CPU Sockets and Slots

Advance Technical Repair of Laptops Motherboard

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Until recently, each generation of CPU (80386, 80486, and so on) has used a unique CPU/motherboard interface method. These have generally come in the form of sockets, in which the CPU has a number of pins on its bottom that fit into a square connector on the O

motherboard (see Figure 2.2). More recent CPUs, including most Pentium-II, Pentium-III, and H

Athlon CPUs, use a slotted design more like that of plug-in cards. (Most Celeron CPUs con- B

tinue to use a socketed design, but some varieties are available in a slotted form.) A

Figure 2.2

A socket on motherboards for socketed CPUs contains many holes into which the pins on the bottom of the CPU fit.

Figure 2.2

A socket on motherboards for socketed CPUs contains many holes into which the pins on the bottom of the CPU fit.

Socketed Designs

Most CPU series have seen the coming and going of several socket variants. For instance, the very earliest Pentium CPUs used a pin layout that's incompatible with what later Pentium

Part i

CPUs used. Early 80486 motherboards used sockets that required great force to insert the CPU, but later 80486 motherboards used zero insertion force (ZIF) sockets, in which a lever on one side of the socket allows you to tighten it around an inserted CPU. (This lever is visible to the right of the socket in Figure 2.2.) The vast majority of Pentium-class motherboards use ZIF sockets. Popular sockets for 80486 and later CPUs include

• Socket 1 169-pin socket for 80486 CPUs.

• Socket 2 238-pin socket for 80486 and Pentium Overdrive CPUs.

• Socket 3 237-pin socket for 80486 and Pentium Overdrive CPUs. Includes support for 3.3v operation, which reduces heat generation by CPUs that also support this feature.

• Socket 4 273-pin socket for 60 and 66MHz Pentiums.

• Socket 5 320-pin socket for 75-133MHz Pentiums.

• Socket 7 321-pin socket for Pentiums of 75MHz and above, including Pentium clones from AMD, Cyrix, and IDT. Some motherboards use a Super7 socket design, which is a Socket 7 layout with support for faster bus speeds, as described shortly.

• Socket 8 387-pin socket for Pentium Pro CPUs.

• Socket 370 370-pin socket used by Celeron CPUs.

• Nx586 The NexGen Nx586 CPU used its own unique pinout. Most Nx586 CPUs were not socketed, however; instead, they were soldered directly to their motherboards.


A Socket 6 design was developed, but never implemented on production motherboards.

It's critically important that you match the socket type to your CPU. You cannot use a CPU that requires one type of socket on a motherboard that uses another. Most 80486 CPUs can be used with Sockets 1 through 3, although some of the more advanced 80486 CPUs require the later Socket 2 or even Socket 3. All Pentium-class motherboards on the market in 2000 are Socket 7 designs (in fact, most implement Super7 features, as well). The motherboard shown in Figure 2.1 is a Socket 7 motherboard. One exception to the interface-matching rule is that Socket 370/Slot 1 adapter cards are available. These allow you to plug a Celeron CPU into a Slot 1 motherboard. Such an adapter might be useful if you want to build a low-cost Celeron computer but plan to upgrade it to a more powerful CPU at some time in the future.

Chapter 2

Socket 7 motherboards are still viable in the marketplace, thanks largely to the AMD K6-2 and K6-III CPUs, which can perform competitively to low-end Intel Pentium-II and Pentium-III CPUs. With AMD's release of the Athlon CPU, though, AMD is likely to eventually abandon Socket 7 CPUs. Likewise, VIA (which bought Cyrix and IDT late in 1999) has announced that its next CPU wil use a Slot 1 interface. Intel now uses Socket 370 for most of its Celeron CPUs, but the most powerful Intel CPUs use l a slotted CPU interface.

Slotted Designs

In early 2000, only three types of CPU slots are in common use:

• Slot 1 This 242-pin design is used by Pentium-II, Pentium-III, and some Celeron motherboards.

• Slot 2 This 330-pin design is used by more recent Pentium-II Xeon CPUs. Most Slot 2 motherboards support at least two CPUs.

• Slot A This 242-pin design is used by AMD for its Athlon CPU. (AMD licensed the technology from Digital Equipment Corporation [now Compaq], which uses the design for its Alpha CPU.)

Each slot design is incompatible with the others. Also, although both the AMD Athlon and the Compaq Alpha CPUs use the same CPU bus, they require different motherboards.

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