13 Types of Motherboard Slots and How They’re Used

Key Points

• The motherboard is the printed circuit board (PCB) containing the core components of an electronic device, such as a computer, tablet, or cellphone.
• Modern motherboards have slots for additional printed circuit boards to connect to the main motherboard.
• CPU sockets are crucial slots in motherboard architecture that contain the connectors for the device’s central processing unit.
• PCI slots used to be the dominant method for adding additional controllers to motherboards, but they have become obsolete in consumer computers.
• PCIe slots are the successor to PCI slots and handle connections to non-core components, offering significant improvements over previous versions of the PCI standard.

The motherboard is the printed circuit board (PCB) containing the core components of an electronic device, such as a computer, tablet, or cellphone. However, most devices include more than one printed circuit board. Modern motherboards have slots for additional printed circuit boards to connect to the main motherboard. We historically used many different types of slots in motherboards, and many are no longer in use today. Let’s examine the different slots used in motherboard technology over the years.

CPU Sockets

While not a traditional slot in the sense of design, the CPU socket is one of the most crucial slots in motherboard architecture. The CPU socket contains the connectors for the device’s central processing unit, the brain of the whole operation.

We can separate modern motherboards into two main CPU sockets, pin grid array (PGA) and land grid array (LGA.) In a PGA socket, the socket has no pins. The manufacturers make CPUs with pins that the user must situate into the socket. An LGA socket uses a socket with a flat pin grid, while the CPU itself has no pins. 

There is no definitive best socket choice. It’s really up to personal preference. I prefer LGA sockets. However, I’ve used PGA sockets, which are just as easy to use.

Hardware enthusiasts may have heard of ball grid sockets, or BGA sockets, a much less common CPU socket type. BGA sockets require soldering to install a CPU. Manufacturers of consumer motherboards don’t use them as much since they’re not user replaceable.

PCI Slots

PCI slots are now obsolete slots that were a core component of traditional consumer DIY desktop motherboards. The term “PCI” stands for peripheral component interconnect. These slots allow users to easily and quickly connect additional component PCBs, like network cards, sound cards, and graphics processing units.

PCI slots used to be the dominant method by which users could add additional controllers to their motherboard’s layout. The slot’s design originated in the 1990s and replaced the Micro Channel architecture and Extended Industry Standard architecture slots in Intel’s server designs.

While Intel quickly replaced its architecture with PCI slots, the slot didn’t explode until 1994, when it replaced the VESA Local Bus. By 1996, the PCI slot was the dominant expansion slot in the computing market.

However, the PCI slot, and its successors, have seen a sharp decrease in its usefulness to the consumer. With the shift in the market moving from standardized desktop computers to sleek, slim, and unstandardized laptop computers, fewer consumers can access the PCI slots on their computers to expand their computers’ PCBs.

PCIe Slots

The successor to the PCI slot is the PCI Express slot or the PCIe slot. The PCIe slots handle connections to non-core components, like the PCI slots. Adding the word ‘express’ to the name of the standard emphasizes and distinguishes the significant improvements made over the previous versions of the PCI standard beyond that of a simple version number.

Users may hear the term ‘PCIe card’ used to refer to many different types of peripheral components. It’s a catch-all term for peripheral components such as memory cards, graphics cards, network controllers, solid-state drives, and even traditional hard drives.

PCIe slots come in four main sizes, determining how many direct connections to your PCIe card’s motherboard will have. The four main sizes are 1×, 4×, 8×, and 16×. The connections are called lanes; the more lanes a card has, the more efficiently the hardware can perform. For instance, you typically want to use a 16× lane slot for a graphics card if you’re using a PCIe graphics card to maximize efficiency and ensure that your graphics are running at the highest possible hardware settings.

Ever since PCIe 3.0, new versions of PCIe slots have increased by factors of eight and doubled the maximum transfer speed of the cards. PCIe 4.0 slots can transfer 64 GB of data per second at 16 Giga transfers per second.

PCI-X Slots

You probably won’t see a PCI-X slot unless you work with servers. However, server maintenance workers and people who simply want to build a server inside their homes will see these slots all the time. PCI-X stands for Peripheral Component Interconnect eXtended. These slots are pretty much exclusively used for servers.

PCI-X slots modify the standard 32-bit PCI bus to accept higher clock speeds of up to 133 MHz. However, the electrical implementation of PCI-X slots is similar to PCIe slots. PCI-X 2.0 further modified the protocol to accept clock speeds of up to 533 MHz and lowered the electrical signal levels.

