Kontron COMe Starterkit Eval T2 - Manual (Page 33)

COM Express Interfaces
2.3.4. Device Up / Device Down and PCIe Rx / Tx Coupling Capacitors
Figure 5: 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 Ex 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. See Section 6.5.1. 'PCI Express Trace Routing Guidelines' on
page 182 for trace length details.
Device Up:
● Coupling caps for the target device PCIe TX lines (COM Ex 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. See Section 6.5.1 'PCI Express Trace Routing
Guidelines' on page 182 for trace length details.
The coupling caps for the Module PCIe TX lines are defined by the COM Express specification to
be on the Module.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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COM Express Module
TX+
TX-
RX+
RX-
TX+
TX-
RX+
RX-
PCIe
Add-in
Device
PCIe
Add-in
Device
COM Express Carrier Board
Device Up
Device Down
RX+
RX-
TX+
TX-
RX+
RX-
TX+
TX-