On Wed, Aug 20, 2003 at 08:09:20AM -0800, Elbert Smit enlightened us
thusly
> Hello members,
>
> I've encountered a strange problem which I would like to share with
> you.
>
> I'm working on a device with a build-in modem. It uses a AC/DC (220V
> to 5V) converter as powersupply. Somehow the modem does not work
> correctly and does not want to pick up the line. Connection with the
> processor seems to be OK, both timing and message content.
>
> The following strange thing happens: Only connecting the ground of a
> scope probe to the ground of the PCB makes the modem work. I suspect
> it to be a grounding problem, but don't have any clue how to solve
> this. The PCB has a full ground plane, so this must be good.
>
> Any ideas?
>
As has been said, ground to a single point.
Also be aware that the dc/dc converter will give a floating voltage,
i.e. it can sit at ground or at +100V or wherever you drag it. It would
only have to go a small distance away from zero to cause trouble. Ground
in these devices is usually a plane for insulation purposes, and not
connected to input or output.
If you have no internal ground in your box, (you seem to be
prototyping) consider earthing the outside of any "D" connector and
providing a ground that way.
In addition, with modules like this, sensitivity can be added by
mismatching logic types - 74hc to 74ls or the like.
BTW, if your grounding is successful, how will you make out with a
signal which isn't grounded? I have seen grounding problems rip through
a complete set of pcbs. We switched off; we switched on; we switched off
again; we switched on again, and everything blew up, smoked, peeled
tracks and sat down in a huge way. There was a 350uF/560V capacitor
bank, with a ground lead missing, and provided by my oscilloscope
negative clip attached to a tender part of the board's anatonomy :-/.
So we got the charge-up pulse for the capacitors directed into the pcb
ground plane. The most amusing thing was the face of the inventor :-D.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
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