text: 1. Field of the Invention
  The present invention relates to image forming apparatus and, more particularly, to such an image forming apparatus as a page printer having a sheet post-processing unit such as a sorter or mail bin into which sheets printed by the image forming apparatus are discharged.
  2. Description of the Related Art
  In a conventional image forming apparatus such as a page printer, a sheet post-processing unit such as a sorter or mail bin for sorting printed recording sheets is connected to the main body of the image forming apparatus, thereby reducing the operation to be performed by the operator. As an example of this type of image forming apparatus, there is one in which a sheet post-processing unit is disposed on and fixed to one side of the main body of the image forming apparatus. In the case of disposing the sheet post-processing unit on the side of the main body of the image forming apparatus, however, an installation area is increased disadvantageously. As an image forming apparatus free from the disadvantage, there has been known one in which the sheet post-processing apparatus is disposed on and fixed to the top of the main body of the image forming apparatus, whereby the installation area is reduced (see, e.g., Japanese Unexamined Patent Publication SHO No. 62-201775, Japanese Unexamined Patent Publication SHO No. 63-160972, and Japanese Unexamined Patent Publication HEI No. 8-208108).
  However, the conventional image forming apparatus is allowed to have only one configuration in which the sheet post-processing unit is fixed either to the side or to the top of the main body of the image forming apparatus. In the image forming apparatus, therefore, it is automatically determined whether sheets are discharged with their printed faces facing upward or downward so that a sheet inverting mechanism is required to be switchable between the sheet discharge mode in which the printed faces of discharged sheets face upward and the sheet discharge mode in which the printed faces of discharged sheets face downward. This increases the complexity of the structure, leading to higher cost. In the latter image forming apparatus wherein the sheet post-processing unit is disposed on the top of the main body of the image forming apparatus, if an optional device such as an additional sheet supply cassette is mounted to the bottom, the height of the overall apparatus is increased and the operability of a sheet discharge portion of the sheet post-processing apparatus is degraded, which prevents the apparatus from being placed on a desk.