farren@well.UUCP (Mike Farren) (12/21/89)
In article <8910@cbmvax.UUCP> valentin@cbmvax.UUCP (Valentin Pepelea) writes: [ talking about differential vs. single-ended signal transmission] >Not quite. Adding the inversion of the inverted signal to the positive signal >will result in the doubling of the amplitude of the signal. However, the noise >is in no way eliminated. It too in fact increseas in magnitude, but does not >double because it is random by nature. That means that some times you get >spikes which are as high as double in usual magnitude, and sometimes you get >lucky ang get a cancel out. This would be true if the noise that one were concerned with were only internally generated random noise. However, the noise that differential transmission is designed to deal with is not so much that as environmental noise, that imposed upon the signal as it travels down the cable. That noise will tend to be imposed on both the non-inverted and inverted signals more-or-less identically, and will cancel out on the other end. Example: suppose a lightning bolt strikes two inches away from your snazzy 1000-meter SCSI differential cable. The data lines were at +12 volts, and the inverted lines at -12. The lightning bolt adds a one million volt spike to every signal on the cable. The result: the data lines are now at 1,000,012 volts, and the inverted lines are now at 999,988 volts. The differential is still just 24 volts, just as if there were no spike whatsoever. Of course, you've just blown both the disk drive and the computer to smoke, but the SIGNAL is o.k. :-) -- Mike Farren farren@well.sf.ca.usa