Quote Originally Posted by PapuaPilot View Post
I'm not totally sure I understand all of your questions, but this may help.

The GPS-20A is your WAAS GPS source required for ADSB-out position and possibly IFR navigation. It's not the altitude source for the transponder as GPS altitude is not overly accurate. The altitude source for the transponder/ADSB and the display(s) (GDUs) comes from the ADAHRS (GSU-25). The GSU-25 provides pressure altitude, which is corrected using the altimeter setting and is very accurate.

I'm not sure how all of these hook up and communicate with each other with the GTX-45R. I have a different transponder. My GPS-20A connects directly to the transponder using a RS-232 channel and sends its data on the CAN bus too. The CAN bus connects to both the GSU-25 and GDU-465. Therefore using both the CAN bus and RS-232 the WAAS GPS position is provided to the entire G3X system.

You are going to need to look carefully in the installation manual chapters 26 and 27 to find the answer to this.

Thanks! Back to reading....

You understood my question\s perfectly. I was assuming the GSU-25 was not needed because I thought WAAS altitude is more accurate than pressure altitude and it seemed kind of silly to provide pressure altitude to the transponder when it already had more accurate altitude provided by the GPS. But sounds like in this configuration its not as accurate so altitude is provided by GSU-25.

So I found on the FCC website the installation manual for the GTX45R that is version 2. Garmin only list version 1 on their website??? So anyone installing a GPS20A and a GTX45R or here is the interconnect drawing. Also if you are wondering what the A/c Suppression BUS is? Answer listed below.


Thanks for your help PapuaPilot. I am running the GTX45R transponder and it does not appear to have a CANBUS connection. I am assuming that the GTX45R communicates to the G3X via the 232 that I hook up to it. Seems weird there is no canbus to it?

Also once you have the GPS-20a connected properly does it provide position source to the G3X if I am not running anything else like a GTN650 or GNS430/530 ect?

Last thing to figure out. I think the GPS-20a can provide gps position to my ELT? Next thing to figure out. I need to dig into the ELT next but maybe the external suppression is used?

The EXTERNAL SUPPRESSION is intended for connection to other L-band equipment, such as a DME
or UAT. The output is active whenever the GTX 35R/45R transmits. When driven by another source, the
GTX 35R/45R transponder receiver and transmitter functions will be suppressed.

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