Opto Isolators & Parallel Ports

I had a person contact me off the list and asked the following:

>> Do you know where I might find information on how to opto isolate the

interface between the drivers and the parallel port? Am now using 4n28's
but these are not working well. <<

Being electronically challenged, I thought I would post tot he list and see
if anyone can help me to understand how to optically isolate the parallel
port.

Thanks,

Tim
[Denver, CO]