  There are many possible interface circuits.  I will show a very
simple circuit/s that I used.  Note: It depends on how you use LPT.

  Note: Since parallel ports in older computers compared to newer computers
are not exactly the same, it is possible that you will have to modify the
circuit.

  "[10k]" = 10K resitor, "PP" = Parallel Port

  INPUT CIRCUIT:

  PP Pin 12 <----[10K]----< TTL device (0/+5V) Recording digital data.

  For "TTL device", one device can be an IR receiver module from Radio Shack.

  OUTPUT CIRCUIT:

  PP Pin 2 or 3 or...9 >----[10K]---> Gnd

  Put a scope (or other) accross the 10K resistor.

  COMMENT:

  I used a laptop that I bought new in early 1998 with above simple circuits.
A magazine called "Poptronics" (formerly "Electronics Now") occasionally
publishes an I/O project. I am sure there are books on I/O projects (possibly
other magazines).
