# 100BASE-T two devices on a single ethernet port

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:

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
(source: netdna-cdn.com)

• Why the heck can't you just slap two standard 100BaseT magnetics (the ones in chunky black gull wing packages) back-to-back for this? Oct 19, 2016 at 22:25
• I am not sure what chunky black gull wing packages are... But I think you are correct about having to use two magentics. Oct 19, 2016 at 23:48

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:

• Thank you for the answer! I think this is exactly what I am going to do. Either I will try and separate the port from the magnetics and use that or I will get two small transformers and just wire them to a single port. The two ports are <4" away from each other and if interfaced separately would also be very short runs <4'. Do you think with such short runs I would be able to do 100Mbps? Could you please expand on this: "or, if your boards are not too far from each other, use a single Ethernet cable and solder two pairs on each board." Don't quite get what the difference is. Oct 19, 2016 at 23:45
• I edited my post with a picture. If you keep the wire pair twisted as long as you can, I think it will be ok. It is not an ideal setup so achieving 100 meters is not guaranteed but on short distance (10-20m) it will be totally ok. I've made some ugly toolings with ethernet and it always worked fine on short distance (< 20m). It's very tolerant.
– zeqL
Oct 20, 2016 at 20:30

Is it possible to wire directly from the PCB's of device 1&2 to a single rj45 mag port

You mean, the magnetics inside the RJ45 housing? No. That is because there is no such thing as a "rj45 mag port". There are a dozen different layouts of magnetics out there and you have to pick the right one for the devices' transceiver.

What you are trying to do is to find a RJ45 connector which has the correct magnetics for device A on pin 1,2,3,6 (ok) but instead having auxiliary connections on pin 4,5,7,8 (anything from magnetics ground connection to resistors to POE feed) it should have another set of Fast Ethernet magnetics as needed for Device B pin 1,2,3,6 BUT on Pin 4,5,7,8. Good luck in finding such an abomination.

The only chance you have is to separate the magnetics from the RJ45 connector.

• Thank you for the answer! I updated the question with the actual port and its schematic as I realize "rj45 mag port" sounds pretty stupid lol. You have a great idea about separating the magnetics. If I butcher the Ethernet port on the SBC and leave only the magnetics in its place it will probably fit. I can then simply break the connection between the magnetics on the shared port on pins (4,5,7,8) and wire it like that. Does that make sense? Oct 19, 2016 at 23:32
• That may work for a while, depending on your dissection skills. Oct 20, 2016 at 7:28