Abstract: The platen (11) is mounted between two side plates (9-10) of equipment which is hinged to two risers (5-6) to rotate about an axis .gamma. running parallel to the axis of the platen. The platen is provided with a freewheel mechanism including an inner ring (28) which is connected to the platen and an outer ring (29) which is provided with a peg (31) engaged in a slot (8) in a fixed member (7). The slot is situated on the opposite side of the platen axis .DELTA. relative to the axis of rotation .gamma. and in substantially the same plane. Detector means (36-37) for detecting the presence of a document in an insertion channel (17) cause the platen to rotate in the direction in which the freewheel mechanism drives the outer ring, thereby pressing the peg against the slot and rocking the equipment about the axis .gamma. thus allowing a document to pass between the print head and the platen until it meets an abutment (14) which is moved into place by the rocking action.