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HMX Automotive Ethernet Setup Guide⚓︎

This document explains how to configure and use the automotive ethernet ports (eth2 / eth3) between two HMX units.

In practice, you should be able to replicate the steps that suit your desired setup by replacing HMX B with another system. Keep in mind that the speed and master/slave roles must be compatible in order to establish a connection, so if one of the endpoints is fixed (e.g. the one that replaces HMX B), the other one (HMX A) must be adapted.

This guide covers:

  • 100BASE-T1 (100 Mbit) and 1000BASE-T1 (1 Gbit) speed selection
  • Master (CMD) / Slave (RSP) configurations
  • IP configuration
  • Throughput testing using iperf3

Both systems must use compatible T1 PHYs.

1. Physical Topology⚓︎

HMX A (eth2/eth3) <---- T1 cable ---->  HMX B (eth2/eth3)

Requirements:

  • Yocto Scarthgap with Linux 6.6 or later
  • Both systems must be configured to use the same speed (100BASE-T1 or 1000BASE-T1)
  • Cable properly connected
  • One side must be Master (CMD), the other Slave (RSP)

2. Verify Link Capability⚓︎

Either port eth2/eth3 is fine but in this guide eth2 will be used.

Run on both systems:

ethtool eth2

You should see:

  • Supported link modes: 100baseT1/Full and 1000baseT1/Full
  • Link detected: yes (when connected)

3. Configure Master / Slave Roles⚓︎

Configuration 1⚓︎

HMX A = Master

HMX B = Slave

On HMX A⚓︎

ethtool -s eth2 master-slave forced-master

On HMX B⚓︎

ethtool -s eth2 master-slave forced-slave

Verify:

ethtool eth2

Expected:

  • master-slave status: master (on HMX A)
  • master-slave status: slave (on HMX B)
  • Link detected: yes

Configuration 2⚓︎

HMX A = Slave

HMX B = Master

On HMX A⚓︎

ethtool -s eth2 master-slave forced-slave

On HMX B⚓︎

ethtool -s eth2 master-slave forced-master

Verify:

ethtool eth2

Expected:

  • master-slave status: slave (on HMX A)
  • master-slave status: master (on HMX B)
  • Link detected: yes

4. Set Link Speed⚓︎

For 1000BASE-T1 (1 Gbit) speed⚓︎

On both systems:

ethtool -s eth2 speed 1000 duplex full

For 100BASE-T1 (100 Mbit) speed⚓︎

On both systems:

ethtool -s eth2 speed 100 duplex full

Verify:

ethtool eth2

Expected:

Speed: 1000Mb/s

or

Speed: 100Mb/s

5. Configure IP Addresses⚓︎

Choose a subnet (example 192.168.10.0/24).

On HMX A⚓︎

ip addr flush dev eth2
ip addr add 192.168.10.1/24 dev eth2
ip link set eth2 up

On HMX B⚓︎

ip addr flush dev eth2
ip addr add 192.168.10.2/24 dev eth2
ip link set eth2 up

Verify:

ip route

You should see:

192.168.10.0/24 dev eth2 scope link

6. Test Connectivity⚓︎

From HMX A:

ping -I eth2 192.168.10.2

From HMX B :

ping -I eth2 192.168.10.1

7. Throughput Testing (iperf3)⚓︎

On HMX B (Server)⚓︎

iperf3 -s

On HMX A (Client)⚓︎

iperf3 -c 192.168.10.2

Expected throughput:

  • \~900--940 Mbit/s at 1000BASE-T1
  • \~90--95 Mbit/s at 100BASE-T1

8. Quick Troubleshooting⚓︎

If link is down:

  • Ensure speed selection is the same on both ends
  • Ensure one side is master and the other slave
  • Reconnect cable
  • Verify with: 
    ethtool eth2

If ping fails:

  • Verify IP addresses are in same subnet
  • Check firewall rules
  • Verify route with: 
    ip route

If HMX fails to switch Speed or Slave/Master roles:

  • Bring down the interface and bring it back up 
    ip link set eth2 down
sleep 1
ip link set eth2 up