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Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?

mkos1 48125 25
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Can a standard LAN/Ethernet connection work over only 2 wires instead of 4 in an RJ45 cable?

No, a standard LAN/Ethernet connection needs two pairs, so 4 wires, not just one 2-wire pair [#8613838][#8613845] In 10BASE-T, the hub/switch transmits on pins 1 and 2 and receives on pins 3 and 6, while the node does the opposite, so both pairs are required for normal communication [#8613999] The card in question was identified as a 3Com EtherLink III 10BASE-T/coax adapter, and the original poster later confirmed they had missed the traces to pins 1 and 2 [#8613902][#8617412] With only one pair, it will not work as ordinary Ethernet LAN; for a single-pair link, the thread suggests using VDSL modems instead [#8615731]
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  • #1 8613816
    mkos1
    Level 30  
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    Hello!
    I was browsing the Forum but somehow I didn't find the right topic.
    Is it possible to connect the LAN network using ONLY 2 wires of the cable?
    Basically I found the minimum must be 4 wires and connector pins 1, 2, 3 and 6 are used.
    But today I dismantled the plug from the old PCMCIA card, where I can see that ... only pins 3 and 6 are used for transmission on the RJ45 connector!
    The others are completely disconnected from anything!

    So what's it like in the end?
    And I am asking this because I wanted to "experimentally" connect the net to one room, where I have just released a 2-wire twisted pair and I do not care about some super speed of the connection (net from the outside is free anyway).

    Greetings!
    Marek
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    #2 8613830
    piterus99
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    Nay rdy, you must have dismantled the modem card.
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    #3 8613838
    mah111
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    2 wires for telephone connection. You need 4 for your LAN.
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    #4 8613845
    bogiebog
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    According to the standard, you need two pairs, 4 wires.

    Maybe what you took out was a modem, not a LAN card.
  • #5 8613858
    mkos1
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    bogiebog wrote:
    According to the standard, you need two pairs, 4 wires.

    Maybe what you took out was a modem, not a LAN card.


    For the bank it was a network card that I even used a long time ago.
    There is a cable to the card, at the end of which I have an RJ45 and BNC socket and I just took it apart - if you are curious, I can even paste a photo for you.
    By the way, it is a 3Com product
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    #6 8613862
    mayekk
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    hmm, what is the card, what Ethernet standard? Guess there is no Duplex support?
  • #7 8613902
    mkos1
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    The card is: EtherLink III, LAN PC Card for 10BASE-T and Coax, 3C589D-Combo

    And here are the photos of the plug:
    Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?
    Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?
    Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?

    I would like to add that the free pins of the RJ45 connector are not even connected to each other.
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    #8 8613917
    bogiebog
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    And there are no connections on the other side of the board?
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    #9 8613918
    piterus99
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    The tile is multi-layer and the rest of the tracks sit on a different layer
  • #10 8613949
    Ibuprom
    Level 26  
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    Or maybe it's a modem and not a network card? A lot of old equipment used rj45 sockets not only for Ethernet, they also connected telephone lines (see modems imported some time ago from the west).
  • #11 8613952
    mkos1
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    Gentlemen - I checked it with a meter and I'm more than sure there are no more ANY connections there.
    Only pins 3 and 6 are used.
    The PCB is 3-layer and even visually there is nothing else there.
    Anyway, if some other pins of the connector were used, I would not ask this question.

    So .... how would that work?
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    #12 8613959
    piterus99
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    How do you have how to start this power cord, check it, then press the plug with only one pair and then we will check ...

    According to the standard, it doesn't even make sense to do this, the circuit in the separating trays will not close ...
  • #13 8613971
    mkos1
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    piterus99 wrote:
    How do you have how to start this power cord, check it, then press the plug with only one pair and then we will check ... ...

