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`Home > Ar cles > Hardware > Upgrading & Repairing
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`Upgrading and Repairing PCs: Memory
`
`By Sco Mueller
`Jan 6, 2010
`
`📄 Contents ⎙ Print + Share This
`
`< Back Page 4 of 12 Next >
`
`This chapter is from the book
`
`Upgrading and Repairing PCs, 19th Edi on
`
`Learn More
`
` Buy
`
`SIMMs, DIMMs, and RIMMs
`Originally, PCs had memory installed via individual chips. They are o en referred to as dual inline
`package (DIP) chips because of their physical designs. The original IBM XT and AT systems had 36
`sockets on the motherboard for these individual chips—and more sockets could o en be found on
`memory cards plugged into the bus slots. I remember spending hours popula ng boards with these
`chips, which was a tedious job.
`
`Besides being a me‐consuming and labor‐intensive way to deal with memory, DIP chips had one
`notorious problem—they crept out of their sockets over me as the system went through thermal
`cycles. Every day, when you powered the system on and off, the system heated and cooled, and the
`chips gradually walked their way out of the sockets—a phenomenon called chip creep. Eventually, good
`contact was lost and memory errors resulted. Fortunately, resea ng all the chips back in their sockets
`usually rec fied the problem, but that method was labor intensive if you had a lot of systems to
`support.
`
`The alterna ve to this at the me was to have the memory soldered into either the motherboard or an
`expansion card. This prevented the chips from creeping and made the connec ons more permanent,
`but it caused another problem. If a chip did go bad, you had to a empt desoldering the old one and
`resoldering a new one or resort to scrapping the motherboard or memory card on which the chip was
`installed. This was expensive and made memory troubleshoo ng difficult.
`
`A chip was needed that was both soldered and removable, which was made possible by using memory
`modules instead of individual chips. Early modules had one row of electrical contacts and were called
`SIMMs (single inline memory modules), whereas later modules had two rows and were called DIMMs
`(dual inline memory modules) or RIMMs (Rambus inline memory modules). These small boards plug
`into special connectors on a motherboard or memory card. The individual memory chips are soldered
`to the module, so removing and replacing them is impossible. Instead, you must replace the en re
`module if any part of it fails. The module is treated as though it were one large memory chip.
`
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`6/19/2017
`Several different types of SIMMs, DIMMs, and RIMMs have been commonly used in desktop systems.
`The various types are o en described by their pin count, memory row width, or memory type.
`
`SIMMs, for example, are available in two main physical types—30‐pin (8 bits plus an op on for 1
`addi onal parity bit) and 72‐pin (32 bits plus an op on for 4 addi onal parity bits)—with various
`capaci es and other specifica ons. The 30‐pin SIMMs are physically smaller than the 72‐pin versions,
`and either version can have chips on one or both sides. SIMMs were widely used from the late 1980s
`to the late 1990s but have become obsolete.
`
`DIMMs are available in four main types. SDR (single data rate) DIMMs have 168 pins, one notch on
`either side, and two notches along the contact area. DDR (double data rate) DIMMs, on the other
`hand, have 184 pins, two notches on each side, and only one offset notch along the contact area. DDR2
`and DDR3 DIMMs have 240 pins, two notches on each side, and one near the center of the contact
`area. All DIMMs are either 64 bits (non‐ECC/parity) or 72 bits (data plus parity or error‐correc ng code
`[ECC]) wide. The main physical difference between SIMMs and DIMMs is that DIMMs have different
`signal pins on each side of the module, resul ng in two rows of electrical contacts. That is why they are
`called dual inline memory modules, and why with only 1" of addi onal length, they have many more
`pins than a SIMM.
