LS2 and POE ?

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Niall Parker

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Jan 28, 2012, 1:35:56 AM1/28/12
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Hello,

I'm just getting started with an LS2 and was wondering what the intent
of the spare pair connections on the RJ45 ... it looks like one is
grounded and the other goes to AVDD33. Is it intended to be used as a
supply connection (in which case how can the 1v8 supply be derived) or
is it for biasing/powering some other circuit on the other end of the
twisted pairs ?

I recognize the jack used on the LS1 was larger and more expensive but
it did give a couple options for using POE, it seems like it would have
been useful to have left the spare pairs unconnected ... I was going to
disconnect the AVDD33 at the ferrite bead but noticed it also supplies a
pin on U4 (eth phys), will need to review the datasheets for it.

Insights appreciated, thanks.

... Niall

Paul Thomas

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Jan 28, 2012, 11:06:12 PM1/28/12
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You need access to the cable side of the the transformers that are
within the rj45 connector, and the connector used does not provide
that access. At the time I did the design the only connectors that did
have it were ~1" long and much more expensive (as the original
linuxstamp used). There are now connectors that are 0.85" long with
that access, but they are a different foot print, and you would still
need to wire it to a POE voltage converter.

All the signals you see on the schematic are going to the board side
of the transformers, and the supplies there are just for biasing.

thanks,
Paul

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Niall Parker

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Jan 29, 2012, 12:47:13 AM1/29/12
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On 12-01-28 08:06 PM, Paul Thomas wrote:
> You need access to the cable side of the the transformers that are
> within the rj45 connector, and the connector used does not provide
> that access. At the time I did the design the only connectors that did
> have it were ~1" long and much more expensive (as the original
> linuxstamp used). There are now connectors that are 0.85" long with
> that access, but they are a different foot print, and you would still
> need to wire it to a POE voltage converter.
>
> All the signals you see on the schematic are going to the board side
> of the transformers, and the supplies there are just for biasing.
Ah, makes sense now ... I was puzzling over the wiring pinout and
overlooked the integrated magnetics. I've used power over the spare
pairs on other boards but in this case I'll need to put a splitter ahead
of the LS2.

Thanks !

... Niall

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