Norman Dunbar makes some magical things to make me read } Marcel wrote : } } >> I'm far from being a hardware genius, but I guess that as the end of } >> the cable is not terminated when there's no drive attached you get } >> noise on the data lines due to reflections at the unused cable end. } } Neither am I (as you probably all know) but how on earth can 'electricity' } get reflections from the end of a cable ? } I mean, it should just leak out the cut end shouldn't it ? :o)
No, it never leak! The signal propagates along the line, and when it reach the end of the line, it partially reflect (just like waves do on harbour) and then propagate back, crossing the new incoming signal... The bad thing is that the device in the middle of the cable is only able to detect the level of the wave, not its component part, neither which component cames from which direction. If you thing of electricity like of water, you must remember that waves DO NOT move the water horizontally, but mostly VERTICALLY. (unless there is a continuous current, but this is not the case with a signal transmission line). The device only has a small cork to monitor on the pool. If the canal is infinitely long, no problem as there will be no reflection. But if there is a wall someway afar, there is reflection, and to avoid mixing 'real signal' and 'reflected signal', the rate of transmission must be severly reduced. Slow rate being inacceptable for the computer industry, the wall must be made in such a way that it absorbs the signal instead of reflecting it. (instead of a vertical wall, a nice soft beach of sand... but very inpractacle for a harbour!) Single wire are not so nice, hence the need to correctly terminate the line with device or a resistor. To make things more complex, even the connection of the wire in the middle for the intermediate device will generate reflection, but it usually perturbate only the generator... hence one reason that IDE is restricted to Two devices per cable, whereas SCSI get a more subtle requirement and copt with reflexion a little better. (at price, of course!)
