Using only parallel ports ?

Is it practical to attempt configuring a 6 axis machine using
only parallel ports (2 or 3 boards) for step motor and encoder
IO or should I invest in a more specialized support board
in order to get acceptable performance. The latency of my
PC is about 20,000 ns

There is a recent discussion in the email lists on the limitations of software encoder counting (you can search the archives for details). If you dealing with a home hobby machine, you may be able to live with the speed limitations of the parrallel ports. If this is for production, why short yourself. You might investigate the Mesa or Pico products and costs first before considering further. the price may not be all that bad for you.