I only use this Garmin (GPS18x LVC, 5m) and have no problem with the RTCM.  I got most of them many years ago but the last one I bought in 2016 and it worked fine also. I just hook them up and they work.  I have not looked at the pulse with a scope.  I recall there is a JP4 that can be changed in the RTCM.  In one position, (1-2), it has thresholds of less than  0.8v and greater than 2.0V and is +-25v tolerant and in the other default position (2-3) it should be standard TTL input and 0 to 5 volt tolerant. 
Chuck

On Thu, Nov 16, 2017 at 9:58 AM, Lee Woldanski <ve7fet@tparc.org> wrote:
Have you looked at:


Specifically, does this Garmin use 5V or 3.3V logic?

I believe the RTCM uses 5V logic only (the VOTER) will do either. 

If your Garmin is 3.3V, it may have issues.

Also, make sure the baud rate is correct, and it is configured to send the required NMEA sentences.


Lee

On Nov 16, 2017 12:40 AM, "Doug Kingston" <dpk@randomnotes.org> wrote:
I can't get the RTCM to recognize the attached GPS unit.

I have determined that pin6 on the DB15 of the RTCM is transmit data to the GPS (using an oscilloscope) and that's hooked to the RxData line of the Garmin (green).  Will the Garmin respond if it does not have lock?

RTCM diagnostics complain:
Running Diagnostics...

Warning!! VF Diode NOT calibrated!!!

Testing PTT/External CTCSS
Error - Failed to read PTT/CTCSS in asserted state
Testing GPS UART
Error - Failed to read data from GPS UART

Any hints? or ways to get more diagnostics?

-Doug-