[App_rpt-users] Garmin GPS18x no longer compatible with RTCM

[Joe Bennett]

I really doubt that a level converter will add enough timing skew to
amount to anything. The jitter you see on the internet will greatly
outweigh the skew of a level converter by orders of magnitude...


-Joe
KA3NAM

On Tue, Jan 26, 2016 at 1:44 PM, James Cizek <james.m.cizek at gmail.com> wrote:
> Hi folks,
>
> Wanted to get this out to you before anyone else bought one thinking they
> would work (since this model is specifically called out in the RTCM docs).
>
> When using a GPS with a Micro-node RTCM for voting,  you need to find one
> that has a 5 volt Pulse-per-second output.  The Garmin lines that is
> recommended (GPS 18X, 18X lvc, etc...)
> NO LONGER do this!
>
> This is what I got back from Garmin:
>
> Mr. Cizek,
>
>
>
> I was just told there was a design change with the GPS 18x LVC. The PPS
> signal voltage is now 3V. This was something we were not told until today.
> We will be making the changes to the online Technical Specifications to
> reflect this change.
>
> I do apologize for the inconvenience. This was a change we were not informed
> of.
>
>
> So this line of GPS's will *NOT* work with an RTCM any longer.  Talking to
> Mark at Micro-node, there is a voltage divider in the RTCM dropping the 5
> volt to 3.3 volt to be compatible with the CPU.  With only 3 volts on the
> incoming line, there isn't enough making it to the CPU to trigger a state
> change and you will get "PPS Timeout" errors and be unable to achieve GPS
> lock.
>
>
> I am not sure what kind of timing skew a level converter would introduce, so
> it's probably a better bet to just get a GPS that has a 5 volt line (Trimble
> Thunderbolt or the similar).
>
>
> There may be a possibility of removing or modifying the voltage divider
> inside as well, but without schematics, and unless you have SMD capable
> gear, it's unlikely that that is a good solution.
>
>
> Just wanted to let folks know so they don't repeat the brain damage I just
> went through figuring this out :)
>
>
> 73
>
> James
>
> KI0KN
>
>
>
>
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