On Tue, 2006-11-21 at 12:04 -0500, Gene Heskett wrote: > On Tuesday 21 November 2006 09:33, Mel wrote: > >Alan wrote: > >> Controller<=====Longest Part of cable===>Drive2<===Short > >> bit===>Drive1 > > > >If I remember my cable theory correctly, all cables should be terminated > >in their characteristic impedance at both ends. If not, then reflections > >occur. > > > >The higher the frequency, the greater effect they have on reliable use > >of the cable. As you said, cable length is also a factor. > > > >Isn't this what "Time Domain Reflectromitry(SP?)" is based on? > > The spelling will do, its reflectrometry, and the answer is yes. The > problem is that to do it on cables as short as the 18 inchers used for > ide connections, it takes some very expensive versions of it since one > must be able to see, and detect an anomoly thats only 1/8" wide. There > are fast samplers, and there are damn fast samplers. For longer stuff, > I've used just a schotkey ttl gate as the signal, and a 100mhz scope to > read it. There I can see with an accuracy of maybe 10 feet over 500 with > that simple a lashup. With the real thing, I can see the individual > connectors and insulators in a length of 3.125" transmission line but I > can only see burnouts with my simple lashup because its not fast enough > by a factor of 100 or so. > -- > Cheers, Gene > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > Yahoo.com and AOL/TW attorneys please note, additions to the above > message by Gene Heskett are: > Copyright 2006 by Maurice Eugene Heskett, all rights reserved. > There is a difference between how digital data is handled and analog data, IF the cable length is <=1 wavelength. Digital drivers are designed to work into an unterminated line, to produce a voltage high when the switch transition reaches the end of the cable. This works because the driver is back terminated with the line impedance (between 20 and 1K ohm depending on the line design), which means that the signal input end instaneously transitions to V/2, propagates at that level to the "open" end, then rises to the input level (as long as the line leakage is sufficiently high and the transmission q is >=10. IDE generally meets these requirements, until you begin to drive it at about 40-45Mhz. At the latter frequencies and higher, IDE must be terminated and the line voltage will be lower. Some of the problems with open digital drive are: 1. radiation due to the harmonic coupling of reflections 2. excess power required to fully charge the line. 3. Pin to pin leakage of the high frequency components. This may be avoided to some degree by controlling the edge rise and fall times as some drivers do. This is my area of expertise for over 40 years of measurement and analysis. Regards, Les H -- fedora-list mailing list fedora-list@xxxxxxxxxx To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
