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I am designing a board with Ethernet input, but would like to have the Ethernet re-routed.

So the board should have two RJ-45 connectors, one which is the main Ethernet (board is using this port for accessing the network), and another which is just re-routing of Ethernet signal.

(Ethernet) --> My Board --> (Ethernet) --> PC(or another board).

I have looked around for a description, circuit or chip for doing this, but with no luck.

I have found the passive 3-port Ethernet hub, but I'm not sure whether that circuit is any good for my application.

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  • \$\begingroup\$ Does it have to work at 10Mbps, 100Mbps or 1Gbps? Do you require full-duplex? \$\endgroup\$ – drxzcl Mar 29 '11 at 11:51
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As Ranieri pointed out, we don't know if you need 10/100/1gig or half/full duplex. We also don't know what kind of CPU you are using. And we don't know how many of these you want to make (I'm assuming 50+, for various reasons). So this answer is sort of a stab in the dark, but...

There are four different approaches you can take. 1. Use a passive hub. 2. Use a 3-port switch chip. 3. Use a CPU that has two Ethernet ports on it. Or 4. forget the 2-port requirement and use commercially available Ethernet switches instead.

I've done enough PCB's with Ethernet to tell you that a passive hub isn't all that great. It might work in some one-off situations, but for a real product they stink. They will limit network bandwidth in weird ways, limit your max cable length, etc.

There are several companies that make Ethernet switch chips. Micrel is one of them, and more importantly Digikey has them in stock. I haven't read the datasheets to know now suitable they are, but it looks promising.

Using a CPU with two embedded Ethernet controllers can be a good option, and would be the option that most companies would choose. But for this to work well, the CPU should be at least a 32-bits, like an ARM or PowerPC. The CPU needs to be running a reasonable TCP/IP stack and be setup to route packets between the two Ethernet controllers. Companies like AMCC and Freescale make these. TI has one on their roadmap, although I don't know if it's available yet.

So I don't know which one will be a good solution for you. My guess is that you'll have to choose between the lesser of three evils, but that's frequently the way engineering works.

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  • \$\begingroup\$ Thank you for your answer. I will just skip the Ethernet hub then, for two reasons: 1. it is not critical for my project (it would just be a cool feature). 2. Getting it to work with all ethernet setups 10/100/1gig ect., will probably be difficult. So I thank you for helping me making the decision. :-) \$\endgroup\$ – JakobJ Mar 30 '11 at 5:47
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The obvious answer is the 3-port passive ethernet hub as seen previously in the question "Building a passive ethernet hub with anti-parallel diodes". Giving you a more specific answer is hard as you don't give any requirements for your application. The passive version is "usually sufficient if you know its limitations".

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  • \$\begingroup\$ Hi, thanks for your answer. I don't think the passive hub will work probably for all Ethernet setups. Therefore I have decided not to build the Ethernet hub on the board, as it seems more difficult than first assumed. \$\endgroup\$ – JakobJ Mar 30 '11 at 5:54
  • \$\begingroup\$ It would be nice if you still accepted an answer even if you don't use any of the answers. \$\endgroup\$ – Trygve Laugstøl Mar 30 '11 at 10:00
  • \$\begingroup\$ Oh, yes... sorry, I'm kinda new to this site. I tried to vote your answer up, but I need more reputation before I'm allowed to do that... \$\endgroup\$ – JakobJ Mar 30 '11 at 19:15
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With the passive version (for reasons explained in the other thread) you will not be able to chain more then one such device (and you seem to want this).

Unfortunately I don't think you can buy cheap Ethernet hub or switch chips in small quantities. Maybe you could get some cheap COTS switches and recycle them into your project.

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  • \$\begingroup\$ Do you know if there are any ICs that can be bought even if they are on the spendy side? ...I'm interested for myself. \$\endgroup\$ – Kellenjb Mar 29 '11 at 13:45
  • \$\begingroup\$ @Kellenjb Not really. I am now working on using COTS WiFi routers and USB hubs since both are cheaper assembled than the chips inside cost at 50+ quantities. \$\endgroup\$ – jpc Mar 30 '11 at 13:32

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