Parallel PCI standards with 5 V and 3.3 V are incompatible. Whatever is electronically possible is allowed mechanically.
Source: Digi PCI Technology Overview.
When looking at the comparison table below, you might be tempted to question whether PCI-X would be rendered superfluous. While PCI Express will mainly be around in desktop systems, PCI-X will remain the prevailing high-performance interface for high-end workstations and server systems. Finally, PCI-X 1066 will be able to provide up to 8.5 GB/s.
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Standard
Bus Width
Clock
Transfer
PCI 2.3
32 Bit
33 MHz66 MHz
133 MB/s266 MB/s
PCI 64
64 Bit
33 MHz66 MHz
266 MB/s533 MB/s
PCI-X 1.0
64 Bit
66 MHz100 MHz133 MHz
533 MB/s800 MB/s1066 MB/s
PCI-X 2.0 (DDR)
64 Bit
133 MHz
2132 MB/s
PCI-X 2.0 (QDR)
64 Bit
133 MHz
4264 MB/s
PCI Express
1 Lines, 8 Bit
2.5 GHz
512 MB/s
PCI Express
2 Lines, 8 Bit
2.5 GHz
1 GB/s (Duplex)
PCI Express
4 Lines, 8 Bit
2.5 GHz
2 GB/s (Duplex)
PCI Express
8 Lines, 8 Bit
2.5 GHz
4 GB/s (Duplex)
PCI Express
16 Lines, 8 Bit
2.5 GHz
8 GB/s (Duplex)
PCI Express
32 Lanes, 8 Bit
2.5 GHz
16 GB/s (Duplex)
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