From last nights run, it appears that only 63 of the 64bytes in the
garmin gps data packet is transmitted. The checksum, insures that the
packet arrived as transmitted. So I suppose, with some high confidence,
that the Garmin Gps/USB really sent 63bytes, rather that 64 bytes with a
1 in 64k chance that the checksum matches the bad 63byte garmin data
packet. I am :-{
Alan Stern wrote:
Don't be surprised if the data you see there is wrong. It's entirely
possible that the Garmin is sending it incorrectly.
Aren't there any device checksum's to catch broken device packets? Its
always the first byte of the garmin data packet that is missing.
There is a checksum. It's a simple 16-bit CRC, but it ought to catch a
missing byte (65535 out of 65536 times, on average). However it is
created by the Garmin, and if the Garmin assembles the output packet
incorrectly then the CRC will be calculated on the incorrect data.
Alan Stern
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