Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <10vvejoo83m2vf5@corp.supernews.com>, "Jake"
<jfoster1@nospamcableone.net> wrote:
> I have noticed on some of the newer boards that have the PCI-E slots. I know
> there are the x8 slots for the video, but what are the x1 slots for.
> Thanks in advance for your time
In the same sense that PCI replaced ISA, the PCI Express
slot is the replacement for the PCI bus slot.
Here is a post from 1995, where some engineers explain how PCI
is better than ISA. In that post, the engineers said ISA could
practically offer about 2MB/sec, while PCI offers 100-110MB/sec
while bus mastering.
http://groups.google.ca/groups?threadm=41sclf%24o4m%40easy2.easynet.com
The PCI bus is 32 bits wide, and the typical desktop PCI bus runs
at 33MHz. This is 132MB/sec, minus the overhead (one address cycle
followed by a burst of data cycles, in the best cases).
PCI Express is similar to the transition from PATA to SATA disk
interfaces. PATA (IDE) used a wide slow parallel bus (the ribbon
cable), while SATA uses a fast serial bus. PCI Express is also a
high speed serial bus. PCI Express has separate serial paths for
transmit and receive, so the paths to the PCI Express x1 card are
full duplex.
The data rate on the serial interfaces is 2.5Gbit/sec. This is in
the form of 8b10b encoded data (a format intended to have a certain
minimum transition density, which makes recovering the clock easier).
Due to that coding format, the usable data after decoding is
2.0Gbit/sec or 250MB/sec. (I haven't been able to find a figure
for what the exact data rate is, down to the last decimal point, so
I hope these numbers are close enough.) That rate can flow in both
directions at once, unlike the PCI bus which is half duplex (one
direction at a time).
A 100MHz clock is fed to each PCI Express slot, as a clock reference.
The 100MHz clock is multiplied up, some how, to make the data come
out at the 2.5Gb/sec rate. (So, the clock isn't applied directly,
but is used to synthesize a higher rate clock to make the data
stream.)
PCI Express has a format like a data packet on a network interface.
This web page has a picture of the format, and the picture shows
some of the overhead involved on that 250MB/sec rate. Just like
PCI, the link will not reach 250MB/sec, but will practically offer
some lower rate. If the data phase had an extremely long burst,
you would get close to 250MB/sec, but many applications will use
shorter transfers than the maximum length allowed.
http://www.esemagazine.co.uk/common/viewer/archive/2004/Jul/26/feature3.phtm
As for what functions to expect first for your PCI Express x1 slot,
I think I've seen PCI Express Ethernet chips announced. Storage
controllers (RAID and otherwise) should also be a popular thing.
Devices with lower data rates may take some time before they are
converted to PCI Express, and smaller companies may not have a
lot of development money to throw at changing their product line
overnight.
Will we ever see a PCI Express modem card or serial port card ?
Dial up modems and RS232 serial ports are kind of a dead business,
so you might have a very long wait for a PCI Express version.
In those cases, when motherboards eventually are fully PCI Express,
a USB based solution to those problems might be a better fit.
Paul
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