Re: [CAD_CAM_EDM_DRO] Parallel Port to Power Supply Grounding Differential

Bill Kichman wrote:

>Thanks for the clarification... I am also planning to use Mach 1 or similar
>app, and will need to interface the limit and other safety circuits, which
>makes the necessity to connect to an outside ground a greater possibility.
>

Don't do that! Make the limit, etc. switches completely isolated from the
machine frame. Use ground or +5 V as needed to power the switches, but
connect it only to the computer. (Even better is to use some form of
opto-isolation for sense inputs, but it isn't mandatory.)

>Bear with me while I go over my setup.....I am using (2) toroid transformer
>power supplies with secondaries completely ground isolated from the AC
>supply, and currently the DC negatives are chassis connected. At present the
>AC chassis ground is not connected to the case (although likely should be
>eventually for safety). I took a look at the parallel port grounds, and
>they are tied in common with the PC chassis ground, at the backplane metal
>connector.
>If I were to install a separate permanent cabled ground, say a stranded
>#14AWG, between the PC and the DC supply ground, and ensure it was always
>connected before powering either device, wouldn't that take care of the
>problem?
>

Yes, that should keep the ground differential very low, unless there is
a short
somewhere. Your #14 wire could still drop tens of volts if a motor wire
were
to short to the machine frame, for instance. The safety ground wires should
carry the current and prevent electrocution, but the computer would not be
so lucky. So, I recommend complete isolation between anything on the
machine
and the machine frame.

> That is, if I ensured I am powering from the same AC socket, can't
>I reasonably expect the grounds to be at the same potential, although there
>will possibly be some ground current through the ground connection?
>Additionally I can use a laptop possibly as a controller, which is fed from
>an isolated wall cube of course, so isn't that similarly approached?
>
>

Yes, the laptop is probably better protected as it doesn't have a wall
socket ground.

Jon