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Voltage drop over the years is because of power saving. Any digital chip power consumed is directly proportional to frequency and voltage squared. Increasingly dense fabrication (such as the latest 20-nm DDR3), minimizing the number of chips going into the end product and reclaiming board space that previous-generation chips would otherwise occupy. Compatibility is generally not an issue in most cases. But you should go through the motherboard manufacturer's guide of supported RAM modules just to make sure if they are compatible or not.
There is DDR, DDR2, DDR3 and DDR4 RAM standards. They are not compatible with each other and can not be mixed. Notebooks utilizing 4th Generation Intel (Haswell) processors require a new type of memory known as DDR3-Low Voltage or “DDR3L”. Opportunities for DRAM memory are vast, and cover almost every device capable of being interconnected and networked. Samsung Computing DRAM delivers a wide range of design advantages, including: Q1: Does the number of RAM chips on the module influences the memory speed, reliability, price, physical size of the module?Our latest-generation Computing DRAM provides an outstanding combination of speed and efficiency. Dedicated on-board power-management features help reduce power consumption during idle memory cycles.
Coming to the PC3 and PC3L part, PC3 RAMs requires an operating voltage of 1.5V whereas PC3L requires an operating voltage of 1.35V. The 'L' in PC3L signifies Low Voltage. So using PC3 RAM in a PC3L RAM slot will not provide it sufficient voltage and it will fail to operate. On the other hand using a PC3L RAM in a PC3 RAM slot may damage it due to overvoltage. So PC3 and PC3L RAMs are not compatible with each other. So, if your system came with PC3L then it probably requires it, so don't try to replace it with PC3. But if your system came with PC3 then you can probably replace it with either PC3 or PC3L. As @davidschwartz pointed out in the comments, the "R" memory is registered which "means it has additional buffers". Sometimes registered memory is called buffered memory. Operate at voltages as low as 1.2V while enabling nearly twice the bandwidth of DDR3 with upcoming DDR4 memory. Generally you can mix 1Rx8 and 2Rx8, as well, though mixing different kinds of RAM sometimes prevents interleaving (i.e., it might be a tad slower).
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PC3 is standard voltage (1.50V) where PC3L is low voltage (1.35V) - A system which requires PC3L cannot support PC3 RAM. PC3L RAM will work in a PC3 based system as it will run at the standard voltage and will inter-operate with PC3 RAM within the system. Very low operating voltages, to help significantly reduce the power consumption and extend the usable duration of battery-powered devices. Built-in mechanisms such as DQ shielding, self-calibration, and fly-by topology for enhanced signal control. The right memory solution can be one of your most important competitive advantages. Samsung leads the industry with cutting-edge DRAM technology, including DDR2, DDR3-and soon, DDR4-to heighten the success of your designs. Leverage Samsung's fastest and highest performance DDR3 memory today with a dual-channel, 128-bit memory module to enable a throughput of more than 25.6GB/s.