Re: [gpsd-users] GPSD, PPS, Garmin 18x LVC and 93 meters of serial cable

[gpsd]

On 25/11/2014 8:47 PM, Sander Smeenk wrote:

Hi Sander,

>
> I'm trying to use a Garmin 18x LVC GPS receiver to provide a stratum1
> time source in our network. The receiver is connected to the server by
> 93 meters of CAT6 UTP over which the GPS' serial outputs are
> patched/connected.
>

Whoa!

A cable length of 93m is WAY outside the spec for conventional RS232 (30 
feet or 10m).

The 18LVC doesn't use RS232 signalling levels (-30 to -3; +3 to +30) 
either - it outputs 'TTL' level 232 (0-5V) - so I'm surprised that 
you're getting any sane data at all, and the skew on your PPS pulse must 
be awful.

Before fighting with GPSd, I would recommend that you get your serial 
cabling right.

I can think of 2 ways:

1. Move the timeserver hardware up to the GPS, use PoE to power it and 
Ethernet link back down to your network. Some hardware like a Raspberry 
Pi or Gumstix (or maybe even a Beagelbone - haven't used these though) 
would all work fine as a timeserver. Then build GPSd and all should be good.

2. If you can't move the timeserver hardware, you're going to have to 
use differential line drivers at each end of your cable to get the RS232 
to travel the distance. Basically you need three RS232 to RS422 
converters at each end - one for TX, one for RX, one for PPS. This is a 
pretty standard setup for transmitting RS232 over long distances.

I'm not sure what doing the RS232-RS422-RS232 translation is going to do 
to your PPS timing though. Gary Miller is likely to have better thoughts 
on this than me (he's the Garmin Guru!).


ciao, Dave


> TX/RX is working fine, i get clear NMEA sentences from the GPS, however
> the pulse is failing and that's exactly what's most important. ;)
>
> The GPS does have a 3DFIX et al, so it's not that it doesn't have any
> fix to pulse from.
>
> I've tried to measure the pulse and it came up with 4.84 volts, however
> the server is not registering the pulse. Also, gpsd 3.11 when starting
> up logs "pps":false consistently while on another box with gpsd 3.10 it
> always starts with "pps":true and gpsmon sporadically registers (logs) a
> PPS pulse.
>
> What makes gpsd 'enable' the PPS 'subsystem'?
> Why is one box (3.10) seeing/trying to see it while the other one (3.11) isn't?
>
> I'm now reading about the ground pins not being exactly zero volts, that
> might cause the signal not to register correctly for RS232-specs?
> Perhaps the one box is more 'sensitive' than the other? I'm just
> guessing...
>
> Thanks in advance for any input!
>
> -Sndr.

--

David Clement
Renfell Technology Pty Ltd