PCI Express vs. PCI-X Bandwidth issues

A

anths

Limp Gawd
Joined
Jun 14, 2001
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491
OK guys I have been doing a lot of reading lately on everything coming out and I have a few questions.

It seems like when PCI Express comes out most people are saying there will be 1 16x slot and the rest 1x slots. 1x slots provide a modest 300MB/sec bandwidth, that is less than current 64/66 PCI slots. PCI-X 1.0 is upto 64/133 which is ~ 1GB/sec and can have multiple slots per board. PCI-X 2.0 is upto 64/533, 4.3GB/sec (or the same as 16x PCI Express) and even still I have read about development of a PCI-X interface with speeds of 64/1066 or over 8 GB/sec. It seems that for people who use equipment beyond the normal users (i.e. faster SCSI or even SATA drives) that PCI-X would be the better interface as of now. With PCI-X being able to match if not surpass the PCI-Express interface in speed, What is the advantage besides getting rid of AGP? I have read PCI-Express provides a more direct path to the processor than PCI, PCI-X or AGP but if the raw speed of PCI-X can go up to double that of PCI-Express, what is the point of the more efficient route?
 
PCI Express saves developmental and production costs by reducing trace numbers and possibly board layers. The connectors also take less PCB real estate. Another small advantage to PCI Express is hot-plugging.

PCI-X's main advantage is backwards compatibility to the most popular PCI implementations (32-bit/33 MHz and 64-bit/66 MHz). This is huge to the intended market of PCI-X. Server and workstation operators spend hundreds or thousands on expansion cards, and they want them to last. PCI Express, on the other hand, inherently lacks backwards compatibility with PCI or AGP devices.

edit: Also, PCI Express will most likely scale higher. It's easier to increase clock frequencies on a serial bus than a parallel one like PCI/AGP/PCI-X. Eventually, PCI Express will come out on top, but by the time this happens, a decade will have passed and server/worstation operators will be able to entertain future expansion card purchases.
 
You will see IO cards that use PCI-Express X-8 and up. Also, X-16 is backwards compatible down to X1 if I am correct.
 
That is all correct, but the backwards compatibility doesn't extend to traditional PCI like PCI-X does.
 
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