In message <[email protected]>, 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 [email protected] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
