600-2705 PCI Express Expansion System User’s Manual
2-1
Revision 1.0, February 2008
CHAPTER 2
THEORY OF OPERATION
2.1
THEORY OF OPERATION
The basic PCI Express Link consists of dual unidirectional differential links, implemented as a transmit
pair and a receive pair. The signaling rate for PCI Express is 2.5 Gigabits/second/Lane/direction. A
link supports at least one Lane.
The PCI Express link from the PCIe-409 over the cable to the PCIe-414 is an eight lane (x8) link. The
PCIe-414 provides three x8 slots and four four lane (x4) slots. Each slot can accommodate either single
lane (x1), x4 or x8 add-in cards. In the case where a x8 add-in card is installed into a x4 slot, only the
first four lanes on the add-in card will be utilized. This situation is termed “down-shifting”. Per the PCI
Express Specification, down-shifting is only allowed in this case. All slot connectors on the PCIe-414
are mechanically x8 with the x4 slots leaving the upper four lanes not connected. Up-plugging, i.e.,
plugging a smaller link card into a larger link connector, is fully allowed.
Once the PCIe-409 is installed into ahost x8 PCIe slot, the cable connected to the PCIe-414, the chassis
plugged into an AC power outlet and any desired add-in cards are installed, the system is ready to be
turned on. A number of things happen at this point. First, the PCI Express links are initialized. This is
a purely hardware initialization where each PCI Express link is set up following a negotiation of lane
widths by the two ends of each link. No firmware or operating system software is involved. Once the
links are initialized or “trained”, there are LED indicators on each of the Cyclone Microsystems’ cards
that indicate both the links that are trained and the individual lanes. A detailed explanation of the LEDs
follows later in this manual.
One essential requirement for system initialization is the ability of the Host system’s BIOS to be able to
enumerate the many bridges inherent in a complex PCI Express design. The links from the PCIe-409 to
the PCIe-414 are created with PCI Express Switches. Each link looks like a PCI-to-PCI bridge to the
Host’s BIOS. The number of bridges can add up quickly. Older BIOS may not have the ability to
handle the number of bridges. Make sure that the BIOS on the host computer has the latest updated
BIOS. If required, contact the host system’s manufacturer to make sure that the BIOS used can handle
the large number of bridges that it will see in the system.
