Abstract: The device embodying the invention is intended to equip a processor-driven printer comprising a hopper capable of containing and supporting a freely rotating roll of paper, a guide and drive means enabling the web of paper to be led from the roll to a drive device, to a line by line print station and then to an output orifice. It comprises at least one first detector capable of detecting marks affix on the web, in an area not used for printing. The invention applies to a printer of the type used in man/machine dialog or in remote transmission.