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I am trying to connect three computers in one network. I have connected the wires like the photo.

I works with two computers but not three computers at the same time.

For example, let's name the ports: A, B, C:

  • A+B works if C is empty.
  • A+C works if B is empty.
  • B+C works if A is empty.
  • A+B+C does not work.

What can be the problem (or, should this work)?

Ethernet hub.

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closed as off-topic by Scott Seidman, Matt Young, placeholder, Chetan Bhargava, Joe Hass Apr 28 at 17:55

This question appears to be off-topic. The users who voted to close gave this specific reason:

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Can you provide actual diagram of wiring? I really can't see what's connected to what in your setup. Also, don't be surprised it doesn't work with 3 devices. –  AndrejaKo Apr 28 at 13:29
3  
No, this can't work with three devices. Period. –  Dave Tweed Apr 28 at 13:36
1  
"Wouldn't this work if you used half duplexing with collision detection?" No. Each computer has one TX pair, and one RX pair. You can connect two computers together by connecting the TX from one to the RX of the other, and the RX of one to the TX of the other. You can't connect two TX to one RX and expect the two TX computers to be able to see each other. –  Adam Davis Apr 28 at 15:15
    
In modern ethernet, the system is a point to point connection, and not a bus. As such per design there is no such thing as collisions in the way the system is operated. –  PlasmaHH Apr 28 at 15:40

2 Answers 2

up vote 7 down vote accepted

Why your current adapter won't work

Each ethernet connection has four wires. They are split into two pairs. Each pair carries one signal, one way. One pair is TX and one is RX.

The ethernet drivers may be capable of swapping the TX and RX pairs during port negotiation - on power on, and when connected to a new hub or switch. But they cannot change the TX and RX lines at other times under normal operation.

When you have two connected to your adaptors, the drivers negotiate which pair means what, and eventually the TX of one connects to the RX of the other, and the RX of one connects to the TX of the other.

Let's say you plug a third one in. Where should its TX be connected?

schematic

simulate this circuit – Schematic created using CircuitLab

Where do you want to connect the TX and RX lines of the third device? You could connect the RX to either of the top pairs, and listen in to what each computer sends, but you'd still be deaf to the other computer the RX isn't on. You could connect the TX to either of the top pairs, and send to one computer, or the other, but not both.

This coupler will work for two computers, but you need an active hub or switch that copies all the TX from every device to all the RX on every device.

Passive ethernet hubs

However, while the 10Base-T specification is specifically a star topology, and requires active powered hubs, a passive hub can be created which works. It has limitations, but it would allow you to connect three ethernet devices together:

enter image description here

Note that it will only work for short cable distances, and due to the voltage drop in each diode will not expand beyond 3 devices. Further it will only negotiate to 10mbps speeds, not any faster. If your ethernet devices rigorously follow the standards, they should connect to each other using this passive hub, however it wouldn't surprise me if some chipsets did not recognize this as a valid ethernet connection.

Switches are so inexpensive though, it seems odd to me that one would choose this route, but there may be specific cases where this is appropriate.

Note that TX is on pins 1 and 2, and RX on pins 3 and 6. At each device there is a transformer on each pair. Consider how this figures into the above circuit to understand how it works - or find out at this question: Building a passive ethernet hub with anti-parallel diodes .

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This should not work because it's not a hub.

The reason why it works if you have only two devices plugged in is that they can auto-negotiate which pair to use in each direction. When you have three devices plugged in "direction" is no longer well-defined.

Back in the day you could just wire devices together in a line without hubs using 10BASE2 cable, although it was slow and stopped working if you forgot the terminators.

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