🛠 Double Ethernet : GbE RTL8211F et 10BASE-T1L sur RJ11

Objectives

The BeaglePlay features two independent Ethernet controllers: a 10/100/1000 RJ45 port behind a RTL8211F PHY, and a long-distance RJ11 port using 10BASE-T1L (single-pair Ethernet) behind a DP83TD510E PHY, limited to 10 Mbit/s but capable of kilometers of cable. We configure both interfaces, verify the link, and document PoDL: Power over Data Line is an output-only 5 V / 250 mA supply for powering a remote device, never an input.

Hardware

• A BeaglePlay board
• An RJ45 cable to a switch
• A twisted pair to a 10BASE-T1L device (or a second board in a loop)

Complete Code

ip -br link          # eth0 (GbE RTL8211F) et eth1 (10BASE-T1L DP83TD510E)
sudo ip addr add 192.168.100.1/24 dev eth1
sudo ip link set eth1 up
sudo ethtool eth1    # Supported link modes: 10baseT1L/Full

# DHCP client sur le GbE
sudo dhclient eth0

# Le PoDL est une SORTIE : 5V / 250mA max vers le périphérique distant
sudo ethtool --show-priv-flags eth1 2>/dev/null | grep -i podl

Expected Verification

ethtool eth0 reports Speed: 1000Mb/s once connected to the switch; ethtool eth1 only accepts 10BASE-T1L full duplex and shows Link detected: yes when the pair is correct (A/B inversion tolerated by the PHY). A cross-ping between two BeaglePlay boards on eth1 passes at ~10 Mbit/s. The RJ11 connector indeed provides 5 V PoDL as an output, limited to 250 mA.

Common Errors

• Injecting 5 V from the outside into the RJ11: PoDL is output-only, you will burn the PHY.
• Expecting gigabit on the RJ11 port: 10BASE-T1L is capped at 10 Mbit/s by the standard.
• Looking for eth1 without the DP83TD510E firmware/overlay: on older images, the interface is missing from ip link.
• Swapping eth0 and eth1 in scripts: the order may depend on overlays, rely on ethtool and the MAC address.
• Forgetting to bring up the interface (ip link set up) and then blaming the cable.