Is it possible to connect two 1000-BaseT transceivers with each other? I have two transceivers, each of them has 4 twisted pairs that actually are meant to be connected to an RJ45 connector including magnetics. Is there anything wrong with making on-board connections between ethernet transceivers?
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\$\begingroup\$ As the others mentioned, the manufacturers often provide app notes where a capacitive coupling is shown. This normally works well but care must be taken that the resistor networks on both side fit to the PHY (voltage mode vs. current mode). You might also want to have a look into 802.3ap, which is a standard specifically for Ethernet on the backplane. \$\endgroup\$– Manu3l0usCommented Jan 10, 2018 at 8:40
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2 Answers
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You may have some trouble if you don't provide a resistive padding network between the two ports when you cross tie them, to simulate the damping you'd normally get with a length of cable / transmission line, and provide enough attenuation to avoid saturating the magnetics. I have not done this with 1000 speeds but I can tell you of several occasions where I've cross connected two 100-Base-T ports, and found that anything less than about 1/2 meter of cable produced unreliable results. 1000-Base-T will surely be even more problematic. To cross connect them "on-board", you will likely have to have "T" pads pon each leg of each differential pair, and at that speed will have to be mindful of the transmission line characteristics of your PC traces. Now all this is worst case! Depending on the magnetics and filtering already on board to interface to the outside world, you may get lucky, or you may find that one resistor on each leg will do (though you'll still need to be careful of your trace layout!). Finally, if you are scratch laying out a board here, you will likely find that the manufacturer of the transceiver has guidelines for simpler direct coupling, when done before the "outside world" interface. There's a little black magic here sometimes, so I'll end by wishing you luck.
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\$\begingroup\$ So you would definitively advise against doing this when designing something that should be reliable and pass CE and FCC compliance tests? \$\endgroup\$– SimonCommented Dec 11, 2014 at 17:46
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1\$\begingroup\$ Wait a second; this answer deals with transceivers that are still magnetically coupled. I think the question asked what to do with direct connections, pin to pin, nothing in between. \$\endgroup\$ Commented Dec 11, 2014 at 17:51
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1\$\begingroup\$ Yes, this was the question. The transceivers are meant to be connected to magnetics. But I want to connect two of them directly (without magnetics). \$\endgroup\$– SimonCommented Dec 11, 2014 at 18:16
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1\$\begingroup\$ You still might at least need damping resistors, and cerainly attention to layout. But for direct connection the manufacture will surely offer some guidelines, and I'd say its even more important in this case. I was once co-working on a simple 100 Base-T unmanaged switch and something like this came up. Both Realtec and Micrel parts were being considered, and the mfgr had different schemes for doing this. In the end we worked with Micrel, mainly so we could talk to their reps without a language barrier. \$\endgroup\$– RandyCommented Dec 11, 2014 at 19:03
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This appnote from Micrel (now microchip) describes how to back-to-back two 10/100 PHY's. You could give it a shot with GBit by implementing their circuit on all four pairs.
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\$\begingroup\$ That method looks familiar, and since the OP said he wanted to have BOTH the external transformer interface AND a direct link, I'd be nervous without the mfgr chiming in on whether its an "iffy" thing to do. Especially at G-bit speeds. \$\endgroup\$– RandyCommented Dec 12, 2014 at 22:21
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\$\begingroup\$ @Randy that's not how I read the question; the OP just wants to back-to-back two GBit PHY's on a PCB. \$\endgroup\$– marktCommented Dec 13, 2014 at 0:01
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\$\begingroup\$ You're probably right... when he said "The transceivers are meant to be connected to magnetics. But I want to connect two of them directly (without magnetics)." I thought worst case that he wanted both on the same ports. One things for sure though... I hope whoever made his chips, they are half as responsive as the folks at Micrel. \$\endgroup\$– RandyCommented Dec 13, 2014 at 3:33
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\$\begingroup\$ Thats right, my actual plan is to back-to-back two GBit PHYs. \$\endgroup\$– SimonCommented Dec 16, 2014 at 16:31