[time-nuts] SLIP vs Ethernet for NTP
bruce.griffiths at xtra.co.nz
Mon Oct 24 00:01:58 EDT 2011
Prell is the Danish equivalent of contact bounce.
Poul-Henning is a Dane so the odd Danish word tends to creep in.
Bill Hawkins wrote:
> 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.
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