you could just use a paralell to serial converter such as what you can get from tandy few dollars and you can use the 5v power rail in your computer for the supply, reduces curcuitry, you say serial input in the question so i presume you only want to go from the computer to your m/c if you want bi-directional communication this complicates things lots, it would be eisier to go and buy a serial card for your pc, you can get pci cards for about AU$60