Originally Posted by
cbcnc
The parallel port coming from your computer has several lines that are grounded. This is assuming you are using the proper power cables to your computer.
Unfortunately that is not always the case, the three cord supply cable coming into your computer is 120v line, 120v neutral, and service ground to the case.
The output of the PC switching supply is isolated and is not normally referenced (connected) to ground at the P.S. output.
It depends on the PC Manufacturer if any of the MB ground plane mounting holes are connected to chassis or just mounted on insulated stand-offs.
I use industrial style PC style units and I ensure they are mounted on metal stand offs so that the ground plane is at ground in all the mounting points.
So your Parallel port common is not always at ground potential.
It is your decision if you ground this at the port or at mounting or both.
In any case all grounding should be bonded to the service ground, shields and power suplly commons , if you wish.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.