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I've been doing some reading about the four sets of twisted pairs in a Cat 5 cable related to 10/100Mb Ethernet connections.

From Wikipedia, it says:

10BASE-T and 100BASE-TX require only two pairs (pins 1–2, 3–6) to operate. Since common Category 5 cable has four pairs, it is possible to use the spare pairs (pins 4–5, 7–8) in 10- and 100-Mbit/s configurations for other purposes.

And it also mentions:

In practice, great care must be taken to separate these pairs as 10/100-Mbit/s Ethernet equipment electrically terminates the unused pins ("Bob Smith Termination").


The primary reason I'm interested in this information is because I have a device that uses 10/100Mb Ethernet and predates gigabit ethernet. Would it be possible to determine if an ethernet wire is connected using one or more of the unused pins? Is there a device or some circuitry I could use to accomplish determining a connection in parallel with the Ethernet cable?

Thanks for any advice.

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  • \$\begingroup\$ There might be. It depends how your device connects to the pins not used for Ethernet. You could use a multimeter to see how the unused pins are terminated. What do you want to achieve with this? \$\endgroup\$
    – Justme
    Dec 30, 2023 at 0:57
  • \$\begingroup\$ so you're asking about a specific device not a general question. questions on how to use appliances are off topic here. \$\endgroup\$ Dec 31, 2023 at 10:38

2 Answers 2

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Would it be possible to determine if an ethernet wire is connected using one or more of the unused pins?

It depends on what the devices did with the center taps on the transformer, but in most devices the taps are connected to POE power or not connected at all.

enter image description here Source: https://e2e.ti.com/support/interface-group/interface/f/interface-forum/958671/bob-smith-termination

Is there a device or some circuitry I could use to accomplish determining a connection in parallel with the ethernet cable?

You could with your own setup if you designed both ends of the cable terminators. The device would be non standard of Ethernet. If it's non standard you could run a parallel twisted pair and use an RJ jack that has 10 pins or something.

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  • \$\begingroup\$ I intend to attach a device to the two twisted pairs required by 10/100 in parallel using a multiplexer or some other circuit to essentially connect the four wires to the ethernet port of the first device when there isn't an ethernet cable connected to the socket. \$\endgroup\$
    – Brogan
    Dec 30, 2023 at 3:52
  • \$\begingroup\$ Interesting, you might also want to look into phy to phy copper bridges \$\endgroup\$
    – Voltage Spike
    Dec 30, 2023 at 4:44
  • \$\begingroup\$ There is a third option too. The unused pairs may be terminated. So a non-PoE device might not just leave the pins disconnected. And I cannot guess how most devices terminate the pins. \$\endgroup\$
    – Justme
    Dec 30, 2023 at 9:21
  • \$\begingroup\$ I believe the device in question is terminating the four extra pins. It also predates PoE. \$\endgroup\$
    – Brogan
    Dec 30, 2023 at 9:22
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    \$\begingroup\$ I guess you could get a DMM, but the real question is why would you want to detect a device if Ethernet has this information accessible in the link information. \$\endgroup\$
    – Voltage Spike
    Dec 30, 2023 at 17:41
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Yes. you might get liucky, otherwise:

No. This is not a reliable way to detect connected devices.

I have seen many devices with only 4 gold fingers in the Ethernet sockets. These devices clearly do not terminate the unused pairs. and there are many others that leave the unused wires unconnected.

the main exception is power-over-etherenet devices. POE requires that all conductors are terminated and the pairs not used for data are typically shorted so if you are not using the POE feature you could probe for resistance. If you are using POE the probing for current (using a non contact current sensor like hall effect) would be an option.

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  • \$\begingroup\$ This might be the case of this device. Or not. I believe you can't simply state "no" for an answer, even if you have see many devices with only 4 pins in the socket. I have seen many devices that have all pins and they might leave them unconnected or terminate them using various methods so in case of any device, it depends. \$\endgroup\$
    – Justme
    Dec 30, 2023 at 11:12
  • \$\begingroup\$ This answer is completely irrelevant as I've already stated in my question how they are likely terminated and I have already confirmed that the four pins are connected and terminated. \$\endgroup\$
    – Brogan
    Dec 30, 2023 at 18:33
  • \$\begingroup\$ This is not a reliable way to detect connected devices. \$\endgroup\$ Dec 30, 2023 at 22:58
  • \$\begingroup\$ @Brogan You asked whether it was possible -- implied, to detect with high or total confidence. I don't see how that can be the case, so this seems to be the correct answer. I've also seen low-profile cables with only two pairs, and there are also industrial connectors with 4 pins (which when adapted to 8P8C, obviously can't all be connected). If you could be more precise (what degree of confidence? what specific device(s)? -- with internal diagrams if possible, or measurements, to prove such is possible), I would suggest asking a new question along those lines. Cheers! \$\endgroup\$ Dec 30, 2023 at 23:24
  • \$\begingroup\$ Saying the answer is no because you've seen hardware with only four pins or that you're aware of the existence of cables with only two twisted pairs isn't an acceptable answer. The question clearly asks about using the unused twisted pairs for detection. \$\endgroup\$
    – Brogan
    Dec 31, 2023 at 1:30

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