is fed back through the return path 105. The document P entering again the feed path 102 through the return path 105 is in a state where the leading end and the trailing end thereof are reversed again, that is, a state where the first side is opposed to the scanning position X. The document P is fed along the feed path 102 and is discharged to the document discharge tray 106 with the first side facing downward. Accordingly, both surfaces of the first side and the second side of the document P are scanned and the document P is discharged to the document discharge tray 106 in the order in which documents are piled on the document input tray 100.
The pickup roller 101, the feed roller 103, and the discharge roller 104 rotate in a predetermined direction with rotary power from a motor. The pickup roller 101 and the feed roller 103 always rotate in one direction, that is, in a direction in which the document P is fed from the upstream side to the downstream side. On the other hand, the discharge roller 106 rotates in two directions so as to perform a switch-back feeding operation. For example, as shown in FIG. 50, when the document P is nipped by the feed roller 103 and the discharge roller 104, the document feeding direction of the feed roller 103 and the document feeding direction of the discharge roller 104 are necessarily equal to each other. When the document is nipped by the feed roller 103 on the upstream side of the scanning position X and the discharge roller 104, the document feeding direction of the feed roller 103 and the document feeding direction of the discharge roller 104 are necessarily equal to each other. Accordingly, for example, a motor for driving the feed roller 103 and a motor for driving the discharge roller 104 are provided separately, the feed roller 103 always rotates in one direction, and the discharge roller 104 changes its rotation direction at a predetermined time. Alternatively, an electronic clutch is provided to transmit driving power to the pickup roller 101, the feed roller 103, and the discharge roller 104 from a single motor and to cut off the driving power to rollers not involved in feeding the document P at a predetermined time.