topic Re: Advice welcomed: am I trying the correct cable? in My Router

Advice welcomed: am I trying the correct cable?

BertCoules — Tue, 16 Dec 2025 18:02:12 GMT

I have a Hub 2 router connected to a BT Mk4 wall box by a short cable. I'd like to move the router further away (approximately 6m) and so I purchased what was advertised as a modem cable.

When I replaced the original cable with the new one and switched the router back on, its lights flashed and it wouldn't stabilise. Checking on a PC, the report was that the connection was faulty.

When I put the original cable back, everything was fine again.

Is it significant that the plugs on the original cable appear to have only two contacts, while the new ones have four?

Also the new plugs are significantly smaller than the socket on the router and (especially) the socket on the BT box (the correct socket: I didn't use the phone socket by mistake). Are those factors enough to cause the connection problem? Googling suggests that the sockets on the BT box and the Hub 2 are both RJ11, and that a "modem cable" uses RJ11 plugs.

I'd be very grateful for any thoughts. Many thanks.

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Tue, 16 Dec 2025 22:57:26 GMT

Have I posted this in the wrong section? Would "My router" be more appropriate? Apologies, if so.

Re: Advice welcomed: am I trying the correct cable?

Baldrick1 — Wed, 17 Dec 2025 08:29:30 GMT

Moderator's note:
Thread moved from Broadband to My Router

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 08:39:59 GMT

Thanks for the move.

Re: Advice welcomed: am I trying the correct cable?

Dan_the_Van — Wed, 17 Dec 2025 09:27:36 GMT

@BertCoules

When you use the new cable, looking at the Hub Light it is flashing orange? This would suggest the new cable is faulty or incorrect wiring..

Do not be concerned about only two pins having wires, as that is all that's needed for a xDSL signal.

So with an RJ11 for a modem only, the plug may only have pins 3 (red) and 4 (green) connected. One pair

The cable should be 1 to 1, 2 to 2 etc with no crossovers. so the colour on each wire should be the same both ends of the cable

I have in my stock of old stuff a DSL cable which looks just like yours, pins 1 and 6 are just blanks as they are never used for phone or modem equipment. Looking into the DSL socket of the Hub there are only four pins.

RJ11 cables for phones would need at least three wires, extra wire is for the bell circuit, which would explain four wires on the new cable.

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 09:39:14 GMT

@Dan_the_Van

Thanks for that. The Hub light was definitely flashing and I think it was orange but being colour blind I can't be absolutely sure. I suspect that the cable is faulty: I noticed that the packaging had been opened and resealed, which could well mean that it had been returned by a previous buyer and simply put back on sale. I'll return it myself and try to get another.

Re: Advice welcomed: am I trying the correct cable?

HPsauce — Wed, 17 Dec 2025 10:52:14 GMT

@BertCoules I think @Dan_the_Van sort of beat me to it, but many cables advertised as "modem cables" are not straight through and/or don't use the "innermost" pair. You really want something specifically advertised as an ADSL cable.

Or it could be just plain faulty........ 🤣

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 12:20:00 GMT

Well, that was interesting. I obtained a replacement cable, connected it - and got exactly the same result, a flashing, possibly orange, light on the router. And once again, returning to the existing cable put everything right.

@HPsauce and @Dan_the_Van Here are the two ends of the new cable. Is this wiring correct for the BT wall box to the Hub 2? The cable is labeled ADSL.

Re: Advice welcomed: am I trying the correct cable?

Dan_the_Van — Wed, 17 Dec 2025 18:43:07 GMT

@BertCoules

Sorry I been side tracked today, had an electrical issue that needed resolving.

The cable look fine to me, seems odd it's not working. Is the cable being sold for use with a modem, as they should be standard.

Silly question you are pushing the plugs fully in, does it align correctly in the DSL socket on the Hub Two?

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 19:13:14 GMT

@Dan_the_Van wrote:

Sorry I been side tracked today, had an electrical issue that needed resolving.

The cable look fine to me, seems odd it's not working. Is the cable being sold for use with a modem, as they should be standard.

Silly question you are pushing the plugs fully in, does it align correctly in the DSL socket on the Hub Two?

No apology necessary; I'm grateful for your help (and that of others).

The box is labeled "Modem cable : ADSL".

It's not at all a silly question: I wondered about that myself. I'm confident that I'm pushing the plugs fully home, but I'm not certain about the alignment: the plugs are noticeably smaller than the sockets in both the Hub and (especially) the BT wall box. I won't be able to try again until tomorrow, but then I'll experiment with wiggling the plugs around and see if I can get a proper contact that way.

Thanks for your thoughts.

Re: Advice welcomed: am I trying the correct cable?

2u2me — Wed, 17 Dec 2025 20:55:21 GMT

Hi there, I don't think its the new cable that is at fault its the actual plug on it, RJ11 plugs and sockets are specific to each other so you need a new cable with the exact plug on it as the one you are replacing, if you look closely at the DSL socket on the router and then you will see it marries up with the same design of the plug, are you sure the plug on the new cable you have purchase is exactly the same as the one you are replacing?

