I have this issue as well apparently its pure luck if your pc lpt port works or not with the cnc connection as on previous threads I have found one guy had 4 dell pc's all the same module and spec yet 2 of them worked and the other 2 didn't (apparently down to the different chips used on motherboards)

only way round is to get a controller to replace the input such as UC100 USB, UC300 and UC400 Ethernet.

Regards
Graham