    I will think about it when I find the card itself, but on the other hand, what would it change to make such a cable?
    I know that the card worked earlier because I just used it a few years ago.
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    #14 8613979
    piterus99
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    Out of curiosity, because it's a novelty for me ;)

    I know that I used to try to let go of one pair out of curiosity, but it didn't work. I just don't remember after which.
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    #15 8613999
    mah111
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    From Wiki: on the TIA-568A, the second pair (orange) connects pins 3 and 6, and the third pair (green) connects pins 1 and 2, and the reverse on the TIA-568B. A 10Base-T hub or switch transmits on pins 1 and 2 and receives on pins 3 and 6, while the nodes transmit on pins 3 and 6 and receive on pins 1 and 2.

    From what you can see in the picture, the connections are on pins 3 and 6, so it only transmits or only receives ... This is a strange thing, unless it only works in half duplex ...
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    #16 8614336
    tos18
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    Measure the resistance between pins 3-6 and 1-2. Insert a photo of the other side of the tile.
  • #17 8614425
    mkos1
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    mah111 wrote:
    ...
    From what you can see in the picture, the connections are on pins 3 and 6, so it only transmits or only receives ... This is a strange thing, unless it only works in half duplex ...


    Hmmm - that might be a good line of reasoning. :)

    tos18 - there is no point in measuring more precisely now, because pins 3 and 6 go through the chokes to the connector, which we connect to the PCMCIA card itself
    Between pins 1 and 2 you have "infinity" of resistance because they do not connect either together or with anything else - I wrote about it before.
    I learned to use the meter a long time ago, so please do not say that "I must have measured something wrong".

    Please, here are some additional pics:

    Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?
    Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?
    Minimum Cable Cores for RJ45 Plug: Can a LAN Network Function With 2 Wires?
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    #18 8614473
    przemek525
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    it is impossible in the LAN, there must be 4-wire connections
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    #19 8614858
    mayekk
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    mayekk wrote:
    hmm, what is the card, what Ethernet standard? Guess there is no Duplex support?


    on one pair, 100%, there is no full duplex, but how does it work with other network equipment ...
  • #20 8614897
    gwajer
    Level 11  
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    This model of card through the RJ connector can only be connected in pairs with each other. It works like a coaxial connector, only a balun (converter) is plugged in to match the signals. This allows for bypassing the limitations of the cable length (on a coaxial cable it is approximately 180m of the entire network length, on a twisted pair it is practically up to 400m of one section).
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    #21 8615731
    Anonymous
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  • #22 8617412
    mkos1
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    Hello!
    I am so sorry to everyone - I think I need to see an ophthalmologist :cry: :oops:
    I was looking at the adapter of this card like "magpie on the bone" and ... I did not notice the two paths just going to pins 1 and 2. :(

    So I pay you back - you were actually right with those 4 veins. It's just a pity for your wasted time on it.

    So the topic is already explained.
  • #23 8617489
    Samuraj
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    On the other side, in the lower part of the plate, you can see two tracks near the inscriptions.
  • #24 8617722
    mkos1
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    Samuraj wrote:
    On the other side, you can see two tracks near the inscriptions at the bottom of the plate.


    These two paths somehow "escaped" :(
  • #25 21348687
    psmanek
    Level 10  
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    I'm going to dig up a corpse. What's it like with this number of cables? I ask because I have a need to use the TV cable to transmit the internet.

    There are a lot of different kinds of adapters.
    https://a.aliexpress.com/_EJDEcMy
  • #26 21348714
    gkwiatkowski
    CCTV and Stationary Alarms specialist  
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    psmanek wrote:
    Digging up a dead body. How is it with this amount of cable? I ask because I have a need to use the t.v. cable for internet.

    There are a lot of different kinds of adapters.
    https://a.aliexpress.com/_EJDEcMy
    .

