Build A PC

A custom PC configuration guide. © 2000,2001,2002,2004 by Jef Poskanzer.

PCI Types

The PCI world is somewhat of a mess right now. There are at least TEN different types of PCI socket in use!

PCI Classic

(case exterior side)
32 bits / 33 MHz
1 Gbps
64 bits / 33 MHz
2.1 Gbps
64 bits / 66 MHz
4.3 Gbps
64 bits / 66 MHz
4.3 Gbps

As you can see, the wide and fast/wide slots are just lengthened versions of the regular slot. In fact they are compatible. You can use regular PCI cards in wide slots if you want to. As for the reverse, using wide PCI cards in regular PCI slots, if there's physical clearance for the extra part of the card then it's supposed to work.

Wide and fast/wide are the same physical slot and should be compatible with each other in both directions. The card and the slot negotiate with each other to determine the highest speed they both support. If you use a fast/wide card in a wide (not fast) slot, the card will just run at 33MHz instead of 66MHz. Same for a wide (not fast) card in a fast/wide slot - it'll just run at 33MHz.

However, while the 3.3V slot is the same length as the 5V wide slots, it is keyed differently, and is generally not compatible with the others. There are exceptions - some PCI cards have all three key cuts in them, and can be used in all three types of PCI slot. They have the smarts to figure out whether to use 5V or 3.3V, 66MHz or 33MHz, and 64 bits or 32 bits. Examples: the syskonnect gigabit ethernet card, and the Adaptec 39160 dual Ultra160 SCSI card.


PCI-X is a generalization of the fast/wide/fast-wide variations. It uses the same physical slots as conventional PCI, but can negotiate speeds of 100MHz, 133MHz, 266MHz, or even 533MHz.

There's also a narrow version of the slot - physically narrow, not the data width.


This was originally called 3GIO. It's supposed to be "software compatible" with previous PCI standards, but is otherwise completely different. The slots are smaller, and come in a variety of lengths. Signalling is both serial and parallel. There's serial signalling on each line, at 2.5GHz with an 8bits in 10bits encoding; and you can have up to 16 lines. It's also full-duplex - you can send data in both directions simultaneously.

The different widths of PCI-Express are upwards compatible - cards should work in any slot equal to or wider than their own size. A 1x card should work in any width slot, a 4x card should work in 4x, 8x, and 16x slots, and so on.

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