PCIe2-2712 PCI Express Gen2 Expansion System User’s Manual
2-1
Revision 1.0, February 2015
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 Gen2 is 5.0 Gigabits/second/Lane/direction.
A link supports at least one lane.
The PCI Express link from the PCIe2-426 over the cable to the PCIe2-462 is a sixteen lane (x16) link.
The PCI Express link from the PCIe2-425 over the cable to the PCIe2-462 is an eight lane (x8) link. The
PCIe2-462 provides ten slots which are populated mechanically with x8 connectors. All sots can
accommodate either a single lane (x1), x4, x8, or x16 add-in cards. Although not expressly permitted by
the PCI Express Specification, all slots accommodate “down-shifting” a x16 card into x8 slot due to the
PCIe2-462 backplane’s usage of open back PCIe connectors. Plugging a smaller link card into a larger
link connector is fully allowed.
Once the PCIe2-426 or PCIe2-425 is installed into the host PC, the cable connected to the PCIe2-2712
chassis, the chassis plugged into an AC power outlet and any desired add-in cards are installed, the
system is ready to be turned on. When the host is turned on, a signal from the PCIe2-426/PCIe2-425
will turn on the PCIe2-2712 chassis. 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 that the links are trained. 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 PCIe2-426/
425 to the PCIe2-462 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 resources
to enumerate a large number of bridges. Make sure that the BIOS packages on the host computer has the
latest updates. 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.