`
`NOTE
`
`There is confusion among users and even in the industry regarding the terms single‐sided and
`double‐sided with respect to memory modules. In truth, the single‐ or double‐sided designa on
`actually has nothing to do with whether chips are physically located on one or both sides of the
`module, and it has nothing to do with whether the module is a SIMM or DIMM (meaning
`whether the connec on pins are single‐ or double‐inline). Instead the terms single‐sided and
`double‐sided are used to indicate whether the module has one or two internal banks (called
`ranks) of memory chips installed. A dual‐rank DIMM module has two complete 64‐bit wide banks
`of chips logically stacked so that the module is twice as deep (has twice as many 64‐bit rows). In
`most (but not all) cases, this requires chips to be on both sides of the module; therefore, the term
`double‐sided has o en been used to indicate that a module has two ranks, even though the term
`is technically incorrect. Single‐rank modules (incorrectly referred to as single‐sided) can also have
`chips physically mounted on both sides of the module, and dual‐rank modules can have chips
`physically mounted on only one side. I recommend using the terms single rank or dual rank
`instead because they are much more accurate and easily understood.
`
`RIMMs also have different signal pins on each side. Three different physical types of RIMMs are
`available: a 16/18‐bit version with 184 pins, a 32/36‐bit version with 232 pins, and a 64/72‐bit version
`with 326 pins. Each of these plugs into the same sized connector, but the notches in the connectors
`and RIMMs are different to prevent a mismatch. A given board will accept only one type. By far the
`most common type is the 16/18‐bit version. The 32‐bit version was introduced in late 2002, and the
`64‐bit version was introduced in 2004.
`
`The standard 16/18‐bit RIMM has 184 pins, one notch on either side, and two notches centrally
`located in the contact area. The 16‐bit versions are used for non‐ECC applica ons, whereas the 18‐bit
`versions incorporate the addi onal bits necessary for ECC.
`
`Figures 6.3 through 6.9 show a typical 30‐pin (8‐bit) SIMM, 72‐pin (32‐bit) SIMM, 168‐pin SDRAM
`DIMM, 184‐pin DDR SDRAM (64‐bit) DIMM, 240‐pin DDR2 DIMM, 240‐pin DDR3 DIMM, and 184‐pin
`RIMM, respec vely. The pins are numbered from le to right and are connected through to both sides
`of the module on the SIMMs. The pins on the DIMM are different on each side, but on a SIMM, each
`side is the same as the other and the connec ons carry through. Note that all dimensions are in both
`inches and millimeters (in parentheses), and modules are generally available in error‐correc ng code
`(ECC) versions with 1 extra ECC (or parity) bit for every 8 data bits (mul ples of 9 in data width) or
`versions that do not include ECC support (mul ples of 8 in data width).
`
`
`
`
`
`Figure 6.3 A typical 30‐pin SIMM.
`
`
`
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`
`
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`
`
`
`Figure 6.4 A typical 72‐pin SIMM.
`
`
`
`
`
`
`
`Figure 6.5 A typical 168‐pin SDRAM DIMM.
`
`
`
`
`
`
`
`Figure 6.6 A typical 184‐pin DDR DIMM.
`
`
`
`
`
`
`
`Figure 6.7 A typical 240‐pin DDR2 DIMM.
`
`
`
`
`
`
`
`Figure 6.8 A typical 240‐pin DDR3 DIMM.
`
`
`
`
`
`
`
`Figure 6.9 A typical 184‐pin RIMM.
`
`
`
`All these memory modules are fairly compact considering the amount of memory they hold and are
`available in several capaci es and speeds. Table 6.13 lists the various capaci es available for SIMMs,
`DIMMs, and RIMMs.
