Why are Ethernet jack center taps connected to VCC?

Question

Taking this schematic as an example:

Can someone explain to me why the center taps of the transformers (TXCT, RXCT) are connected to VCC? I've seen this connection in many (although not all) schematics on the Internet.

What would happen if they would be connected to GND?

Where do you see this? Nowhere in the schematic you link do I see center taps connected to VCC... — marcelm, Feb 28 '17 at 14:33

score 3 · Accepted Answer · answered Feb 28 '17 at 12:03

3

TXCT and RXCT are connected to each other and then to C19. The connection to Vcc (3V3A) is to provide power to the chip outputs TXOP and TXON because these are open collector outputs: -

answered Feb 28 '17 at 12:03

Andy aka

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Thank you! Can you show me where is specified that TXOP, TXON are open collector? And why is it necessary to provide 3V3 to RXIP and RXIN? – Cristian M Feb 28 '17 at 12:33
How about a link to the data sheet? – Andy aka Feb 28 '17 at 12:40
It doesn't mention anything. https://cdn.sparkfun.com/datasheets/Dev/Arduino/Shields/W5500_datasheet_v1.0.2_1.pdf – Cristian M Feb 28 '17 at 12:55
No it doesn't appear to yet most PHY interfaces like this are open collector. – Andy aka Feb 28 '17 at 14:04

score 1 · Answer 2 · answered Feb 28 '17 at 20:39

The schematic depicts a PoE (power over Ethernet) tap on the magnetics of the Ethernet connection. The connection is NOT to Vcc, but to a "V+" and "V-" pair of terminals, available at a socket, which presumably are available for some other (not shown) circuitry.

The provision for connection of jumpers, at JP1, is somewhat odd; PoE power ought not to directly be grounded, but should feed a DC/DC converter that provides some isolation. It is possible that the (not shown) circuitry is intended as a power source for some distant accessories (in that scenario, grounding one terminal makes sense; the other would be driven with -48VDC).

Why are Ethernet jack center taps connected to VCC?

2 Answers2