A door operator to be disposed at a door leaf, a door frame, a door transom, or a wall, with at least one mounting plate, on which the door operator assembly is disposed. The assembly has a modular design and has at least one connector module, a drive module and a main module, which are latched onto the mounting plate by latching elements. The main module includes a system carrier, which accommodates at least one supply unit and at least one control unit, fastened by latching elements.