`
`Table 6.13. SIMM, DIMM, and RIMM Capaci es
`
`Capacity
`
`Standard DepthxWidth
`
`Parity/ECC DepthxWidth
`
`30‐Pin SIMM
`
`256KB
`
`1MB
`
`256Kx8
`
`1Mx8
`
`256Kx9
`
`1Mx9
`
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`
`Capacity
`
`4MB
`
`16MB
`
`72‐Pin SIMM
`
`1MB
`
`2MB
`
`4MB
`
`8MB
`
`16MB
`
`32MB
`
`64MB
`
`128MB
`
`168/184‐Pin DIMM/DDR DIMM
`
`8MB
`
`16MB
`
`32MB
`
`64MB
`
`128MB
`
`256MB
`
`512MB
`
`1,024MB
`
`2,048MB
`
`240‐Pin DDR2/DDR3 DIMM
`
`256MB
`
`512MB
`
`1,024MB
`
`2,048MB
`
`Standard DepthxWidth
`
`Parity/ECC DepthxWidth
`
`4Mx8
`
`16Mx8
`
`256Kx32
`
`512Kx32
`
`1Mx32
`
`2Mx32
`
`4Mx32
`
`8Mx32
`
`16Mx32
`
`32Mx32
`
`1Mx64
`
`2Mx64
`
`4Mx64
`
`8Mx64
`
`16Mx64
`
`32Mx64
`
`64Mx64
`
`128Mx64
`
`256Mx64
`
`32Mx64
`
`64Mx64
`
`128Mx64
`
`256Mx64
`
`4Mx9
`
`16Mx9
`
`256Kx36
`
`512Kx36
`
`1Mx36
`
`2Mx36
`
`4Mx36
`
`8Mx36
`
`16Mx36
`
`32Mx36
`
`1Mx72
`
`2Mx72
`
`4Mx72
`
`8Mx72
`
`16Mx72
`
`32Mx72
`
`64Mx72
`
`128Mx72
`
`256Mx72
`
`32Mx72
`
`64Mx72
`
`128Mx72
`
`256Mx72
`
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`
`SIMMs, DIMMs, and RIMMs | Upgrading and Repairing PCs: Memory | InformIT
`
`Capacity
`
`4,096MB
`
`184‐Pin RIMM
`
`64MB
`
`128MB
`
`256MB
`
`512MB
`
`1,024MB
`
`Standard DepthxWidth
`
`Parity/ECC DepthxWidth
`
`512Mx64
`
`512Mx72
`
`32Mx16
`
`64Mx16
`
`128Mx16
`
`256Mx16
`
`512Mx16
`
`32Mx18
`
`64Mx18
`
`128Mx18
`
`256Mx18
`
`512Mx18
`
`Memory modules of each type and capacity are available in various speed ra ngs. Consult your
`motherboard documenta on for the correct memory speed and type for your system. If a system
`requires a specific speed memory module, you can almost always subs tute faster speeds if the one
`specified is not available. Generally, no problems occur in mixing module speeds, as long as you use
`modules equal to or faster than what the system requires. Because there's li le price difference
`between the various speed versions, I o en buy faster modules than are necessary for a par cular
`applica on, especially if they are the same cost as slower modules. This might make them more usable
`in a future system that could require the faster speed.
`
`Because SDRAM and newer modules have an onboard serial presence detect (SPD) ROM that reports
`their speed and ming parameters to the system, most systems run the memory controller and
`memory bus at the speed matching the slowest module installed.
`
`NOTE
`
`A bank is the smallest amount of memory needed to form a single row of memory addressable by
`the processor. It is the minimum amount of physical memory that is read or wri en by the
`processor at one me and usually corresponds to the data bus width of the processor. If a
`processor has a 64‐bit data bus, a bank of memory also is 64 bits wide. If the memory runs dual‐
`or tri‐channel, a virtual bank is formed that is two or three mes the absolute data bus width of
`the processor.
`
`You can't always replace a module with a higher‐capacity unit and expect it to work. Systems might
`have specific design limita ons for the maximum capacity of module they can take. A larger‐capacity
`module works only if the motherboard is designed to accept it in the first place. Consult your system
`documenta on to determine the correct capacity and speed to use.
`Registered Modules
`SDRAM through DDR3 modules are available in unbuffered and registered versions. Most PC
`motherboards are designed to use unbuffered modules, which allow the memory controller signals to
`pass directly to the memory chips on the module with no interference. This is not only the cheapest
`design, but also the fastest and most efficient. The only drawback is that the motherboard designer
`must place limits on how many modules (meaning module sockets) can be installed on the board, and
`possibly also limit how many chips can be on a module. So‐called double‐sided modules that really
`have mul ple banks of chips onboard might be restricted on some systems in certain combina ons.
