Kontron COMe Starterkit Eval T2 - Manual (Page 53)

COM Express Interfaces
The x16 connector usually is used to drive the PCI Express Graphics Port (PEG) consisting of 16
PEG lanes, which are connected to the appropriate x16 connector pins. For more information
about the signal definition of the PEG port, refer to Section 2.4. 'PEG (PCI Express Graphics)' on
page 48 above.
Note: Auxiliary signals
The auxiliary signals are provided on the PCI Express connectors to assist
with certain system level functionality or implementations
. Some of these
signals are required when implementing a PCI connector on the Carrier Board .
For more information about this subject, refer to the PCI Express Card
Electromechanical Specification, Rev
. 1.1 Section 2.
2.4.4. Routing Considerations
Please refer to Section 2.3.6 'PCI Express Routing Considerations' on page 47 above.
2.4.4.1. Polarity Inversion
Per definition, PCI Express supports polarity inversion by each receiver on a link. The receiver
accomplishes this by simply inverting the received data on the differential pair if it detects a
polarity inversion during the initial training sequence of the link. In other words, a lane will still
work correctly if a positive signal 'PEG_TX+' from a transmitter is connected to the negative
signal 'PEG_RX-' of the receiver. Vice versa, the negative signal from the transmitter 'PEG_TX-'
must be connected to the positive signal of the receiver 'PEG_RX+'. This feature can be very
useful to make PCB layouts cleaner and easier to route.
Polarity inversion does not imply direction inversion, this means the 'PEG_TX' differential pairs
of the Module must still be connected to the 'PEG_RX' differential signal pairs of the device.
2.4.4.2. Lane Reversal
During the PCB layout of a COM Express Carrier Board, it is quite possible that the signals
between the Modules connectors and the PCI Express device on the Carrier Board have to be
crossed. To help layout designers overcome this signal crossing scenario, PCI Express specifies
Lane Reversal. Lane Reversal is the reverse mapping of lanes for x2 or greater links.
For example, on a link with a width of x16, which supports Lane Reversal, the TX0, TX1, ...
TX14, TX15 of the transmitting device have to be connected to RX15, RX14, ... RX1, RX0 of the
receiving device, and vice versa. See Figure 19 below.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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