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- Ethernet Without Magnetics? 2 answers
We are designing a small payload which will contain several ethernet capable devices connected via a backplane we must design.
All guidelines concern the scenario where the PHY is connected to an RJ45 connector, but in my case I am connecting ethernet accross a backplane.
The Major Components in payload enclosure:
- Embedded Gigabit Switch (Example: Embedded Switch) (includes Ethernet Magnetics)
- 2x Embedded Computer & Expansion module (Example: COMe-mAL10 (includes Ethernet Magnetics)
- Interface Board to connect to enclosure connectors. Industrial connectors with/ twisted cables)
- Backplane connecting all the above boards
Note: Both the computers and the switch have integrated magnetic and are designed for 1000BASE-T so implementing 1000BASE-KX is not really an option. I realize that 1000BASE-KX is made for back-plane connection but the PHY transceiver needs to be modified to support that (and magnetic removed)
Usual PCB PHY to RJ45 Guidelines:
The usual guidlines are for PCB routing between the transceiver (PHY) and the magnetic, some common ones listed below:
- Signal after magnetics should be above chassis ground plane, no overlap with board ground plane.
- Signal should be routed as differential pairs (100Ohms controlled impedance)
- The use of vias is to be minimized. If vias are used, keep them to a minimum and always match vias so the differential pairs are balanced.
- ideally the magnetics are included in RJ45 connectors or that the RJ45 connector is very close to the magnetics.
Example Guideline Documents
LAN7500/LAN7500i Layout Guidelines (Section 5)
Answer: Small PCB section, wont have much effect, follow general layout guidelines.
Answer: Implement backplane ethernet (1000BASE-KX)
Answer: If the correct reference plane is under the signals the PHY<->Magnetics or Magnetics<->RJ45 then longer length can be run, else keep as short as possible.
Answer: Use capacitors and implement 1000BASE-KX.
From all of the above my take home message is that I can successfully connect my different devices directly via back-plane using 1000BASE-T as long as I have the separate ground below each signal and the traces are of controlled impedance, is this correct?
If the above is correct, The impedance controlled trace on the backplane will be run above a ground plane which is only capacitively coupled to the center tap of the magnetic at each end? I guess this is the equivalent of the shielding in shielded cables?
Does anyone have a link to a document/guideline describing how 1000BASE-T can be implemented on backplanes?