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 21:13:38 GMT

@2u2me wrote:

Hi there, I don't think its the new cable that is at fault its the actual plug on it, RJ11 plugs and sockets are specific to each other so you need a new cable with the exact plug on it as the one you are replacing...

Thanks for that. I haven't closely compared the new cable's plug to the existing ones; I'll do so in the morning. But I'm rather surprised by what you say: if RJ11 plugs and sockets have to be specifically matched to each other in order to work, doesn't that make nonsense of the standard?

Re: Advice welcomed: am I trying the correct cable?

2u2me — Wed, 17 Dec 2025 21:27:53 GMT

No, what I am saying is you can insert a RJ11 plug into a RJ45 socket and vice versa, but the connections will not marry up

you need something like this.

RJ11 to RJ11 ADSL Cable for BT Broadband Router | DSL Modem Internet Lead UK – Esellertree

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 21:40:46 GMT

@2u2me wrote:

No, what I am saying is you can insert a RJ11 plug into a RJ45 socket and vice versa, but the connections will not marry up.

That wasn't what you said. But in any case - as I thought I'd made clear - I'm not trying to insert an RJ11 plug into an RJ45 socket, much less vice versa. Why on Earth would I? I might not be as technically clued up as a lot of people, but I certainly know better than to try that.

My point was that the R11 plugs on the new cable are significantly smaller than the R11 sockets into which I'm plugging them, which could perhaps be causing poor or non existent contact.

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 22:00:15 GMT

Sorry, just to clarify: my point was that the bodies of the R11 plugs on the new cable are significantly smaller than the R11 sockets into which I'm plugging them, which could perhaps be causing poor or non existent contact.

Re: Advice welcomed: am I trying the correct cable?

HPsauce — Wed, 17 Dec 2025 22:05:18 GMT

@BertCoules The retaining latch on the cable ensures it is inserted correctly in the centre of the socket.

Are you sure it's pushed fully home at each end? Also compare the plugs on the longer cable that fails to those on the shorter one that works. Take close-up photos and magnify them if necessary.

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Wed, 17 Dec 2025 23:22:45 GMT

@HPsauce wrote:

@BertCoules The retaining latch on the cable ensures it is inserted correctly in the centre of the socket.

Are you sure it's pushed fully home at each end? Also compare the plugs on the longer cable that fails to those on the shorter one that works. Take close-up photos and magnify them if necessary.

The plugs seemed to go home with a satisfying click but at the same time didn't feel particularly secure; I'll check again tomorrow. I'll also compare the plugs on the two cables. Thanks for the thought.

@2u2me Apologies for my post above (message 14), which on rereading struck me as unnecessarily rude.

Re: Advice welcomed: am I trying the correct cable?

Dan_the_Van — Thu, 18 Dec 2025 08:09:38 GMT

No, what I am saying is you can insert a RJ11 plug into a RJ45 socket and vice versa, but the connections will not marry up.

For what it worth the Openreach MK4 and Hub two both have RJ11 sockets.

@BertCoules

If you hold the old and new cables end to end; does the molding which holds the plug in position (the bit with the locking tab on) how do they compare in width, do the the wires line up?

Do you still have the ADSL inline filter which came with the Hub Two? If you do maybe try removing the faceplate from the Openreach Master socket and use the test socket with the filter.

It might help towards finding where the issue is, it might work now or it might be still the same.

If you have any phone extensions connected to the master socket they will stop working with the faceplate removed.

Re: Advice welcomed: am I trying the correct cable?

BertCoules — Thu, 18 Dec 2025 12:20:49 GMT

@Dan_the_Van wrote:

If you hold the old and new cables end to end; does the molding which holds the plug in position (the bit with the locking tab on) how do they compare in width, do the the wires line up?

Yes, they do. But I've found what I think must be a significant clue to what's wrong: I've been able to borrow an RJ11/RJ45 cable tester and checked both the working and the non-working cables. These were the results:

The working cable lit up the following lights: on the main body of the tester, 3 then 4. And on the remote unit, 4 then 3. Both sets of lights continued to cycle, always in that order.

The non-working cable produced lights which cycled in this order: main body, 2, 3, 4, 5; remote, 2. 3. 4, 5.

It seems to me that (quite apart from the number of wires, and I know that only 3 and 4 are vital in this instance) the two cables must be configured differently, which would presumably explain the problem. Am I correct?

Re: Advice welcomed: am I trying the correct cable?

Dan_the_Van — Thu, 18 Dec 2025 12:47:54 GMT

@BertCoules

A good idea to check the cables.

From what I understand the incoming phone line is not polarity sensitive, so it does not mater if the A and B connectors in the master socket are crossed, so the cross over cable should not be an issue.

Do you reboot the hub when you use the new cable?

I'm wondering if the contact pins in the RJ11 sockets have become compressed on the master socket or Hub, so the pins are not touching fully the contacts of the plug. That was my thinking behind trying a ADSL in-line filter in the test port, which is exposed when the faceplate is removed which is just clipped on.

As an experiment you could always swap over the A & B incoming copper line wires, MK4 should use cam-lock connectors which should require special tool, just a terminal screwdriver to gain access to the incoming cable.