A method and apparatus for transmitting print data signals serially to a printing device is disclosed. Print data for printing dots selectively line by line is stored as a plurality of bytes in which each byte represents the number of contiguous binary bits to be output to the printing device having the same binary value. The bytes are output to a comparator which receives the output of a counter driven by clock signals. The counter is started when a byte is output and the clock signals clock a data line held at a level representing the value of the bits. When the count equals the byte, the comparator toggles the data line so that it is at a level representing the next binary bits to be output. The apparatus is disclosed for use in a franking machine.