Buffered RAM and Voltage

www.oempcworld.com

Buffered RAM and Voltage :

Recently larger 64MB and 128MB SIMM's and DIMM's have been introduced. These modules frequently stack multiple chips (loads) in what is known as a composite architecture (See the Composite section of this page). This increases the total load on the memory bus in the form of increased capacitance. This is a bad thing. If you recall, a capacitor in a DC circuit will charge like a battery when a voltage is applied across it and it will discharge a current when the voltage is removed. If a pulse of voltage is sent through a circuit with a capacitance its signal will rise slowly and decay slowly. Capacitance in a circuit adds a delay in response Signals, such as memory address lines, are sent encoded as a series of voltage pulses. Therefore, an increased capacitance has the effect of slowing the overall rate at which signals may be conducted. The ability of the motherboard to overcome this effect is know as the load driving capacity of the board. This is generally not sufficient to drive larger modules with higher capacitance. Rather than redesign the motherboard and increase its load driving capacity, the industry has opted to modify the memory module by adding a line driver (buffer) such as an 74F244. This is a single chip on the memory module which is much smaller than a normal RAM chip. You can identify buffered modules by looking for this chip. The line driver handles all loading of the RAM chips and redirects all incoming and outgoing signals. It introduces some delay but this is more than offset by the increase in signal rise and fall times. So the net effect is to increase the speed of memory access. In practical terms, using a line driver (buffer) is more important when you install more than one bank of very high density SIMM's or DIMM's.

An alternate method of dealing with the issue of increased capacitance is to merely lower the voltage of the signal sent to the memory module. It takes less time for a signal to rise and fall if the voltage is lower. If the voltage has to be increased from 0 to 5v to send a signal across a capacitor it will take a long time for the capacitor to charge and decay. Likewise if the signal it sent to a memory module with a higher capacitance it will take a long time for the signal to rise and fall. If the pulse is merely from 0 to 3.3v than it will take much less time for the signal to rise and fall. Therefore lowering the voltage of the memory module from 5v to 3.3v has the effect of speeding up the system just like the case of adding a memory buffer. You can even combine lower voltage and a buffer to augment signal speed.

Older SIMM's use 5 volt technology. Newer DIMM's are available in both 3.3v and 5v formats. Some "mixed-mode" modules are actually 3.3v modules that can run at 5v. Many 3.3v DIMM's can actually run at 5v and therefore can be mixed with 5v DIMM's but in practice it is not a good idea to mix 3.3v DIMM's with 5v SIMM or DIMM's unless your absolutely sure about the manufacturer's IC specifications. On newer motherboards that use both 5v SIMM's and DIMM's it is a good idea to use 5v DIMM's if you plan to use the SIMM socket. If you don't plan to use SIMM's at all you can use all 3.3v DIMM's. The choice of voltage you use is not dependent on the motherboard chip set.