What Is PCI?
PCI (Peripheral Component Interconnect) is a parallel computer bus standard introduced in 1992 for attaching expansion cards directly to a system's motherboard. It replaced earlier ISA and VLB slots and became the dominant expansion interface for desktop and industrial systems through the 1990s and 2000s. A conventional PCI slot is a 32-bit or 64-bit parallel bus running at 33 MHz or 66 MHz, operating at either 5 V or 3.3 V signalling. A standard 32-bit, 33 MHz PCI slot provides a theoretical peak bandwidth of about 133 MB/s, while the later 64-bit, 66 MHz variant (often labelled PCI-X at higher clock rates) reaches well over 1 GB/s. Keyed notches on the connector prevent a 3.3 V card from being inserted into a 5 V-only slot, which is why voltage support must be checked when specifying legacy hardware.
PCI vs PCI Express
PCI Express (PCIe) is the serial successor to PCI and is not electrically compatible with it. Where PCI shares one parallel bus among all slots, PCIe uses dedicated point-to-point lanes, so bandwidth scales with lane count and generation rather than bus sharing. The table below summarises the practical differences.
| Feature | Conventional PCI | PCI Express (PCIe) |
|---|---|---|
| Signalling | Parallel, shared bus | Serial, point-to-point lanes |
| Typical widths | 32-bit / 64-bit | x |
