[time-nuts] SLIP vs Ethernet for NTP
bill at iaxs.net
Mon Oct 24 01:50:43 UTC 2011
Poul-Henning Kamp mentions "contact prell."
Google can't find it. Even quoted, I get shampoo and people with that name.
I understand well contact bounce and contact dwell, but what is the meaning
From: Poul-Henning Kamp
Sent: Sunday, October 23, 2011 1:40 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] SLIP vs Ethernet for NTP
In message <4EA45815.5080705 at earthlink.net>, Jim Lux writes:
>I'd have to go back to some pretty old
>databooks, but I'll bet the x8 thing has been around since the 70s. Why
>8, and not 4, is a better question...
The original standards text describes this in some detail, but I can't
remember which one of them it was (Not V.24, possibly V.28 ?)
Since the other end might be electromechanical, the system had to
be imune to a rate tolerance in the several %, as well as flank-jitter
and contact prell.
With 4x oversampling, your sampling point on the start bit
would be somewhere in the [37.5...62.5]% interval.
A 2.5% rate difference would eat 25% over 10 symbols, and you would
be left with +/-12.5% for jitter/prell.
8x oversampling gives you +/-18.75%, a full 50% better.
It was argued at the time, that the sampling point of the start bit
should be 75% into the start bit, because the prell is not symmetric,
but this was not adopted.
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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