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To: gnuman who wrote (72321)	5/9/2001 1:54:31 PM
From: Bilow	Read Replies (1) \| Respond to of 93625

Hi Gene Parrott; The error in your argument is this line: "The wave in the direction of the controller subtracts 15 ma, so we now again have a 0.4V signal heading for the controller." Think about this one carefully, and I'm sure you'll see the error. (Basically, what you're saying is that two wrongs make a right, LOL.) Or that dumping "+" charge onto a wire can cause its voltage to go down. When two positive current sources are simultaneously injected into a transmission line, it is true that the wavefronts that approach each other are going in the opposite direction, and that therefore the currents are opposed. But when the wavefronts pass through each other (or collide and bounce off of each other, depending on how you want to look at it), the voltage steps add, and since they are both positive, they do not cancel. Instead they double. The situation is equivalent to the voltage doubling that occurs at the end of an unterminated transmission line. The currents add, and since they are opposite in direction, they cancel, just as you said. Maybe a way of thinking about this that would be useful for you is to remember that in the case at hand, since it is entirely linear, the law of "superposition" applies. This means that you can solve the problem for the wave forms from a single current pulse from RDRAM chip "A", and solve it again for a single current pulse from RDRAM chip "B". After solving the two systems for current and voltage at each point in the bus at each point in time, you can then add the resulting solutions to get a solution for the combined current sources. When you do this with superpositions, it is obvious that the voltages add, and that therefore the voltage peaks at 3 * 0.4 = 1.2 volts. If you simply write out the wave forms from the two RDRAM chips for the situation I described, you will see the obvious 1.2 volt bump. This should not be a seriously difficult homework assignment for you, but trying to analyze electronics (or physics or math) with words is a waste of time, written language is not the natural language for this reasoning. Instead, just draw out the waveforms. -- Carl