Electrical – 100BASE-T two devices on a single ethernet port

ethernetsbc

I am trying to embed a SBC inside a device. Power, weight and space are very limited. The two devices need to communicate with each other via Ethernet using a crossover cable. I would like to solder wires directly from the PCBs of both devices to a single Ethernet port. By using two pairs from each device I would be able to either interface them separately by splitting the 4 pairs again before connecting to a router or allow them to communicate via a crossover cable feeding back into the same port. This would allow me to remove an Ethernet port from one of the devices and save space.

For the crossover cable:

            |----ETHERNET PORT-----|
Device 1 -> CableSideA -> CableSideB -> Device 2
1           1             7             3
2           2             8             6
3           3             4             1
6           6             5             2

Here is a diagram I quickly made that might help explain:

Two Ethernet ports one device
Legend

  • The crossover cable can be seen at the top left. This is an RJ45 connector (MALE) with wires crimped as seen in the diagram
  • Below the crossover cable is the RJ45 (FEMALE) Port that both pcbs are wired to
  • At the bottom there are both PCB pinouts
  • The right diagram shows the cable that will be built to split the connection if I need to interface either one or both of the devices.
  • The cable has on one end an RJ45 connector(MALE) that is split on the other end into two RJ45 connectors (MALE)

Is it possible to wire directly from the PCB's of device 1&2 to a single RJ45 port (HR911105A) using two pairs from each device (100BASE-T only utilizes two pairs) and then split this connection outside of the port?

Data Sheet of the port in question: HR911105A
HR911105A DATA SHEET
(source: netdna-cdn.com)

Best Answer

If you are lacking space, you can wire directly the Ethernet cable to the board.

What you are trying to do is to use a cable to transmit the signal from the PHY to the transformer which is not recommended as you still have the DC part of the signal.

But instead of using a RJ45 connector with an integrated magnetics you can put two discrete 100 Mbps magnetics on each board and connect the wire to a simple RJ45 connector, or, if your boards are not too far from each other, use a single Ethernet cable and solder two pairs on each board.

Also, keep in mind that your final Ethernet cable with two 100 Mbps will not be standard !

As an example of Ethernet in a very space-constrained board, I made a board for a 1Gbps PHY only 3.5 cm long and 2.5 cm wide. A FPC ribbon is used for RGMII link to the CPU board and I used a very thin transformer from Pulse: H5120. And the Ethernet cable is directly soldered to the board.

100 Mbps works totally fine but as the RGMII ribbon is very long, Gigabit speed is not working on all boards.

Edit following sbc comment #1:

Here is a picture of my thinking: enter image description here