rtd cpuModules CME34BT, User Manual, Page: 30 / 110

22 CME34BT cpuModule BDM-610000088 Rev A
Selecting the Stack Order for the CME34BT
There are several things to consider when selecting the order of boards in the stack. Before selecting the
order, be sure to determine which bus connector on each board is the “Active” bus. Typically, if a peripheral
module has both PCIe and PCI bus connectors, only the PCIe is active and the PCI is pass-through. The
following is a list of rules to use to determine the stack order:
1. The PCIe connectors above and below the CPU have completely separate signals. Therefore it is
possible to attach boards to the PCIe connector above and below the CPU.
2. Any board that uses a SATA link must be within one board of the CPU.
3. Any board that uses a PCIe link must be within six boards of the CPU. Peripheral boards that
repopulate a PCIe link should be placed closer to the CPU than any peripheral boards that use a
PCIe link without repopulating.
4. To preserve power integrity, it is recommended that there be no more than six boards between the
CPU and the power supply.
5. In order to maintain maximum performance over the full temperature range, it is recommended
that a PCIe spacer be used between the CPU and any board immediately above it.
6. A maximum of four PCI boards may be attached to any PCI bus. If more than four PCI boards are
needed in a system, a PCIe to PCI bridge is required.
7. The CPU or PCIe to PCI bridge (if one exists in the stack) must be at one end of the PCI bus
segment, and all of the peripheral cards at the other end. There may be up to eight PCI pass-through
connectors between the CPU or PCIe to PCI bridge and the peripheral cards.
8. There must be no more than two boards between the first PCI peripheral and the last PCI
peripheral. If there are four PCI peripheral cards in a PCI bus segment, there may not be any PCI
pass-through connections between them.
9. The standard CPU has the PCI connection below the board only. This means that all boards with
an active PCI bus interface must be below the CPU. Contact RTD sales if it is required to have a
PCI board above the CPU.