Hi What ever degradation the serial stream sees on the LAN, the resulting NTP output will see once it's on the same LAN. It's unlikely you will see more than a 2:1 net degradation no matter what is going on. The flywheel in the NTP algorithm will likely help you in this case to actually improve things a bit.
Bob -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Chris Albertson Sent: Wednesday, May 23, 2012 3:13 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Serial port server .. any interest in a write up on using ? On Wed, May 23, 2012 at 9:08 AM, Bob Camp <li...@rtty.us> wrote: > Hi > > If the timing involved is NTP, I'm not so sure that a normal home lan with > gigabit switches would be a problem. You can indeed saturate the poor thing. > Unless you have a very unusual system, it is unlikely you will saturate it > for very long or saturate it very often. NTP is pretty tolerant of the > occasional burp of a few ms. Depends on your accuracy goals. Of course it would work to some degree. But we are talking several orders of degradation if the PPS second is carried over even a dedicated either net that uses only a crossover cable. The problem is not so much the network but the buffer in the ethernet interface. This is the same thing that happens when petiole try and send the PPS over a serial to USB cable. Those cables typically cost you two orders of magnitude timing error. The reason the direct connected PPS is so good is because of the simple hardware design and the even simpler software interrupt handler. There are only four of five lines of code that get executed. It is so simple. Both Either net and USB are "packetized" where the data arrive in chunks and the entire chunk has to get inside the computer before it can be looked at. While PPS to a serial port is just an edge triggered logic gate that forces an interrupt. Again you can do it but you take a 10E2 or 10E3 "hit". If you do need to send a PPS over Ethernet cable you can do it, but don't use networking. Cat-5 cable has "extra" pairs that ethernet does not use. Those can be used to send a balanced, differential signal down the wire. A pair of RS-422 transceiver chips and terminating resistors would work well for this. (Don't try to send an unbalanced TTL level pulse down 100+ feet of Cat-5 cable, it works very poorly.) This is all easy to test. All you need is uSec level test gear. You don't need hyper expensive counters. Compare the transmitted PPS to your Thunderbolt's PPS or simply look at NTP's log files. -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.