Pcie Signals Ac Couple; Figure 3 Pcie Rx Coupling Capacitors - Emerson COM Express Carrier Design Manual

3.4.2 PCIE Signals AC Couple

Figure 3 PCIe Rx Coupling Capacitors

Device Down refers to a PCIe target device implemented down on the carrier board. Device Up refers
to a PCIe target device implemented on a slot card (or mini PCIe card, ExpressCard, AMC card).
There are several distinctions between a PCIe Device Down and Device Up implementation.
Device Down:
 Coupling caps for the target device PCIe TX lines (COM Express module PCIe RX lines) are down on
the carrier board, close to the target device TX pins;
 Trace length allowed for PCIe signals on the carrier board is longer for the Device Down case than for
Device Up.
Device Up:
 Coupling caps for the target device PCIe TX lines (COM Express module PCIe RX lines) are up on the
slot card.
 Trace length allowed for PCIe signals on the carrier board is shorter than for the Device Down case,
to allow for slot card trace length.
The coupling caps for the COM Express module PCIe TX lines are defined by the COM Express
Specification to be on the module.
Page 22 of 103 COM Express Carrier Type 2
Design Guide