Peripheral Component Interconnect

Peripheral Component Interconnect (PCI) is a series of expansion card standards that have been used in Apple's various Macintosh lines.

History
In June 1995, the Power Macintosh 9500 became the first Mac to incorporate a PCI slot, replacing the NuBus architecture that had been in use since the Macintosh II in 1987. The Power Macintosh G4 adopted the PCI eXtended (PCI-X) specification along with a single AGP slot to accommodate faster video cards. Both were replaced by the more advanced PCI Express (PCIe) specification in the Power Macintosh G5.

1st generation Mac Pro models between 2009 and 2012 contain two PCIe 2.0 x4 slots, a single-wide PCIe 2.0 x16 slot, a double-wide PCIe 2.0 x16 slot for a GPU card, and a Mini-PCIe slot for a wireless card.

The 3rd generation Mac Pro, introduced in 2019, features 8 PCIe 3.0 slots which can accommodate proprietary MPX modules. One half-length slot is occupied by an Apple I/O card.

Alternatives
Some compact and portable Macintosh models without room to accommodate expansion cards have included a Thunderbolt port since 2011, which merges PCIe and DisplayPort into a pair of high-speed serial signals, allowing for the addition of external expansion rigs and eGPUs. More recent models have merged Thunderbolt, DisplayPort, and USB 3.x into a single Thunderbolt 3 / USB-C port.