Expansion Slots

An expansion slot provides a fast link to a computer’s processor, accommodating cards for enhanced video, a modem, sound processing or Ethernet. Several types are used, although the PCI and PCMCIA varieties are the most common.

Advanced Graphics Port (AGP)

This kind of expansion port, found in many modern computers, accommodates a video card that provides the video signals for a monitor. Traditionally, the display connects via a Super Versatile Graphics Array (SVGA) port, although more recent cards provide a Digital Video Interface (DVI) or Apple Display Connector (ADC) output for an LCD screen.

Unlike PCI slots (see below), which use a 32-bit or 64-bit data path, AGP slots provide a 64-bit connection to the processor. Cards and slots come in various speeds, as shown in below:-

SpeedBus Clock (MHz)
Standard66 •
2 Times (2x)133
4 Times (4x)266
8 Times (8x)533
Same speed as an Apple PCI video card

Peripheral Component Interconnect (PCI)

This high-speed slot, used in earlier Mac OS machines and PCs can be used for co-processors, multiple-processors or video compression hardware. Apple’s original version, known as Apple RISC Bus (ARBus), uses a bus clock of 33 or 66 MHz, although older cards and slots only run at 33 MHz. The 33 MHz version accommodates a peak transfer rate of 133 MB/s whilst the 66 MHz variety conveys up to 266 MB/s. The bus is 32 bits wide.

Some older PowerPC-based Mac OS machines have a special slot for the use of a video card, which provides accelerated graphics or 3D graphics for games and support for DVD Video discs. This kind of slot in G3 models runs at 66 MHz, twice the speed of other PCI slots, but in later machines is superseded by the faster AGP or PCIe forms of connection.

Cards come in 7-inch and 12-inch versions, although some slots can’t accept the larger cards. There can be up to 10 slots, accommodating up to 5 cards with 32-bit or 64-bit addressing. In practice, most machines can only take three 64-bit cards, often in addition to a 32-bit slot for video. Fortunately, you can also insert a 32-bit card into a 64-bit slot. The more common 32-bit cards and buses only use pins 1 to 62, whilst 64-bit hardware uses all the pins up to pin 94. Cards and buses may operate with 5 or 3.3 volt signalling, so key notches are provided to prevent the insertion of inappropriate cards.

PCI in a PC

Many modern PCs have PCI/ISA slots that provide connectors for both a PCI and an ISA card (see below). In addition, some older PCs employ a PCI accelerator card to accommodate the modern Windows operating system.

Some cards also add extra features, built into later machines, but not found in older models. For example, a typical multi I/O controller card provides a hard disk controller, floppy disk controller, parallel port, two serial ports and a game port. A card of this type usually needs to use an extra slot for the actual connections, in this instance for one of the serial ports and for the game port. Some PCI cards are supplied with a paddle board that must be plugged into an adjacent ISA slot.

Peripheral Component Interconnect Extended (PCI-X)

A more modern variation of PCI that operates at 133, 266 or 533 MHz, giving transfer rates of around 1014, 2035 and 4070 MB/s. Note the installation of an older PCI card results in the entire bus running at the rate required by the earlier card.

Peripheral Component Interconnect Express (PCIe or PCI-E)

This is the latest replacement for the earlier PCI and PCI-X slots, formerly known as 3GIO or Arapaho. It isn’t compatible with earlier cards, since it uses multiple serial connections instead of a parallel connection. Coneying data at 250 MB/s over 16 serial channels it offers a total transfer rate of 4 GB/s. There can be 1, 2, 4, 8, 12, 16 or 32 serial channels or ‘lanes’ in use, although there must be a sufficient number of lanes in a slot to support the attached card.


This slot conforms to standards set by the Personal Computer Memory Card International Association (PCMCIA) and is a variant of the PCI bus, but with a 68-pin connector. Originally used in Apple’s discontinued Newton organiser, it often appears on modern portable computers.

Older PC Cards only handle 6-bit data at rates of up to 20 MB/s. However, an improved form of card known as CardBus accommodates 32-bit data at up to 132 MB/s. These newer cards must be fitted in a compatible slot, although a non-CardBus card works in any slot, even one that supports CardBus.

Card Types

The cards come in three basic types:-

Some portable computers accept two Type II cards or one Type III card, the latter usually in the lower slot to prevent you inserting a second card. A Type III card may contain a miniature hard disk drive or consist of a zoomed-video (ZV) card, the latter used to generate or process video signals, as extracted from a DVD-ROM drive or a camera.

PC Card Products

Various kinds of cards are available, including:-

Depending on your computer, you may need extra software. For example, a USB card designed for use in a portable PC won’t work in an Apple PowerBook with an older system unless you have the necessary USB class driver software installed.