`
`Systems designed to accept extremely large amounts of RAM (such as servers) o en require registered
`modules. A registered module uses an architecture that has register chips on the module that act as an
`interface between the actual RAM chips and the chipset. The registers temporarily hold data passing to
`and from the memory chips and enable many more RAM chips to be driven or otherwise placed on the
`module than the chipset could normally support. This allows for motherboard designs that can support
`many modules and enables each module to have a larger number of chips. In general, registered
`
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`6/19/2017
`modules are required by server or worksta on motherboards designed to support more than four
`sockets. One anomaly is the ini al version of the AMD Athlon 64 FX processor, which also uses
`registered memory because its Socket 940 design was based on the AMD Opteron worksta on and
`server processor. Subsequent Socket 939, AM2, and Socket F versions of the Athlon FX no longer
`require registered memory.
`
`To provide the space needed for the buffer chips, a registered DIMM is o en taller than a standard
`DIMM. Figure 6.10 compares a typical registered DIMM to a typical unbuffered DIMM.
`
`
`
`
`
`
`
`Figure 6.10 A typical registered DIMM is taller than a typical unbuffered DIMM to provide
`room for buffer chips.
`
`TIP
`
`If you are installing registered DIMMs in a slimline case, clearance between the top of the DIMM
`and the case might be a problem. Some vendors sell low‐profile registered DIMMs that are about
`the same height as an unbuffered DIMM. Use this type of DIMM if your system does not have
`enough head room for standard registered DIMMs. Some vendors sell only this type of DIMM for
`par cular systems.
`
`The important thing to note is that you can use only the type of module your motherboard (or chipset)
`is designed to support. For most, that is standard unbuffered modules or, in some cases, registered
`modules.
`SIMM Details
`The 72‐pin SIMMs use a set of four or five pins to indicate the type of SIMM to the motherboard.
`These presence detect pins are either grounded or not connected to indicate the type of SIMM to the
`motherboard. Presence detect outputs must be ed to the ground through a 0‐ohm resistor or jumper
`on the SIMM—to generate a high logic level when the pin is open or a low logic level when the
`motherboard grounds the pin. This produces signals the memory interface logic can decode. If the
`motherboard uses presence detect signals, a power‐on self test (POST) procedure can determine the
`size and speed of the installed SIMMs and adjust control and addressing signals automa cally. This
`enables autodetec on of the memory size and speed.
`
`NOTE
`
`In many ways, the presence detect pin func on is similar to the industry‐standard DX coding used
`on modern 35mm film rolls to indicate the ASA (speed) ra ng of the film to the camera. When
`you drop the film into the camera, electrical contacts can read the film's speed ra ng via an
`industry‐standard configura on.
`
`Presence detect performs the same func on for 72‐pin SIMMs that the serial presence detect (SPD)
`chip does for DIMMs.
`
`Table 6.14 shows the Joint Electronic Devices Engineering Council (JEDEC) industry‐standard presence
`detect configura on lis ng for the 72‐pin SIMM family. JEDEC is an organiza on of U.S. semiconductor
`manufacturers and users that sets semiconductor standards.
`
`Table 6.14. Presence Detect Pin Configura ons for 72‐Pin SIMMs
`
`Size
`
`Speed
`
`Pin 67
`
`Pin 68
`
`Pin 69
`
`Pin 70
`
`Pin 11
`
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`SIMMs, DIMMs, and RIMMs | Upgrading and Repairing PCs: Memory | InformIT
`
`1MB
`
`100ns Gnd
`
`1MB
