[time-nuts] SLIP vs Ethernet for NTP

Bill Hawkins bill at iaxs.net
Sun Oct 23 21:50:43 EDT 2011

Fascinating thread.

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
of prell?

Bill Hawkins

-----Original Message-----
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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