On Sat, Jan 4, 2014 at 1:51 PM, Tom Van Baak <t...@leapsecond.com> wrote:
> > Pulse quality of single-ended RS232 over unbalanced twisted pair is going > > to be pretty bad beyond a few feet. If you want to transport the 1pps > over > > twisted pair there are a couple of options: > > Hi Brian, > > I suspect this is true at one level, but what would be helpful to to > *quantify* it. What is "pretty bad"? What is "few" feet? You are implying > that 1PPS timing is dependent in cable quality and cable length. I would > agree. But please provide some numbers, even rough numbers,... OK, in this use case it is easy to divide "good" for "bad". Bad means it does not work. I could not get interrupts to trigger. That is "poor" timing. If they reliably trigger then for NTP the details past that hardly matter. NTP can have very good timing using even not some good jitter on the clock because it looks at many clock poses over many minutes. Also on most PCs the time stamp resolution is one microsecond with about 2uSec accuracy. The shape of a PPS pulse does not matter, but they DO have to get to the PC and trigger an interrupt. If you only have a four wire cable, you are NOT using balanced pairs. For me, sending t/l level serial over 60+ feet of install cat-5 cable did not even result in reliable data transfer. Bosting the levels to RS-232 worked much better. Looking at waveforms on a scope using different lentghs of cable, will NOT be very instructive. Because in the real world a 100 foot cable is installed inside a wall and ceiling next to all kinds of real-life things also found in walls and ceilings, like fluorescent lights, other data cables, AC mains wire and "whatever". You'd get real clean signals looking at wire in a lab. But in a real cable the result might depend on the HVAC cycle or if it is day or night. -- 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.