This question is not GPSBabel dependent but I think I could get some help in this mailing list:

I am developing a software driver to communicate with GARMIN GPS. The driver is able to read the GPS' position and set a waypoint. This must/should work with all available Garmin devices. The device is connected over RS232.

Not such a big problem, if i would have enough information about the interface specifications.
Up to now i used the IntfSpec.pdf (
, Garmin Device Interface Specification - May 19, 2006) for developing the interface driver. But obviously this document doesn't contain all the needed information.
I tested my driver with several devices (eTrex Vista, GPSMap 60CS, GPSMap 76C and GPS 12 XL) and it worked well. But the thing which surprises me: GPSmap 60CSx (SW-Version: 3.00) behaves quite strange, so of course my driver doesn't work with that device.
When I set the device into DEMO-mode it sends some strange data every second without a request. The packet-id of this data is 0x99 - but this is not mentioned in the PDF I got.
Another thing: When I request the PVT-data from the device it first sends a packet with the id: 0x72 (this packet-id is also not listed in the PDF, what the heck is that?) and just afterwards the expected PVT-data 0x33.

For me it seems quite strange my device sends packets which are not mentioned in the manufacturer's official interface specifications.

Is there someone out there who knows more about this issue? Or someone who could support me with another interface specification PDF or a website with interesting information about the protocol or "hidden" packets?

Thanks in advance!

Fabian Braun