On 06/25/19 19:35, William Unruh wrote:
On 2019-06-25, Chris<xxx.syseng....@gfsys.co.uk> wrote:
...
Thanks again for the replies. Did a bit of digging this morning and
find that the 1pps sync stuff has been done before. Well, many
years ago in fact and more or less how I had visualised it - ntp
data augmented by the 1 pps signal. Several pointers to the way
forward and it's also supported in the FreeBSD kernel, using either a
pin on a serial or parallel port for the 1 pps. Probably go for
the parallel port, as that avoids the hardware to convert 1 pps
ttl to rs232 levels.
Except most serial ports actually used on computers handle ttl levels
just fine.
True, but would not rely on that sort of hack for anything serious,
as it won't have good noise immunity for what is a nS scale timing
signal. The 1pps from the gps is at ttl or cmos level on a different
piece of kit in another box. A diff line driver and receiver might
be the best way, though that would need a bit more hardware. Devil
is in the detail as usual, but just need to get it working to start
with.
Stratum 1 says nothing about accuracy. You could have stratum 1 come off
smoke signals and have an accuracy of minutes, and a stratum 7 have an
accuracy of usec. All it indicates is how many steps the time server is
from a hardware time source. It says nothing about how good that time
source is, or how good the connection is between servers.
The docs i've looked at seem to say that stratum 1 is generally
assumed to be caesium or gps, though the number of links would affect
it. Just trying work it out so what appears on the net is as good
as the standard it refers to...
Chris
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