[time-nuts] GPS USB dongle for time server

[jim s]

On 11/8/2010 5:14 PM, jim s wrote:
> This is probably the thousandth time this was asked, but I googled and 
> didn't get a direct answer.
>
Here is the reference to the PPS that made me make statements about RS232

http://forums.freebsd.org/showthread.php?t=16507

Here is an actual implementation using a Garmin 18LVC, which was 
referenced by another poster.

http://time.qnan.org/

The difference between the RS-232 and the USB is that there is a wire 
(DCD or CTS) so you can do a precise hack if you can overcome the issues 
of system load, etc.  Perhaps a driver patch to grab the exact system 
clock time in a driver when either changes would be one method, immune 
to system load.

No way to do so as pointer out on USB.  It has not only some difference 
depending on the number of USB hubs, but also total number of devices.  
Assuming you hack the USB driver to get a time hack exactly when a 
certain string comes from the USB, you would need to figure out how to 
estimate the latency from the device to the interrupt service routine to 
eliminate that, as it would jitter.

The other thing I didn't state was that I got the USB receiver out of a 
discard frenzy at the place I'm working, and got it to work with a 
freebe program on windows XP.  It's a Holux GM-210.

They do make an RS232 version, but I got the USB one.  It says it uses 
the Sirf Star II in the document.  I suspect if one pried open the box 
it would have pinouts for RS232 in the pod I have.

I suspect it was given to them as a starter kit for a project way before 
I arrived.

Jim