`
`80ns
`
`Gnd
`
`1MB
`
`70ns
`
`Gnd
`
`1MB
`
`60ns
`
`Gnd
`
`—
`
`—
`
`—
`
`—
`
`Gnd
`
`Gnd
`
`—
`
`Gnd
`
`Gnd
`
`—
`
`—
`
`—
`
`2MB
`
`100ns —
`
`Gnd
`
`Gnd
`
`Gnd
`
`2MB
`
`80ns —
`
`Gnd
`
`—
`
`Gnd
`
`2MB
`
`70ns —
`
`Gnd
`
`Gnd
`
`2MB
`
`60ns —
`
`Gnd
`
`—
`
`—
`
`—
`
`4MB
`
`100ns Gnd
`
`Gnd
`
`Gnd
`
`Gnd
`
`4MB
`
`80ns
`
`Gnd
`
`Gnd
`
`—
`
`Gnd
`
`4MB
`
`70ns
`
`Gnd
`
`Gnd
`
`Gnd
`
`60ns
`
`Gnd
`
`Gnd
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`4MB
`
`8MB
`
`100ns —
`
`8MB
`
`80ns —
`
`8MB
`
`70ns —
`
`8MB
`
`60ns —
`
`16MB
`
`80ns
`
`Gnd
`
`16MB
`
`70ns
`
`Gnd
`
`16MB
`
`60ns
`
`Gnd
`
`16MB
`
`50ns
`
`Gnd
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`Gnd
`
`Gnd
`
`—
`
`Gnd
`
`—
`
`—
`
`—
`
`—
`
`—
`
`—
`
`Gnd
`
`—
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`Gnd
`
`—
`
`—
`
`Gnd
`
`Gnd
`
`Gnd
`
`Gnd
`
`Gnd
`
`32MB
`
`80ns —
`
`Gnd
`
`—
`
`Gnd
`
`Gnd
`
`32MB
`
`70ns —
`
`Gnd
`
`Gnd
`
`32MB
`
`60ns —
`
`Gnd
`
`—
`
`—
`
`—
`
`Gnd
`
`Gnd
`
`32MB
`
`50ns —
`
`Gnd
`
`Gnd
`
`Gnd
`
`Gnd
`
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`
`— = No connec on (open)
`
`Gnd = Ground
`
`Pin 67 = Presence detect 1
`
`Pin 68 = Presence detect 2
`
`Pin 69 = Presence detect 3
`
`Pin 70 = Presence detect 4
`
`Pin 11 = Presence detect 5
`
`Unfortunately, unlike the film industry, not everybody in the computer industry follows established
`standards. As such, presence detect signaling is not a standard throughout the PC industry. Different
`system manufacturers some mes use different configura ons for what is expected on these four pins.
`Compaq, IBM (mainly PS/2 systems), and Hewle ‐Packard are notorious for this type of behavior. Many
`of the systems from these vendors require special SIMMs that are basically the same as standard 72‐
`pin SIMMs, except for special presence detect requirements. Table 6.15 shows how IBM defines these
`pins.
`
`Table 6.15. Presence Detect Pins for IBM 72‐Pin SIMMs
`
`67
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`—
`
`Gnd
`
`Gnd
`
`68
`
`—
`
`69
`
`—
`
`—
`
`—
`
`Gnd —
`
`Gnd —
`
`70
`
`—
`
`—
`
`—
`
`—
`
`SIMM Type
`
`IBM Part Number
`
`Not a valid
`SIMM
`
`1MB 120ns
`
`2MB 120ns
`
`n/a
`
`n/a
`
`n/a
`
`2MB 70ns
`
`92F0102
`
`—
`
`—
`
`Gnd —
`
`8MB 70ns
`
`64F3606
`
`Gnd —
`
`Reserved
`
`n/a
`
`Gnd Gnd —
`
`2MB 80ns
`
`92F0103
`
`Gnd Gnd —
`
`8MB 80ns
`
`64F3607
`
`—
`
`—
`
`—
`
`—
`
`Gnd
`
`Reserved
`
`n/a
`
`Gnd
`
`1MB 85ns
`
`90X8624
`
`Gnd —
`
`Gnd
`
`2MB 85ns
`
`92F0104
`
`Gnd —
`
`Gnd
`
`4MB 70ns
`
`92F0105
`
`—
`
`Gnd Gnd
`
`4MB 85ns
`
`79F1003 (square notch) L40‐
`SX
`
`—
`
`—
`
`Gnd Gnd
`
`1MB 100ns
`
`n/a
`
`Gnd Gnd
`
`8MB 80ns
`
`79F1004 (square notch) L40‐
`SX
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`SIMMs, DIMMs, and RIMMs | Upgrading and Repairing PCs: Memory | InformIT
`
`67
`
`—
`
`Gnd
`
`Gnd
`
`68
`
`69
`
`70
`
`SIMM Type
`
`IBM Part Number
`
`Gnd Gnd Gnd
`
`2MB 100ns
`
`n/a
`
`Gnd Gnd Gnd
`
`4MB 80ns
`
`87F9980
`
`Gnd Gnd Gnd
`
`2MB 85ns
`
`79F1003 (square notch) L40SX
`
`— = No connec on (open)
`
`Gnd = Ground
`
`Pin 67 = Presence detect
`
`1 P
`
`2 P
`
`3 P
`
`4
`
`in 68 = Presence detect
`
`in 69 = Presence detect
`
`in 70 = Presence detect
`
`Because these pins can have custom varia ons, you o en must specify IBM, Compaq, HP, or generic
`SIMMs when you order memory for systems using 72‐pin SIMMs. Although very few (if any) of these
`systems are s ll in service, keep this informa on in mind if you are moving 72‐pin modules from one
`system to another or are installing salvaged memory into a system. Also, be sure you match the metal
`used on the module connectors and sockets. SIMM pins can be n or gold plated, and the pla ng on
`the module pins must match that on the socket pins; otherwise, corrosion will result.