The following points are worth noting:-

Special Mac Slots

Older ‘classic’ Mac OS machines have various special slots that can be used to add extra features. For example, some older PowerPCs have a video-in slot for the Apple Video System and a TV tuner connector, whilst older PowerBooks have internal slots for a modem card, video output card or Ethernet card. The following types are often encountered:-

Cache Slot

A special slot that accommodates a Level 2 cache in an older machine that doesn’t have one. The installation of a cache card can considerably improve the speed of such a computer.

Communications Slot II

This type of slot is often used for to accommodate an Ethernet card, which provides a connection via thin coax (10Base-2), twisted-pair (10Base-T or 100Base-T), an AAUI interface, or a combination of these systems.

Digital Audio Video (DAV)

This connector, fitted to the motherboard of some Mac OS computers, lets you install an extra card for compressing or decompressing digital audio and video material. This type of card is often used for 24-bit video processing, providing support for a range of video formats, including YUV, JPEG, MPEG, DVI or H.261. Some machines come with such a card already fitted.

Mezzanine Slot

Also known as a Perch slot, this only appears on the ‘A’ version iMac, hidden behind a panel near other sockets. Although unsupported by Apple, it’s used by some third-party products. Unlike the PCI Mezzanine Card (PMC) or Personality Slot found in desktop PCs, which uses four 64-way sockets, this has a 160-way socket conforming to the PCI electrical standard.


This kind of slot was fitted in later 680x0-based Macs. With suitable hardware, up to 15 cards can be accommodated, each with an address numbers from 0 to 14, the computer itself having the number 15. Up to eight 12-inch cards of the NuBus 87 variety can be accommodated in a Mac II.

Although the cards talk to the processor at 3 MB/s, they correspond with each other at between 10 and 20 MB/s. The backwards-compatible NuBus 90 card, introduced with the Quadra 700 and 900, runs at 30 MB/s. Unfortunately, the speed reverts to the slower rate if a NuBus 87 card is present.

Processor Direct Slot (PDS)

The PD slot or PDS is the oldest form of expansion slot. It uses a special 96-way DIN connector, giving direct access to the data lines and address lines of the 680x0 processor used in older Mac OS machines. Different types of PDS are used for different processors, so you must have the correct type of card to suit the machine. The 16-bit forms of PDS include SE-Bus (Mac SE), 030 Direct Slot (Mac SE/30), PDS (Mac Portable) and 020 Direct Slot (Mac LC).

Special PC Slots

Various slots have been fitted in PCs over the years. With the notable exceptions of AGP and PCI (see above), none are normally used with Mac OS computers. The less common formats are listed below.

Industry Standard Architecture (ISA)

This standard slot is commonly used to accommodate a modem or other devices. The 8-bit variety, also known as an XT expansion card, has a 2-row 62-way connector for 8-bit data, address lines and control circuits. A card of this type can also incorporate links to adjust its operation to suit a real XT machine or a modern ISA slot. The 16-bit version, also known as an AT expansion card, has an extra 2-row 36-way connector (furthest from the faceplate) for the additional connections.

Most modern PCs have these obligatory ISA slots, although these may be supplemented or replaced by PCI/ISA slots that have sockets for both PCI and ISA cards. An ISA card can also be inserted into an EISA slot or VL-bus slot (see below). Since ISA uses only 24 address lines its addressing range is limited to 16 MB. The maximum data transfer rate varies from 1.5 to 5.3 MB/s.

Extended ISA (EISA)

This 32-bit slot provides compatibility with older 16-bit ISA cards by using two-tier connectors, with the lower set carrying the extra 32-bit circuits and the upper contacts conveying the normal ISA circuits. Unfortunately this is expensive and few EISA cards are encountered. The bus runs at 8.33 MHz, but provides burst transfers of up to 33 MB/s.

Micro Channel Architecture (MCA)

This 16-bit or 32-bit slot, devised by IBM, provides an average transfer rate of 20 MB/s (similar to NuBus 97) and accommodates any number of processors. Its incompatibility with ISA and EISA has led to its gradual downfall.

VESA Video Local Bus (VL-bus or VLB)

This 32-bit or 64-bit slot was originally devised by the Video Electronics Standards Association (VESA) for the 486 processor. Up to three slots can be accommodated, and each slot also accepts an ISA card, but not an MCA or EISA card. Connectors are the same as an ISA card but with additional narrow-pitch connectors at the rear. The bus runs at the processor’s own clock, up to a maximum of 40 MHz. Typical burst speed transfer rates are 130 MB/s (32-bit) or 250 MB/s (64-bit).

©Ray White 2006.