Advantech SOM-ETX Design Guide
SOM-ETX
LAN_RXD-
ACTLED
LAN Connector
RJ45
LAN_TXD-
SPEEDLED
Magnetic
Module
(Transformer)
Transmit
Differential Pairs
Receive
Differential Pairs
EX: DEATA LF8505
Figure 5-25 10/100M Ethernet Connections
5.8.2.2 Power Consideration and Ethernet LED
In general, any section of traces that is intended for use with high-speed signals
should observe proper termination practices. Many board layouts remove the ground
plane underneath the transformer and RJ45 jack to minimize capacitive coupling of
noise between the plane and the external Ethernet cable.
And some RJ-45 including LED need 3.3 volts to drive link and action LEDs, so we
need provide 3.3 volts on carrier board. Link and activity LED can be implemented by
using the SOM-ETX module’s LILED and ACTLED pins. These pins sink current and
are intended for attachment to a LED cathode. The anode of the LED should be
pulled to 3.3 volts through a resistor of 330 Ohms or greater.
5.8.3 Layout Guideline
The transformer should be placed close to the RJ-45 connector to limit EMI
emissions. Each differential pair of data signals of TX+/TX-, RX+/RX-, T+/T- and
R+/R- is required to be parallel to each other with the same trace length on the
component (top) layer and to be parallel to a respective ground plane. Four 49.9-ohm
pull-down resistors for both RX+/- and TX+/- nets respectively are suggested being
located as close to the transformer as possible.
Figure 5-26 Ethernet Layout Guidelines
5.8.3.1 Transformer
SOM-ETX
LAN_RXD-
ACTLED
RJ45
Connector
LAN_TXD-
SPEEDLED
TD+
TD-
RD+
RD-
CT_RD
CT_TD CT_TX
CT_RX
TX+
TX-
RX+
RX-
4
5
7
8
1
2
3
6
0.01u
/1KV
75ohm
0.1u
0.1u
0.1u(NL)
49.9ohm
(NL)
49.9
ohm
(NL)
330 ohm
330 ohm
Route to Minimum
Route to Minimum
LED
+3.3V
Magnetic
Module
(Transformer)
90
Chapter 5 Carrier Board Design Guidelines