`
`CAUTION
`
`To have the most reliable system when using SIMM modules, you must install modules with gold‐
`plated contacts into gold‐plated sockets and modules with n‐plated contacts into n‐plated
`sockets only. If you mix gold contacts with n sockets, or vice versa, you are likely to experience
`memory failures from 6 months to 1 year a er ini al installa on because a type of corrosion
`know as fre ng will take place. This has been a major problem with 72‐pin SIMM‐based systems
`because some memory and motherboard vendors opted for n sockets and connectors while
`others opted for gold. According to connector manufacturer AMP's "Golden Rules: Guidelines for
`the Use of Gold on Connector Contacts" (available at
`www.tycoelectronics.com/documenta on/whitepapers/pdf/aurulrep.pdf) and "The Tin
`Commandments: Guidelines for the Use of Tin on Connector Contacts" (available at
`www.tycoelectronics.com/documenta on/whitepapers/pdf/sncomrep.pdf), you should match
`connector metals.
`
`If you are maintaining systems with mixed n/gold contacts in which fre ng has already
`occurred, use a wet contact cleaner. A er cleaning, to improve electrical contacts and help
`prevent corrosion, you should use a liquid contact enhancer and lubricant called Stabilant 22
`from D.W. Electrochemicals when installing SIMMs or DIMMs. The company's website
`(www.stabilant.com) has detailed applica on notes on this subject that provide more technical
`details.
`
`SDR DIMM Details
`SDR (single data rate) DIMMs use a completely different type of presence detect than SIMMs, called
`serial presence detect (SPD). It consists of a small EEPROM or flash memory chip on the DIMM that
`contains specially forma ed data indica ng the DIMM's features. This serial data can be read via the
`serial data pins on the DIMM, and it enables the motherboard to autoconfigure to the exact type of
`DIMM installed.
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`DIMMs can come in several varie es, including unbuffered and buffered as well as 3.3V and 5V.
`Buffered DIMMs have addi onal buffer chips on them to interface to the motherboard. Unfortunately,
`these buffer chips slow down the DIMM and are not effec ve at higher speeds. For this reason, most
`PC systems (those that do not use registered DIMMs) use unbuffered DIMMs. The voltage is simple—
`DIMM designs for PCs are almost universally 3.3V. If you install a 5V DIMM in a 3.3V socket, it would be
`damaged, but fortunately keying in the socket and on the DIMM prevents that.
`
`Modern PC systems use only unbuffered 3.3V DIMMs. Apple and other non‐PC systems can use the
`buffered 5V versions. Fortunately, the key notches along the connector edge of a DIMM are spaced
`differently for buffered/unbuffered and 3.3V/5V DIMMs, as shown in Figure 6.11. This prevents
`inser ng a DIMM of the wrong type into a given socket.
`
`
`
`
`
`Figure 6.11 The 168‐pin DRAM DIMM notch key defini ons.
`
`
`
`DDR DIMM Details
`The 184‐pin DDR DIMMs use a single key notch to indicate voltage, as shown in Figure 6.12.
`
`
`
`
`
`Figure 6.12 The 184‐pin DDR SDRAM DIMM keying.
