35

Are a

  • 4GB 1Rx8 PC3L 12800S

and a

  • 4GB 2Rx8 PC3 12800S

compatible?

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

4

7 Answers 7

36

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.

4
  • 4
    PC3L RAM can be used in PC3 May 4, 2018 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, 2018 at 4:29
  • 1
    In my case, 4 ranks across 2 channels may cause compatibility issues.
    – James Ray
    Sep 27, 2018 at 4:37
  • FWIW, basicnetworkingconcepts.blogspot.com/2017/01/… appears to be plagiarized from this question and answer, and it showed up for me as a "featured snippet" at the top of Google search results, above the SuperUser page.
    – Wyzard
    Jul 30, 2019 at 16:19
10

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).

6

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.

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.

4

Lets make everything clear about DDR RAM. Please, help me to finish all information about DDR3 RAM. And please, correct me if I am wrong.

  1. DDR full name is DDR SDRAM (Double Data Rate Synchronous Dynamic Random-Access Memory). It is a main PC system memory.

  2. For desktop PC DDR RAM standard - DIMM, for Laptop PC - SO-DIMM.

  3. There is DDR, DDR2, DDR3 and DDR4 RAM standards. They are not compatible with each other and can not be mixed.

  4. In our days (year 2019) DDR3 and DDR4 RAM modules are more widely used. DDR2 is rare. DDR is almost never found in working PCs. DDR2 is marked PC2 on physical memory stick/ module/ PCB. DDR3 is marked PC3 or PC3L (L - for low voltage). DDR4 is marked PC4.

  5. DDR - PC works on 2,5V voltage. DDR2 - PC2 works on 1,8V voltage. DDR3 - PC3 works on 1,5V voltage. DDR3 - PC3L works on 1,35V voltage. There are PC3 RAM modules that works on both voltages. DDR4 - PC4 works on 1,2V voltage. Voltage drop over the years is because of power saving. Any digital chip power consumed is directly proportional to frequency and voltage squared. Q1: What will happen if user will use 1,5V DDR3 in 1,35V system? Q2: What will happen if user will use 1,35V DDR3 in 1,5V system?

  6. Marking 1R means 1 rank of memory in one RAM module/ stick. Rank is a set of memory chips working as a one group. Marking 2R means 2 ranks of memory in one RAM module/ stick, 4R - 4 ranks. All ranks in one module is always directly connected to the same memory channel/ bus in parallel. System can access only one memory rank/ set at a time. There is a limit of how many memory ranks can be connected to the same memory channel (it depends on PC system). System will see 2R memory module as if it were 2 modules connected on the separate slots to the same channel. 4R memory module - as if it were 4 modules connected. Q1: What will happen if user exceeds the memory rank limit of the system? Q2: Will PC work faster with 1R memory module instead of 2R or 4R memory module? System can access all the memory of the DDR module at a time, not at 2 or 4 clocks...

  7. x8 (for example PC3 1Rx8) means that physical memory chips on the DDR module/ PCB holds 8 bit long memory. DDR memory is always accessed in 64 bits wide data by the system (72 bits with ECC - error correction code). x8 DDR module has to hold 8 of physical RAM chips soldered in serial per 1 rank (to become a 64 bit wide memory). 9 physical chips for DDR with ECC. Respectively 2 ranks of DDR module has to hold 16 RAM chips (18 with ECC), 4 ranks of DDR module has to hold 32 RAM chips (36 with ECC). There are also x4 and x16 DDR modules. Their DDR modules has to hold 16 and 4 of physical RAM chips soldered per 1 rank respectively (to become a 64 bit wide memory). Q1: Does the number of RAM chips on the module influences the memory speed, reliability, price, physical size of the module?

  8. The number in the end of marking shows the speed of DDR RAM in MBps (Mega Bytes per second).

  9. DDR RAM working frequency can be counted from it's speed. DDR RAM makes two transfers of data on one cycle/ clock (on rising front and falling front of the clock signal). Every transfer is 64 bit wide. So, for exsample, 8500 MBps / 8 "bytes per transfer" / 2 "transfers per clock" = ~ 531 MHz (Mega Herz).

For example "PC3 2Rx8 8500 no ECC" means:

  1. DDR3 memory 1,5V (can be double voltage RAM).
  2. 2 ranks of memory per module/ stick.
  3. 8 physical memory chips per rank, 16 chips in total soldered on the RAM module.
  4. The speed of DDR3 RAM is 8500 MBps. RAM works on 531 MHz bus frequency.
  5. DDR3 RAM module is without error correction code function.
  6. System sees this DDR3 module as if it were two modules on the same memory channel.

If you have something to add, please add it as a comment. It will be an organized information about DDR RAM in one place.

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. drd3 and drd4 is different

-2

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.

3
  • is ATI still around? DDR3 would indicate its recent.
    – Journeyman Geek
    Oct 21, 2015 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, 2015 at 7:15
  • That's backwards. 2Rx8 are dual-rank, and usually lower density. Dec 10, 2016 at 15:21

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