[time-nuts] GPS USB dongle for time server
lists at rtty.us
Tue Nov 9 01:39:47 UTC 2010
The gotcha is that the NEMA string has a poorly defined transit due to the USB stack. The jitter is still there. On a loaded system it can be fairly large.
The next layer to the onion is that the location of the PPS within the NEMA string is also poorly defined on a lot of the cheap(er) hardware out there. There are some devices where it's off by a large fraction of a second.
Better to figure out a way to get a "real" pps into the box.
On Nov 8, 2010, at 8:14 PM, jim s wrote:
> This is probably the thousandth time this was asked, but I googled and didn't get a direct answer.
> I want to do a crude (as in to the second or so) time server inhouse to add into a group of high accuracy servers. This is so that I can go off grid and still get updates.
> I see that there is a way to get a feed called PPS or with PPS via RS232. The discussions of using USB instead are concerned with having too much jitter.
> since USB by its nature won't have an accurate exact dedicated line to let the GPS toggle to do a time hack to the software, I can see why RS232 is preferable with the hardware signal lines they have.
> If I just go with the NEMA stream as it gets to me via an USB HID Com port, I assume that there will be some jitter baring a way to send in the physical time hack.
> Any pointers to reading or comment on this would be appreciated. I am probably asking the query before I should, as far as research, and appreciate any replies.
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