Hi! I've just checked the high and low voltages on the port by setting pin D0 to 1 and 0 and they are 4.65V and 0.05V respectively, so this seems fine. I've also done a check on the port ground and the battery supply ground by connecting the two via a 10K resistor and measuring the voltage across it and... it reads 0V! This makes perfect sense as ground should mean ground whatever the device. When doing this test before, I think I simply had my wires crossed..! ;o) Ah well, you live and learn as they say!
As a bit of background, I am building my own design for a general purpose data aquisition unit for automotive use with sensors for tachometer, temperature, pressure, etc. The software is my own written in VB for display and data logging. The interface is the 'glue' between these two 'components', which now that these few niggles have been ironed out I can forge ahead with! ;o)
Thanks again for your help!