PCIe slots have largely replaced PCI-X slots in most applications. PCIe slots have an entirely different electrical connector and use one or more fast serial connector lanes rather than slower connections in parallel.

Accelerated Graphics Port

AGP or Accelerated Graphics Port slots were meant to replace PCI slots regarding adding graphics cards to motherboards. The AGP slot is a superset of the PCI slot. Its design is intended to overcome some notable limitations of the PCI slot regarding processing graphical information. The AGP slot was mainly used for 3D graphics.

The AGP slot offered many advantages over the traditional PCI slot. Firstly, it doesn’t share the PCI bus. So it provides a direct, point-to-point connection between the graphics card and the motherboard chipset. Secondly, it uses split transactions. Splitting the address and the data phases allows the card to send multiple addresses to the chipset for processing. This process also avoids lengthy delays when the bus is idle during read operations. 

Thirdly, its simplifies bus handshaking over PCI bus handshaking. PCI cards negotiate bus transaction lengths using the FRAME# and STOP# signals. AGP transactions are always eight bytes long; the total length is included in the request. The IRDY# and TRDY# signals are also removed in favor of transferring data in blocks of four clock cycles, and pauses are only allowed between blocks.

Finally, the AGP slot allows for sideband addressing, separating the address and data buses so that the address phase does not use the main address/data lines. The AGP slot achieves this by adding a “SideBand Address” bus that accepts eight-byte requests while other AGP data flows over the 32-bit main bus. This SideBand Address bus significantly improved overall AGP data transfers.

In 2004, the more popular PCIe slot replaced the AGP slot. Manufacturers then stopped including the AGP slot on commercial motherboards.

Industry Standard Architecture

Industry Standard Architecture is a discontinued architecture initially pioneered by IBM PC/AT. The name of the slot is usually shortened to “ISA.” ISA slots were based on Intel and intended to replace them as the standard. However, other bus standards, such as the VESA local and PCI buses, quickly replaced the ISA bus.

ISA buses initially took the name “PC bus” or “AT bus.” The ISA bus was an 8-bit slot on the motherboards of 8088-based IBM PCs, such as the IBM PC/XT and other IBM-compatible PCs. The term “Industry Standard Architecture” or ISA was a retronym brought about by IBM clone companies who wanted to continue using the slot after IBM phased it out in favor of the Micro Channel architecture, which was incompatible with the ISA bus.

The 16-bit ISA slot continued to see use with 32-bit processors for some time. Consumer PCs rarely use Industry Standard Architecture today. The more popular PCI and PCIe standards replaced the ISA slot in consumer PCs. However, industrial computers continue to use the ISA slots as many expansion cards used in industrial computers never transitioned to the PCI and PCIe standards.

Extended Industry Standard Architecture

Extended Industry Standard Architecture, or EISA slots, began as an unsuccessful attempt to transition the ISA slot to a 32-bit bus. The Gang of Nine, a collection of IBM clone companies, announced the EISA slot in 1988.

EISA slots are an expanded 32-bit version of the ISA bus, retroactively renamed by the Gang of Nine to avoid infringing upon IBM’s PC/AT bus. The EISA slot increases the bus size from 16 bits to 32 bits. The EISA bus also allows more than one CPU to share a single expansion bus. EISA bus slots are backward compatible and can accept older ISA cards if necessary.

Extended Industry Standard Architecture slots became a favorite of the server market, and manufacturers tended to prefer these slots where they would traditionally use an MCA slot. EISA slots are also exceptionally good at handling bandwidth-intensive tasks, making them quite popular for server building. However, due to the high price necessary to implement EISA slots, the slots never became popular in the desktop PC market.

Micro Channel Architecture

The Micro Channel Architecture bus superseded the ISA bus in IBM machines. The MCA slot is a proprietary technology of IBM. While other companies could license and use the MCA slot, the slot’s primary use was in IBM machines. The PCI slot ultimately superseded the MCA slot.

IBM designed the MCA slot to address some notable technological limitations of the ISA slot. Notably, the ISA slot struggled with a slow bus speed, a fixed, hardware-side number of interrupts, fixed, hardware-side limits on input and output addresses, complex hardware-side configuration, and deep links to the 80×86 chip family.  ISA slots also suffered from poor power distribution and grounding and many poorly documented interface standards that often varied between system and manufacturer.

