User manual

Advantech SOM-Express Design Guide
Chapter 5 Carrier Board Design Guidelines 83
SOM-Express
TX
RX
RX
TX
PCI
Express
Device
L2L3
L1
Figure 5-49 Topology #1 – SOM Express to PCI Express Device Down
Table 5-28 SOM Express to PCI Express
L1 L2 L3 Capacitor Value
Max = 14.75 inches Min = 0.25 inches
Max = 14.5 inches
Max = 14.75
inches – L2
75 nF to 200 nF,
Tolerance = 20%,
Package: recommended to use 0603,
or use 0402 if necessary
5.10.3.4 PCI Express Topology #2 and #3 – Device Down Routing
Both the ExpressCard and the docking topologies allow a maximum of 10 inches
from SOM-Express pin to the pin of the down device. This maximum length takes into
account all routing, including the breakout region, which should not exceed 0.25
inches per device. The TX and RX pairs must be routed interleaved, such that the
pairs alternate between TX and RX on the carrier board to reduce crosstalk for
microstrip and stripline topologies.
SOM-Express
TX
RX
RX
TX
Express
Card or
Docking
connector
L1
L2
Figure 5-50 Topology #2 and #3 – SOM Express to Express Card or Docking Conn.
Table 5-29 SOM Express to Express Card
L1 L2 Capacitor Value
Max = 10 inches Max = 10 inches 75 nF to 200 nF,
Tolerance = 20%,
Package: recommend to use 0603,
or use 0402 if necessary