Editing Peripheral Component Interconnect (section)

===Mixing of 32-bit and 64-bit PCI cards in different width slots===
[[File:PCI-X in a 32-bit slot.jpg|thumb|A semi-inserted PCI-X card in a 32-bit PCI slot, illustrating the need for the rightmost notch and the extra room on the motherboard to remain backward compatible]]
[[File:64bit PCI SCSI card in a 32bit PCI slot.jpg|thumb|64-bit SCSI card working in a 32-bit PCI slot]]
Most 32-bit PCI cards will function properly in 64-bit PCI-X slots, but the bus clock rate will be limited to the clock frequency of the slowest card, an inherent limitation of PCI's shared bus topology. For example, when a PCI 2.3, 66-MHz peripheral is installed into a PCI-X bus capable of 133&nbsp;MHz, the entire bus backplane will be limited to 66&nbsp;MHz. To get around this limitation, many motherboards have two or more PCI/PCI-X buses, with one bus intended for use with high-speed PCI-X peripherals, and the other bus intended for general-purpose peripherals.

Many 64-bit PCI-X cards are designed to work in 32-bit mode if inserted in shorter 32-bit connectors, with some loss of performance.<ref>{{cite web
 |url          = http://docs.znyx.com/products/hardware/zx370.htm
 |title        = ZX370 Series
 |access-date   = July 13, 2012
 |author       = ZNYX Networks
 |date         = June 16, 2009
 |quote        = The ZX370 Series is a true 64-bit adapter, widening the network pipeline to achieve higher throughput while offering backward compatibility with standard 32-bit PCI slots.
 |archive-url  = https://web.archive.org/web/20110502042329/http://docs.znyx.com/products/hardware/zx370.htm
 |archive-date = May 2, 2011
 |url-status   = dead
}}</ref><ref>{{cite web
| url          = http://docs.znyx.com/products/pdf/ZX370_Datasheet_02_web.pdf
| title        = ZX370 Series Multi-Channel PCI Fast Ethernet Adapter
| access-date   = July 13, 2012
| author       = ZNYX Networks
| quote        = Backward compatible with 32 bit, 33&nbsp;MHz PCI slots
| archive-url  = https://web.archive.org/web/20130720174917/http://docs.znyx.com/products/pdf/ZX370_Datasheet_02_web.pdf
| archive-date = July 20, 2013
| url-status   = dead
}}</ref> An example of this is the Adaptec 29160 64-bit [[SCSI]] interface card.<ref>{{cite web
| url         = http://www.adaptec.com/en-us/support/scsi/u160/asc-29160/_docs/29160_users_reference_pdf.htm?nc=/en-us/support/scsi/u160/asc-29160/_docs/29160_users_reference_pdf.htm
| title       = Adaptec SCSI Card 29160 Ultra160 SCSI Controller User's Reference
| access-date  = July 13, 2012
| author      = Adaptec
|date=January 2000
| format      = pdf
| page        = 1
| quote       = Although the Adaptec SCSI Card 29160 is a 64-bit PCI card, it also works in a 32-bit PCI slot. When installed in a 32-bit PCI slot, the card automatically runs in the slower 32-bit mode.
}}</ref> However, some 64-bit PCI-X cards do not work in standard 32-bit PCI slots.<ref>{{cite web
 |url=http://www.lacie.com/support/support_manifest.htm?id=10525&article=1164 
 |title=LaCie support: Identify a variety of PCI slots 
 |access-date=July 13, 2012 
 |author=LaCie 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20120404080344/http://www.lacie.com/support/support_manifest.htm?id=10525&article=1164 
 |archive-date=April 4, 2012 
}}</ref>{{Unreliable source?|date=July 2012}}

Installing a 64-bit PCI-X card in a 32-bit slot will leave the 64-bit portion of the card edge connector not connected and overhanging. This requires that there be no motherboard components positioned so as to mechanically obstruct the overhanging portion of the card edge connector.{{clear}}