However, IBM didn’t maintain the market share they expected to regain from the MCA slot for long. Though the MCA slot addressed the technical concerns of the ISA slot, the PCI slot replaced the MCA slot upon its introduction.

VESA Local Bus

The Video Electronics Standards Association designed the VESA Local Bus to supersede the ISA bus on motherboards and provide an affordable alternative to the MCA bus for accelerated graphics. Manufacturers used the VESA Local Bus alongside the more popular ISA bus to standardize and expand the buses used for accelerated graphics.

VLB slots add a standardized fast path for video cards for accelerated video-mapping inputs and outputs and direct memory access. Another feature of the VLB slot was that users could still use ISA buses to handle essential device functions. ISA buses would handle functions such as interrupts and port-mapped inputs and outputs.

Around the time of the VESA Local Bus’ development, the 8.33 Mb/s standard had become a bottleneck for graphical user interfaces, which dominated the PC market, artificially increasing the need for accelerated graphics within the consumer PC market. IBM’s Micro Channel Architecture offered a 66 MHz alternative to the ISA slot. However, the steep licensing fees that IBM required for manufacturers to implement the MCA slot caused the slot to fail on the market ultimately.

VLB’s speeds depended on the CPU’s bus speed. However, the bus began at rates of 100 Mb/s for CPUs with a bus speed of 25 MHz. VLB speeds went up to 200 Mb/s for CPUs with a bus speed of 50 MHz. These rates were a significant increase from the ISA slot’s speeds. The ease of implementation of the VLB slot made it a tempting choice for manufacturers during its lifespan. However, the PCI slot ultimately replaced the VLB, MCA, and EISA slots upon implementation.

RAM Slots

RAM slots are where you plug in your random access memory. Random access memory is the “short-term memory” of the computer. As the computer processes data for your session, the computer stores various information in the random access memory.

Your computer stores all data that passes through it during the session to the RAM. Open programs, files, and graphics if you don’t have a dedicated GPU with VRAM. Your computer then accesses the RAM to allow you to switch between programs and files without loading them in their entirety whenever you want to move between windows.

However, each time you turn the computer off, cutting power to the RAM, the RAM deletes everything in its memory. This feature can be helpful or frustrating, depending on what you were doing when the power was cut. Unsaved documents that exist only in the RAM are lost when the RAM wipes. Thus, you must save the document’s data to the hard drive to keep a hard copy that doesn’t get wiped when you cut the power to the computer. 

RAM slots are relatively easy to use. They have two locks, one on either side of the slot, and you only have to push the RAM stick into the slot to engage the locks, locking the RAM stick in place. Dual-channel RAM slots color code the slots, showing which slots correlate in a channel.

The only thing you must remember regarding RAM slots is the slot’s generation. RAM of a previous generation is not compatible with a newer slot. So, ensure you have RAM of the correct generation to swap in.

Audio/Modem Riser

An audio/modem riser or AMR slot is a riser slot. Risers are an expansion PCB type that allows users to connect more expansion cards to them. Intel designed AMR slots to interact with the chipset to provide computers with analog functions such as audio and modem connectivity.

Riser technology never took off. So, it’s uncommon to see AMR slots on motherboards, especially since an AMR slot would take up the space of a typical PCI slot. Additionally, including a PCI slot is more versatile for the user than the AMR slot. AMR slots are a depreciated technology replaced by Advanced Communications Risers and Intel’s Communications and Networking Risers.

Advanced Communications Riser

The Advanced Communications Riser is a riser slot developed to supersede Intel’s Audio/Modem Riser and provide competition for Intel’s Communications and Networking Riser. The ACR slot sits on the motherboard alongside the PCI or PCIe slots. ACR slots are a low-cost way of adding certain expansion cards to your motherboard.

ACR quickly replaced AMR slots since ACR slots were both technologically superior and backward compatible with AMR cards. However, ACR slots have become entirely obsolete due to the inclusion of discrete communications and networking ports hardwired onto motherboards.

Communications and Networking Riser

The communications and networking riser was an Intel proprietary riser slot that allowed chipsets to interface with riser cards for expanding your computer with analog ports, mainly for audio and networking. The CNR replaced the AMR standard and was Intel’s proprietary riser card for audio and networking cards.

CNR slots were the direct competitor of the ACR slot. Like ACR slots, discrete hard-wired parts on motherboards have replaced CNR slots, rendering them obsolete.

The image featured at the top of this post is ©Pafnuti/Shutterstock.com.