`
`
`
`DDR DIMMs also use two notches on each side to enable compa bility with both low‐ and high‐profile
`latched sockets. Note that the key posi on is offset with respect to the center of the DIMM to prevent
`inser ng it backward in the socket. The key notch is posi oned to the le , centered, or to the right of
`the area between pins 52 and 53. This is used to indicate the I/O voltage for the DDR DIMM and to
`prevent installing the wrong type into a socket that might damage the DIMM.
`DDR2 DIMM Details
`The 240‐pin DDR2 DIMMs use two notches on each side to enable compa bility with both low‐ and
`high‐profile latched sockets. The connector key is offset with respect to the center of the DIMM to
`prevent inser ng it backward in the socket. The key notch is posi oned in the center of the area
`between pins 64 and 65 on the front (184/185 on the back), and there is no voltage keying because all
`DDR2 DIMMs run on 1.8V.
`
`DDR3 DIMM Details
`The 240‐pin DDR3 DIMMs use two notches on each side to enable compa bility with both low‐ and
`high‐profile latched sockets. The connector key is offset with respect to the center of the DIMM to
`prevent inser ng it backward in the socket. The key notch is posi oned in the center of the area
`between pins 48 and 49 on the front (168/169 on the back), and there is no voltage keying because all
`DDR3 DIMMs run on 1.5V.
`RIMM Details
`The 16/18‐bit RIMMs are keyed with two notches in the center. This prevents a backward inser on and
`prevents the wrong type (voltage) RIMM from being used in a system. Currently, all RIMMs run on
`2.5V, but proposed 64‐bit versions will run on only 1.8V. To allow for changes in the RIMMs, three
`keying op ons are possible in the design (see Figure 6.13). The le key (indicated as "DATUM A" in
`Figure 6.13) is fixed in posi on, but the center key can be in three different posi ons spaced 1mm or
`2mm to the right, indica ng different types of RIMMs. The current default is op on A, as shown in
`Figure 6.13 and Table 6.16, which corresponds to 2.5V opera on.
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`SIMMs, DIMMs, and RIMMs | Upgrading and Repairing PCs: Memory | InformIT
`
`
`
`Figure 6.13 RIMM keying op ons.
`
`
`
`Table 6.16. Possible Keying Op ons for RIMMs
`
`Op on Notch Separa on Descrip on
`
`A
`
`B
`
`C
`
`11.5mm
`
`2.5V RIMM
`
`12.5mm
`
`Reserved
`
`13.5mm
`
`Reserved
`
`RIMMs incorporate an SPD device, which is essen ally a flash ROM onboard. This ROM contains
`informa on about the RIMM's size and type, including detailed ming informa on for the memory
`controller. The memory controller automa cally reads the data from the SPD ROM to configure the
`system to match the RIMMs installed.
`
`Figure 6.14 shows a typical PC RIMM installa on. The RDRAM controller and clock generator are
`typically in the motherboard chipset North Bridge component. As you can see, the Rambus memory
`channel flows from the memory controller through each of up to three RIMM modules in series. Each
`module contains 4, 8, 16, or more RDRAM devices (chips), also wired in series, with an onboard SPD
`ROM for system configura on. Any RIMM sockets without a RIMM installed must have a con nuity
`module, shown in the last socket in Figure 6.13. This enables the memory bus to remain con nuous
`from the controller through each module (and, therefore, each RDRAM device on the module) un l the
`bus finally terminates on the motherboard. Note how the bus loops from one module to another. For
` ming purposes, the first RIMM socket must be 6" or less from the memory controller, and the en re
`length of the bus must not be more than it would take for a signal to go from one end to another in
`four data clocks, or about 5ns.
`
`
`
`
`
`Figure 6.14 Typical RDRAM bus layout showing a RIMM and one con nuity module.
`
`
`
`Interes ngly, Rambus does not manufacture the RDRAM devices (the chips) or the RIMMs; that is le
`to other companies. Rambus is merely a design company, and it has no chip fabs or manufacturing
`facili es of its own. It licenses its technology to other companies who then manufacture the devices
`and modules.
`
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