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Technology Stocks : Rambus (RMBS) - Eagle or Penguin
RMBS 92.72+5.2%Nov 24 3:59 PM EST
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To: John Walliker who wrote (32524)10/22/1999 2:26:00 AM
From: Alan Bell  Read Replies (2) of 93625
 
John,

I guess I don't see where you would get a 1.5x signal. Lets look at the drivers in the memory chips, not the ones in the RAC which operate differently. The Rambus drivers are current sinks. They have no ability to source current or pull up. All the pullup current is supplied by the terminator resistor which pulls up to 1.8 v, the nominal high value.

When a driver wants to assert a high, it doesn't do anything. When it wants to assert a low value, it sinks exactly enough current to pull the characteristic impedance (and terminator value) down 0.4 v to 1.4v which is the reference voltage level.

This signal propagates in both directions. In the outer direction, it is absorbed by the terminator. In the inner direction, it bounces off the RAC, thereby doubling the value to 1.0v at the RAC. (Because the RAC is the only device that will examine this value, the 1.4v signal at the other RDRams data pins doesn't matter.)

So the Rac will see a solid 1.0v, well below the 1.4v reference level.

If two RDrams drive current should overlap slightly, the resultant incoming signal would be 1.8-0.8 until it hits the RAC which would then double it to be 1.8-1.6 or 0.2v. This is above gnd so the signal value is always between 0.2 and 1.8 thereby within the safe operating range of the Ram's and rac's input circuits.

May I point you at a somewhat simplistic explanation in the pdf located at - tmo.hp.com

-- Alan
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