[time-nuts] Serial port server .. any interest in a write up on using ?

Hal Murray hmurray at megapathdsl.net
Wed May 23 00:28:47 UTC 2012


lists at rtty.us said:
> I'd bet at least a cold order of french fries that you would be below 1 ms
> on a modern home wired LAN. Certainly everything I ping locally is sub 1 ms
> unless wireless is involved.

The delay doesn't matter much.  You can correct for it.  I can measure sub ms 
even over a wifi link.

The jitter will be the problem.  That will depend upon the load and your 
setup.

Here is an interesting sample:

64 bytes from xo-c2 (192.168.1.105): icmp_seq=7 ttl=64 time=1.66 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=8 ttl=64 time=1.63 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=9 ttl=64 time=1.58 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=10 ttl=64 time=49.2 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=11 ttl=64 time=67.9 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=12 ttl=64 time=1.59 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=13 ttl=64 time=1.58 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=14 ttl=64 time=63.6 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=15 ttl=64 time=50.0 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=16 ttl=64 time=1.64 ms
64 bytes from xo-c2 (192.168.1.105): icmp_seq=17 ttl=64 time=1.71 ms

Note that 4 samples are way off scale.  The target machine is idle, but the 
packets are going over a wifi link that has other traffic but is far from 
overloaded.  Yes, this is wifi rather than a simple ethernet switch.

I suspect I could make similar problems with a pair of ethernet switches.



pete at petelancashire.com said:
> I've never though about using one to distribute the 1PPS for NTP. Its a pity
> there isn't enough umph inside one of these little Linux boxes to implement
> NTP. 

If it's smart enough to run Linux, it should be able to run NTP.  If you have 
an example that doesn't work, please contact me off list.

The usual problem is that there isn't a handy way to get a PPS into the box.  
Depending on your goals, PPS over USB might work.  gpsd uses an ioctl to 
wakeup on modem signal change.  It works for some USB serial chips.  So a box 
with only USB can probably keep time in the low ms range.  (Beware of hanging 
bridges.)



-- 
These are my opinions.  I hate spam.






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