PCI Express - Coming Soon to a PC Near You

Max Romantschuk writes "I've been following the emerging of PCI Express for some time now. PCI Express, previously known as "Third Generation I/O" or "3GIO", is the technology set to replace PCI. PCI has been with us for around ten years now, and is rapidly running out of bandwidth. Last week Anandtech ran an interresting story on PCI Express. The techology has previously been covered by Hexus and ExtremeTech aswell. I feel this technology looks all set to replace PCI, and we really do need some new bus technology to keep up with the bandwidth demands of today's applications. Or is this just yet another way to force us into a new upgrade cycle?"

Comments

It will not just replace PCI

[motown]

Due to its high bandwidth, it's expected to replace AGP as well.

Re:Speed

[ViXX0r]

As I understand it, using serial there is no having to worry about whether all the bits arrive at the same time (as there obviously is with parallel), and so the speed of transmission can be dramatically increased past the point at which it becomes faster than the "equivalent" parallel technology... bits arrive in the order they were sent - guaranteed.

Re:Speed

[KrishnaACD]

I wondered this too, so went digging. the most concise and, to me, most credible answer was the following (Credit to K. Adam's at Geek.com)

Serial Faster than Parallel... (5:41pm EST Wed Jul 25 2001) The problem with parallel (ribbon) data transfer cables is the crosstalk that occurs between adjacent conductors at very high clock/transfer speeds. IBM developed a work-around for ATA-66 and ATA-100 by using an 80-conductor cable with a 40-pin interface, by stringing a "ground" conductor between each "signal" conductor. Capacitance issues, "standing waves," and impedance (electrical resistance as relates to rapidly-changing voltages) matching problems become more evident in parallel (ribbon) cables as you crank up the clock/transfer speeds, also. It's a lot easier to match the impedance of a few conductors in a serial cable to its interface, than trying to match impedance for 40 conductors. Parallel schemes actually have a lot less "processing" overhead than serial schemes, but you're ultimately limited by physics a lot more quickly than with serial... - by K. Adams

Kacd.

Parallel faster than Serial

[gorjusborg]

using serial there is no having to worry about whether all the bits arrive at the same time (as there obviously is with parallel), and so the speed of transmission can be dramatically increased past the point at which it becomes faster than the "equivalent" parallel technology... bits arrive in the order they were sent - guaranteed.

I'm afraid this might add to the confusion about serial interfaces being 'faster' than parallel. While it is true that you don't have to worry about data/clock skew when using serial interfaces, enabling you to clock them faster, a parallel interface running at the same clock speed as a serial interface will always be faster in terms of data throughput. The reason for this is simple: serial == 1 bit per clock, parallel = > 1 bit per clock.

So, saying that serial is faster than the "equivalent" parallel interface is confusing, and incorrect, because one could be referring to equivalent clock rates being used for each interface, in which case parallel will provide at least twice the data throughput. On the other hand, "equivalent" could be referring to identical throughput rates, in which case the serial and parallel interfaces would provide, by definition, identical data rates.

The real advantage that PCI Express has over PCI/PCI-X is that it is a point-to-point, rather than a multi-drop, bus. This setup requires less time between pin transitions, meaning that it can be clock faster. Also, like Ethernet, a serial protocol can imbed the clock into the data stream so clock/data skew is no problem whatsoever.

Serial is not better than parallel anymore than digital is better than analog, there are just physical reasons why implementing point-to-point serial at significantly higher clock rates is easier than multi-drop parallel.

Anyone still awake?
Didn't think so :-l