3
\$\begingroup\$

I am working on the board that suppose to have 1 GbE port (10/100/1000BASE-T). Looking at connectors all I seem to find is standard RJ45 connector.

However in my design the board itself is not directly exposed to the outside, there is a backplane which will include some other ports as well.

Now, I could put the standard RJ45 on the board and and 2 of them on backplane and connect board to backplane with standard CAT5 cable. However the RJ45 connector is rather bulky comparing to everything else on board.

So my question is -- are there any alternatives to this approach, how do commercial products that do not expose main board ports directly work around that?

PS: I've seen this question which recommends micro USB 2.0 port as chip alternative, but that means only 4 wires per port (so 2 ports per board for single RJ45?) I foresee some impedance matching issues right there.

\$\endgroup\$
  • 1
    \$\begingroup\$ If you already connect your board to the backplane, just pass the Ethernet over the backplane connector (if you can define it yourself that is). \$\endgroup\$ – Manu3l0us Jan 10 '18 at 10:15
  • \$\begingroup\$ @Manu3l0us right, just how? Standard pin headers? Might get complicated with same issue of impedance matching as this can seriously interfere with the signals, for 100BASE-T probably not a big issue, but for 1000 I guess it would have some impact already. \$\endgroup\$ – Alexey Kamenskiy Jan 10 '18 at 16:44
  • \$\begingroup\$ One of the cheapest off-the-shelf connectors with reasonable high-speed characteristics are PCIe connectors if that's an option for your usecase. But I don't know what other requirements you have for the connector (electrical, mechanical, environmental)... \$\endgroup\$ – Manu3l0us Jan 11 '18 at 8:19
  • \$\begingroup\$ @AlexKey There are standards for backplane Ethernet somewhere. \$\endgroup\$ – user253751 Jan 17 '18 at 22:27
7
\$\begingroup\$

You need to use a different ethernet standard than 802.3ab, that doesn't need RJ45 jacks. Like IEEE 802.3ap “Backplane Ethernet”, which supports gigabit speeds over a backplane type of connection. This standard was designed for your application in mind.

Also see On-Board connection of ethernet transceivers for 10/100 speed connections on the same board. The Micrel now Microchip app note AN-120 http://ww1.microchip.com/downloads/en/AppNotes/Capacitive%20Coupling%20Ethernet%20Transceivers%20without%20Using%20Transformers.pdf describes how non-transformer (and connector) ethernet connections are designed on board.

\$\endgroup\$
  • \$\begingroup\$ Thanks for your answer. Sorry, I gave a cat5 as an example of copper wire cable, still for short connection (board to backplane) cat5 should be good I think :) as for the transceiver I will be using RGMII to Marvell's 88E1512, which does provide GbE speeds (and documentation for that is rather detailed). The only concern is actually connection to backplane from transceiver itself. I searched online quite a bit and cannot see any other connector type than RJ45. \$\endgroup\$ – Alexey Kamenskiy Jan 10 '18 at 7:13
  • \$\begingroup\$ Thanks again, I misread that you recommended to look up 802.3ab. So after realizing this I looked at 802.3ap, which seems to be what I sort of looked for, but rather bulkier than RJ45 in this case. Since the target device does not require superb networking performance this will be an overkill (and budget bummer), so I guess I will stick with RJ45 on board and copper wire cable to backplane (with RJ45 female to RJ45 female on it). \$\endgroup\$ – Alexey Kamenskiy Jan 10 '18 at 7:29
  • 1
    \$\begingroup\$ @Passerby Cat5e is okay for gigabit Ethernet \$\endgroup\$ – Manu3l0us Jan 10 '18 at 10:27
  • 1
    \$\begingroup\$ "Also, don't use Cat 5 for Gigabit. Just... no." - It's worth noting that 1000BASE-T is designed to accept 100 meters of Cat-5 (not e) as a valid medium. In fact, the 1000BASE-T standard was ratified several years before the Cat-5e standard was. (see e.g. wikipedia or the IEEE 802.3ab standard (document behind a paywall, but the title reveals enough). \$\endgroup\$ – marcelm Jan 10 '18 at 13:02
  • \$\begingroup\$ Noted and corrected. \$\endgroup\$ – Passerby Jan 10 '18 at 21:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.