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. In August 1999, the high-end Power Mac G4 ("Sawtooth") added a single AGP slot (along with existing PCI slots) to accommodate faster video cards. In June 2003, high-end Power Mac G5 models replaced PCI with PCI eXtended (PCI-X) slots. In October 2005, dual and quad-core G5 models replaced both PCI-X and AGP with the more advanced PCI Express (PCIe) specification in the Power Mac 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 December 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.