11

Are a

  • 4GB 1Rx8 PC3L 12800S

and a

  • 4GB 2Rx8 PC3 12800S

compatible?

What do 1Rx8 and 2Rx8 mean? Can I put them together?

14

1Rx8 means it is a single-rank module and 2Rx8 means it is a dual-rank module module. Rank is a data block which is 64 bits wide without Error Correction Code (ECC) created using some, or all of the memory chips on a module. The x8 in them specifies the number of banks in the memory module. Higher the number of banks, the fewer the chips in the memory module, the better the reliability and power consumption.

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.

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 in short 1Rx8 PC3L 12800S and 2Rx8 PC3 12800S RAMs are not compatible with each other. But if it is only the 1Rx8 and 2Rx8 in concern, they may be compatible with each other but it's best to check the motherboard manufacturer's list of compatible RAMs.

  • 1
    PC3L RAM can be used in PC3 – Nick Shvelidze May 4 '18 at 10:54
  • I'm looking to replace two 2 GB 1Rx8 PC3 1066 MHz cards with two 4 GB PC3 1066 MHz cards for my old Dell Latitude E6410, as it is very slow, lags and freezes. However after looking at the manual, calling Dell technical support, and looking up the motherboard part number, HNGW4, to try and check compatibility (but couldn't find such info), I still am not sure if 2Rx8 is compatible. I might just take the risk and see if I can try one out from a local shop, or failing that buy online. Edit: just read Timothy's comment; 1rx8 is safer. – James Ray Sep 27 '18 at 4:29
  • 1
    In my case, 4 ranks across 2 channels may cause compatibility issues. – James Ray Sep 27 '18 at 4:37
8

These modules aren't compatible with each other.

  • PC3 is standard voltage (1.5 V) DDR3 memory, while PC3L is DDR3L low-voltage (1.35 V) memory. Newer Intel processors (since fourth-generation Haswell) require DDR3L memory; regular DDR3 modules are out of specification for these processors and can cause the system to fail:

Notebooks Cannot Complete Power On Self Test (POST) When Used With DDR3 Memory Module

Notebooks utilizing 4th Generation Intel (Haswell) processors require a new type of memory known as DDR3-Low Voltage or “DDR3L”.


Upgraders: 1.35V memory is required!

Please note we fit 1.35V memory into our G750s. New Haswell CPUs require this and using 1.5V memory can cause problems along the lines of:

  • Overheating
  • Damage to the CPU IMC (due to the increased potential difference)
  • Greater battery use than our stated values.
  • 1R and 2R refer to the number of memory ranks on the module. Dual-rank (2R) modules appear to the system as if two memory modules were inserted in that memory channel, which may cause compatibility issues if you have multiple such modules on the same channel.

  • The highest density memory modules (32 GB or more) are intended for server use and generally will not work on consumer systems that use Intel Core processors. Consumer CPUs are typically limited to two or four ranks per channel while server processors can often handle eight or more ranks; this explains why many Intel Xeon processors can accept extremely large amount of memory (768 GB for Xeon E5 V3 parts).

5

Ummmmm it does NOT mean they are either on one side or 2 sides...

A memory rank basically is a block of data that is created using some or all the memory chips on a memory module (your sticks). It must be 64 bits of data wide (error corrected modules: 72 bits). A memory module can contain one, two, or four areas of 64-bit wide data areas, depending on how they are engineered. So "Ranks" = "Number of 64-bit wide data areas" .

R1x8 therefor means this module is a single-rank module, the other module, R2x4 being a dual-rank module.

Some Intel chipsets limit the number of ranks that you can put into your computer. For your Dell Precision Workstation P670 it is probably limited to 8 ranks in total, so with the sticks you have, you're not yet maxed out yet (2x2 + 2x1 = 6 ranks).

Single rank chips are usually more expensive because of the way they are built. They also allow you to put in more memory because of the rank limitation of your chipset.

3

PC3L can generally replace PC3, but not vice-versa. PC3L is DDR3 RAM spec'd to run at 1.35v, but is generally also fine at 1.5v. PC3 is spec'd to run at 1.5v; it might not work at 1.35v.

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.

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).

I do recommend running a memory test after RAM upgrades, though. Windows Memory Diagnostic is probably sufficient, though MemTest86 or MemTest86+ is better.

-1

1R means the chips are all on one side, 2R means they're higher density on both sides. Some boards will not use high density with low density RAM chips or vise versa. Those built off the ATi chipset are one. Other boards are ok with it, you'll be best off checking the motherboard documents.

In general best performance is achieved by having identical chips. Same capacity, same everything.

  • is ATI still around? DDR3 would indicate its recent. – Journeyman Geek Oct 21 '15 at 7:09
  • I don't actually know sorry, it's just the one that popped into my head as an example as not being able to take high and low at the same time. – Kilisi Oct 21 '15 at 7:15
  • That's backwards. 2Rx8 are dual-rank, and usually lower density. – Dave Burton Dec 10 '16 at 15:21

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