On 2012-01-10, David J Taylor <[email protected]> wrote: > "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.
Agreed waveforms would be nice. I guess the problem could be common mode interference. Eg, if the RX line carries a 1 bit (-12V) enough of that -12 volts could spill over onto the PPS line. to lower the pulse height that the serial card would not recognize it as a pulse. But as you say, a waveform on the CDC line would be nice. Not only of the start of the pulse itself, but also of the full one second to see if there is crosstalk from the other lines. It should not really matter since the nmea sentences usually come between the PPS pulses, not overlapping, but the oncore uses its own system and its output may overlap. Better might be to carry the PPS on a completely separate line from the serial signals to reduce crosstalk. > > Cheers, > David > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
