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Technology Stocks : Rambus (RMBS) - Eagle or Penguin -- Ignore unavailable to you. Want to Upgrade?

To: Ian Anderson who wrote (81664)	3/22/2002 2:55:33 AM
From: Ian Anderson	Read Replies (1) \| Respond to of 93625

I would still like to know what technique is being used to terminate the stock price? I've never seen it with such a low noise level.

To: Ian Anderson who wrote (81664)	3/22/2002 4:24:25 AM
From: John Walliker	Read Replies (1) \| Respond to of 93625

Ian, Rambus has always used a 28 Ohm bus for designs which use RIMMs. Standardisation is necessary because the impedance must be the same on the RIMM as on the motherboard. When memory chips are soldered directly to the motherboard the designer has the freedom to choose a higher impedance. The standard devices automatically adapt to the higher impedance, so there is no need for different versions. 28 Ohm tracks minimise the extent to which discontinuities such as RIMM connectors and the memory chips themselves cause reflections compared with the higher impedances typically used in conventional memory designs. The disadvantage is that they take up more space because the tracks are wider. (50 Ohm tracks on the surface of a PCB are roughly 1.5 times wider than the thickness of the insulating layer below them whereas 28 Ohm tracks are closer to 3 times wider than the insulator thickness.) Rambus does not use AC termination, although it behaves in a similar way. One end of the bus has a DC connection through a 28 Ohm termination resistor to a reference voltage. The other end (nearer the controller) is open circuit. Any device can be in one of two states 1) open circuit or 2) pulling a constant current to ground from the bus. If all devices are in the off state then no DC is drawn through the termination resistor. When a device is in the on-state then a current is drawn through the termination resistor. The lack of a termination at the controller end of the bus means that signals are reflected before finally being absorbed at the terminated end. Because of this the voltage on the bus seen by a driving device may vary, which is one reason for the need to use constant current bus drivers. Another reason is that a conventional bus driver looks behaves a short circuit, so there could be multiple reflections between a driving device and the open circuit end of the bus. A constant current driver does not cause reflections in this way. (I had better mention that nothing is perfect so some small amount of reflection may occur even so.) Using only one termination resistor on each bus line reduces power consumption compared with using two. John