Quote Originally Posted by irving2008 View Post
... those optos need driving and the parallel port on some PCs isnt up to the job.
Of course! I was forgetting some crappy PCs only output 3.3V
Quote Originally Posted by irving2008
The question I would ask is where are the LEDs and the 74HC14 being powered from?

Also what are the pull up resistors either side of the opto?
At the present they are being powered from the 5V rail of a smorgasbord bench PSU. I'll confirm the R values tomorrow.