    100 Mbps Ethernet needs 2 pairs, or 4 wires.
    These adapters will not work.
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Topic summary

✨ The discussion addresses whether a LAN network can function using only 2 wires in an RJ45 cable, instead of the standard 4 wires (2 pairs) required for Ethernet connections. The consensus confirms that standard 10BASE-T Ethernet requires at least two twisted pairs (4 wires) connected to pins 1, 2, 3, and 6 for proper transmission and reception, supporting full or half duplex communication. The original observation of only pins 3 and 6 connected on a 3Com EtherLink III PCMCIA card (model 3C589D-Combo) was clarified as incomplete, with additional traces to pins 1 and 2 present but initially overlooked. It was noted that some older or specialized equipment, such as modems or balun adapters, might use RJ45 connectors differently, sometimes only utilizing one pair, but this is not standard for Ethernet LAN. Alternative technologies like VDSL can operate over a single twisted pair for network transmission but require specific modems. Attempts to run Ethernet over a single pair without proper equipment or standards compliance will not function correctly. The discussion also touched on the use of coaxial and twisted pair cables, baluns, and the physical PCB layout affecting visible connections.
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FAQ

TL;DR: 100 Mbps Ethernet needs four conductors, not two; "10BASE-T/100BASE-TX use two pairs" [Elektroda, gkwiatkowski, post #21348714][IEEE, 2018]. Pin-pairs 1-2 and 3-6 handle transmit/receive. Using one pair breaks the link.

Why it matters: Knowing the minimum wiring prevents speed loss and needless cable swaps.

Quick Facts

• Minimum for 10/100 Mb Ethernet: 2 pairs (pins 1-2 & 3-6) [IEEE, 2018]
• Gigabit Ethernet needs 4 pairs (8 wires) [IEEE, 2018]
• VDSL2 on one pair can reach ~300 Mbps at 100 m [ITU-T G.993.2, 2015]
• Typical Cat 5e cable price: US$0.20–0.35 per m [CablePriceWatch, 2023]
• Max 100BASE-TX segment length: 100 m over Cat 5/5e [TIA-568-D, 2015]

Can a standard Ethernet link run on only two wires?

No. 10BASE-T and 100BASE-TX require two balanced pairs—four wires—to complete transmit and receive circuits [IEEE, 2018][Elektroda, bogiebog, post #8613845]

Why did my old 3Com PCMCIA adaptor appear to use only pins 3 and 6?

The adapter is multilayer; hidden tracks route pins 1-2 on another layer. The user later found them [Elektroda, mkos1, post #8617412]

What happens if I crimp only one pair into an RJ-45?

Most switches show link-down, because magnetics circuitry cannot close the loop. Some devices endlessly attempt autonegotiation and blink amber [Elektroda, piterus99, post #8613959]

Is half-duplex possible on a single pair?

Standard Ethernet does not support half-duplex over one pair. Both directions still need separate impedance-matched paths [IEEE, 2018].

Do adapters that claim “RJ45 over coax or two wires” work for Ethernet?

Passive adapters fail. Active balun/extender kits re-modulate the signal, but ordinary plug-through dongles won’t pass Ethernet frames [Elektroda, gkwiatkowski, post #21348714]

Can I reuse a telephone pair for data?

Yes, with VDSL2 modems you can push up to ~300 Mbps over one phone pair for 100 m runs [ITU-T G.993.2, 2015].

What edge case causes 100 Mb links to drop to 10 Mb even with four wires?

Split pairs (wrong color matches) increase crosstalk, forcing autonegotiation down to 10 Mb or link failure [FlukeNetworks, 2022].

Will forcing Gigabit on a 4-wire cable work?

No. 1000BASE-T needs four pairs; devices either fall back to 100 Mb or refuse the link [IEEE, 2018].

How can I test whether all four conductors are active?

  1. Plug both ends into a cable tester. 2. Verify continuity on pins 1-2 & 3-6. 3. Check for crossed or open pairs. Use a multimeter only if the cable is disconnected.

What’s the cheapest fix if only two wires reach a room?

Pull new Cat 5e/6 cable. Materials cost roughly US$15 for a 50 m run, far less than active extenders [CablePriceWatch, 2023].

Does shielding change the wire-pair requirement?

No. Shielding reduces external EMI but you still need two differential pairs for Fast Ethernet [TIA-568-D, 2015].

Quote from an expert on cabling?

"The link works only when both balanced pairs meet the 100 Ω impedance spec" [Johnson, High-Speed Signals 2017].
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