[SI-LIST] Re: routing of Ethernet pairs
- From: "Joe Paul" <joepaulm@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 9 Feb 2005 19:59:36 +0550
If possible I would have gone for transformer near the Ethernet controller and
used a connector without transformer .
My concerns are
1).
Long track with connectors in between can induce a lot of cross talk in a fully
loaded system.
A tightly coupled routing, and good spacing/shield from other signals , can
ensure that the cross
talk induced are mostly commode mode. But high CMRR of Ethernet systems depends
to a considerable
extend on CMRR of transformer.
Keeping a big cross talk receiver ( 2 x (1- 3 inch ) trace and 2x DIN
connectors) may be too much
for the Ethernet controller to handle alone.
2)
Placement of passives is another trouble.
I think , termination resistors near silicon is better ,other wise the traces
may be un-terminated
at the controller.
Further the phy datasheets of Intel and Realtek , I have seen, specifies a bias
VCC at center tap
of TX transformer.
Providing a clean bias VCC will be easier when the transformers are on board
3)
EMI concerns ? .
I know only a little , but Connectors with an EMI filter can help.
Irrespective of above arguments by my suggestions are
1).
Use adjacent pins in connector for pairs , surrounded by corresponding grounds
A B C
G G x
RX1 G x
RX2 G x
G G x
TX1 G x
TX2 G x
G G x
or
A B C
G G G
G RX1 G
G RX2 G
G G G
G TX1 G
G TX2 G
G G G
2).
If more than one interface is there on the card, interleiving Rx and TX may be
bad for Rx lines
(In Rx signals are weeker as it had travelled through the cables and connectors)
Joe
Ordyn,India
> Hello out there,
>
> I'd like to ask a question about routing Fast Ethernet differential
> pairs trough DIN41612 connector.
>
> I'm considering a contraption of a controller board accommodating
> 10/100M Fast Ethernet controller, and a backplane with RJ-45
> connector(magnetics integrated). Both boards are interconnected using a
> right angle DIN connector. The trace length between RJ-45 to DIN, and
> DIN to Controller could be in order 1 - 3 inches. Each pair would be
> routed on A row of DIN connector pins(short bend) with two pairs (RX,
> TX) separated by a single GND pin. The tracks would be routed as
> controlled pairs with defined impedance and symmetricity.
>
> I'm concerned parasitics of DIN connector may cause us some sleepless
> nights. Could you comment, please?
>
>
> Thanks in advance
>
> Roman Seifert
> Aristocrat Technologies
> Sydney
>
>
>
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