A Short Note on PCI, PCI-X and PCI Express

PCI refers to Peripheral Component Interconnect. PCI is a standard that provides high performance I/O interconnect technology which enables data transfer between a motherboard/CPU and its peripherals/add-on components. In this article let us look at a short introduction of the three main PCI standards – PCI, PCI-X and PCI Express.

But before that let us look at some devices that could be connected to the CPU using PCI standards – Wi-Fi Cards, Bluetooth devices, Network Interface Cards (NIC), Modems, Sound/Graphics cards, Storage/disk controllers, TV tuner cards, Analog/Digital interface cards for IP Telephony, etc. Actually, PCI slots are available in PC/server and PCI connectors are available with these devices. One can fix the PCI connectors into the PCI slot to make a connection between the CPU and the device.

There is a industry association called PCI-SIG (PCI Special Interest Group) that forms and distributes PCI specifications. Since PCI is an open-standards based technology, PCI card made by one vendor can be fixed into a PCI slot made by a different vendor. PCI-SIG ensures this interoperability, mostly for similar PCI standards.


PCI is the oldest interconnect standard. A 32 bit PCI bus supports a maximum speed of 33 Mhz and 133 Mbps of data throughput.  There are two types of PCI cards – Cards that support 5V and cards that support 3.3V. There are certain universal PCI cards that support both 5V and 3.3V. The PCI cards that support 5V cannot be fixed into PCI slots meant for 3.3V and vice versa. PCI cards do not fit into PCI Express (PCIe) slots as well.


PCI-X is also referred to as PCI Extended and it is based on PCI architecture. PCI-X is backward compatible with PCI 3.3V and universal PCI boards. A 64 bit PCI-X bus supports a throughput of 1 Gbps and even more. With the increasing popularity of PCI Express, PCI-X standard has become less popular.

PCI Express (PCIe)

PCI Express (PCIe) is the latest standard that is already available on many computer and server motherboards that supports a high throughput (Multiple Gbps per slot) sufficient for many bandwidth demanding applications required by today’s peripheral devices.

The older PCI standards uses a shared architecture in which the CPU and all peripheral devices share address/data/control lanes. PCI Express technology follows a point to point topology with separate serial links connecting each device to a common switch that connects to the CPU. In PCIe, each peripheral device has exclusive access to the switch (and hence the CPU). PCIe bus supports full duplex communications between any two endpoints connected to it.

PCIe based devices are not compatible with older PCI/PCI-X slots. So, in many computers/servers both PCI slots and PCI Express slots are provided. There are different versions of PCI Express cards/slots – x1, x2, x4, x8, x16, x32. Slot size, connectors and supported speeds are different for each version. Smaller PCIe cards fit into larger PCIe slots (x4 based PCIe card can fit into x16 based PCIe slot), but not vice-versa. Have a look at the photo of three different PCIe slots and one PCI slot, available here.

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