Hi all,
I know that there is an electrical limitation to the number of slots
that can be loaded on a single PCI bus. Does this limitation exist also
in PCI Express? If so how many slots can be loaded?
TIA.
PCI express is a point to point interface. Lanes are serial connections
at extremely high speeds, and the more lanes you bond together to drive
a slot, the higher the bandwidth. Typical motherboard interface bonded lane
sizes x1, x4, x8, x16. Each lane is 250MB/sec, bidirectional, almost like
gigabit ethernet in bandwidth, only twice as fast.
Chipset
/ | | \
/ | | \
/ / \ \
slot1 slot2 slot3 slot4
In terms of fanout, if you started with a chipset that had 40 lanes,
and connected them to 40 small x1 slots, you can have about 8 times
as many slots as on a typical PCI motherboard. There has to be a
terminating logic block, to take all the decodes for a particular
address and send them to the appropriate lanes.
I believe that there are also PCI Express switching products, implying
the ability to extend the fanout even further. That would be handy,
say, if you had exceeded the practical number of serial interfaces
on a single chip.
A single chip will have thermal limitations, noise limitations (from
all the chunks of logic driving that stuff), and pinout limitations. But
in terms of building a computer product with simple planar distribution
of signals, there is no limit from the chipset, at least if you wanted
to emulate the same performance level as the PCI bus. If we took
our 40 lane chipset and connected to 40 slots of x1 size, that would
give a pretty big motherboard.
You can see in the table here, for example, a chipset with a total of
46 lanes, which can be used to address 16, 16, 8, 1, 1, 1, 1, 1, 1
slots. That is enough I/O for the motherboard slots, plus some PCI
Express integrated chips like Ethernet chips, Firewire chips right
on the motherboard. You could wire a chip like that to 9 of the
tiny x1 slots, as there are nine endpoints (throwing away most of
the total bandwidth).
http://www.nvidia.com/page/nforce5_specs_amd.html
If you need more slots, look at the products here. A huge
fanout is possible, for a lot of money. Maybe some day, there
will be PCI Express to PCI Express expansion here.
http://www.mobl.com/expansion/products/pcie_expansion/index.html
Paul
