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Topic: Industrial 101: Ethernet wiring guide wrong? (Read 230 times) previous topic - next topic

feklee

On arduino.org, I found: How to realize an Ethernet connection using an Arduino Industrial 101

According to that guide, one needs to connect the pins of a female RJ45 connector as follows:
  • 1: TD+
  • 2: TD-
  • 3: VCC
  • 4: RD+
  • 5: RD-
  • 6: GND


That's what I did, with a milled PCB, and it doesn't work. I connected the board to my laptop's Ethernet port. The status lights on my laptop's connector don't turn on. When my laptop is connected e.g. to the switch in a DSL router, Ethernet works fine. So there seems to be something wrong with the board built around the Industrial 101.

Now in other locations on the Internet, e.g. in the Wikipedia article on TIA/EIA-568, I find a different pinout for the RJ45 connector:
  • 1: TX+ (TD+?)
  • 2: TX- (TD-?)
  • 3: RX+ (RD+?)
  • 4: -
  • 5: RX- (RD-?)
  • 6: -


How do I connect the RJ45 connector for Ethernet?

ShapeShifter

Full disclosure: I have no direct experience with the Industrial 101, but I have worked on a number of Ethernet interfaces.

According to that guide, one needs to connect the pins of a female RJ45 connector as follows:
  • 1: TD+
  • 2: TD-
  • 3: VCC
  • 4: RD+
  • 5: RD-
  • 6: GND
Looks reasonable - the Tx/Rx direction is reversed from the labels on the connector's data sheet, but that's not unusual: it all depends on your point of view, because what one side considers Tx and Rx has the opposite meaning on the other end of the cable.

Quote
Now in other locations on the Internet, e.g. in the Wikipedia article on TIA/EIA-568, I find a different pinout for the RJ45 connector:
  • 1: TX+ (TD+?)
  • 2: TX- (TD-?)
  • 3: RX+ (RD+?)
  • 4: -
  • 5: RX- (RD-?)
  • 6: -
That also looks reasonable. But why are the connections so different? Did you look at the datasheet for the connector used in the original guide you referenced?

It sounds like you might be using a plain old 8P8C RJ-45 jack: it's probably a chunk of plastic with a some wires that go directly from the socket contacts to the board. The hookup guide you are referencing using a socket that is very different: it has a series of integrated transformers that are wired between the socket contacts and the board contacts. This type of socket is said to have "integrated magnetics."

Open the data sheet for the specified socket, and look at page 6.  You can see the transformers that are inside the connector, and you can see how there isn't a direct mapping between the input pins on the left that connect to your board, and the output pins on the right that make up the socket connection. A tip-off that this requires a socket with integrated magnetics is that there are connections for Vcc and Ground - these are not connections that appear on the output side of the socket, but they are connections needed by the transformers within the socket.

You must use a socket with the proper integrated magnetics. You cannot use a plain RJ-45 socket, unless you are going to manually add the required transformers and interconnections to your adapter board. That would be a lot of work, it is much easier to use the integrated magnetics. A quick search found the required connector for less than US $3

feklee

Thank you very much for this thorough and instructive explanation!

It sounds like you might be using a plain old 8P8C RJ-45 jack: it's probably a chunk of plastic with a some wires that go directly from the socket contacts to the board.
Indeed. I am using an 8P8C RJ45 jack by Würth Elektronik. It didn't cross my mind that there are jacks that include additional electronics, i.e. other than for the status LEDs.

Guess I have to redo the board…

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