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

[James Cizek]

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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