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Table 10 : PCI Express* Slot Card / Device Down Trace Length Guidelines
Parameter
Signal Group
Differential Impedance Target
Single End
Isolation to equivalent pairs
Isolation to other signal groups
Tx/Rx Spacing
LA + LB
Lc
Max length of LA+LB+LC
Length matching
Reference Plane
Carrier Board Via Usage
AC coupling
Notes:
1. H is height above reference plane.
2. Equivalent pairs are TX to TX or RX to RX(Noninterleaved)
3.
AC caps are recommended to be placed close to PCIe device side (avoid placing AC cpas on
mid-bus).
Main Route Guidelines
PCI Express* expansion
85Ω±10%
55Ω±10%
11H (MS) and 3H(DS)
11H (MS) and 5H (DS)
11H(MS) and 5H (DS)
Please see the
SOM-5992 Layout Checklist
Carrier Board Length
Slot Card:
14"
Device
Down:4~ 16"
Differential pairs (intra-pair): Max.
REFCLK+ and REFCLK- (intra-pair):Max. 5mils
GND referencing preferred
Min 40-mil trace edge-to-major plane edge spacing
GND stitching vias required next to signal vias if
transitioning layers between GND layers
Power referencing acceptable if stitching caps are
used
Max. 2 vias per TX
Max. 4 vias per RX (to device)
Max. 2 vias per RX (to slot)
The AC coupling capacitors for the TX lines are
incorporated on the COM Express Module. The AC
coupling capacitors for RX signal lines have to be
implemented on the customer COM Express Carrier
Board. Capacitor type: X7R, 100nF ±10%, 16V, shape
0402.
４１
Notes
1,2
1
1
2
mils
3
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