[ntp:questions] Re: Garmin gps18: The best DIY gps refclock?

Roman Mäder newsXXF.10.rmaeder at spamgourmet.com
Fri Nov 25 15:17:07 UTC 2005

Terje Mathisen wrote:

> I decided to look into this interesting unit, both due the the low
> price, and the fact that nearly all my other refclocks are Motorola
> Oncores.
> The gps18lvc is an OEM model of the Garmin 12-channel/WAAS receiver,
> packed together with an antenna into a small puck device. The PPS signal
> is specified as "< 1 us", which means that the stability of the server
> system clock will determine the actual performance.
> At the other end of the (default) 3 m cable there's just a set of
> colour-coded wires, carrying rs232-level send/receive/ground & pps
> signals, as well as a pair of (slightly thicker) red/black power supply
> wires.
> I got the gps in a couple of days from a local distributor here in Oslo,
> and picked up a 9-pin female rs232 connector with housing and a 1 m
> small-size USB cable: Total cost was less than 1000 NOK, in the US I'm
> guessing you could get the same for approx $100.
> ...
> I then cut the USB cable and identified the +5V power wires (also
> red/black coded!), I cut off the USB signaling wires with a small offset
> between them to avoid any risk of a short circuit.
> The only remaining task was to solder the signaling wires onto the rs232
> connector:

it never occurred to me to use USB power, good idea. I used an old mobile
phone charger that happened to output the correct voltage. My unit is over
10m away from the computer room, so I made the connection with CAT5 network
cable that terminates in a RJ-45 wall socket, from where a patch cable
leads to an RJ45-DB25 adaptor. These adaptors come with 8 pins connected to
the RJ-45 socket that can be inserted into any position in the DB25 plug,
so it is easy to make nonstandard connections. I drilled a hole for the
power leads and soldered them to the phone charger. Works fine for me.

With temperatures stable, jitter can be as low as 0.001ms.

Roman Maeder

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