"unruh" <[email protected]> wrote in message
news:yb%[email protected]...
[]
I was expecting that, for timing purposes, the four pairs would be used
as:
- TXD/ground
- RXD/ground
- DCD/ground
- unused
i.e. that every relevant RS-232 line was carried with a ground on the
twisted pair. This would be better than some random "pushing of pins"!
The problem of course is that all of those grounds are common grounds at
the DB9 interface ( and probably at the gps as well) which means that
the ground current would be carried on all three ground wires, giving a
common mode signal on each pair.
But I think that would be less of a problem as:
- the signals are carried on individual twisted pairs
- ground is now 1/3 of the resistance it was before
If possible, I would not be earthing the GPS other than via the cable (and
use the unused pair to carry +5V to the GPS).
I would, of course, be interested in any actual waveforms, which have been
unfortunately absent in this discussion so far. We have no idea whether
the problem is related to the cable length or not, and if so, why.
Cheers,
David
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