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Definitive Answer to UK FXS Pinouts

I've always had trouble wiring up UK phones and faxes to FXS ports.

I've read endless documents and threads about the subject, but would like to hear the definitive answer from someone who definately knows.

I'm pretty sure that I should only wire up the middle two pins in the FXS port. From here, we normally run this into a patch panel, then whereever the fax is use a PBX master/secondary adapter to connect a standard phone or fax to the wall socket.

So I'd like to know which pins the two middle pins on the RJ11 should hit on the RJ45, and which pins on the PBX adapter/phone these should come to.

Also, is there any difference between phones and faxes? I didn't think there was, but I've several occasions where the cable from a phone (BT plug to RJ11) works fine for phones but not for fax machines - are these not standard?

Thanks for any help, hope this makes sense....

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Definitive Answer to UK FXS Pinouts

OK the FXS ports on Cisco routers will present the line on pins 3/4 of an RJ11, as you say the middle 2 pins.

If you then pass this through to a patch panel for RJ-45 then connect it to 4/5 again the middle 2 pins.

These pins should terminate on pins 2/5 in a BT socket (not the middle pins).

As for the cable that works for a phone and not a fax, this is probably down to the manufacturer of the phone/fax putting a crossover in the cable. Some manufacturers (typically UK specific products) use straight cables RJ11 1-6 (or 2-5 if they only use 4 wire cable/plugs) to BT 1-6 (2-5 with 4 wire cable), and then pickup the pins within the device to connect the line internally, whereas other manufacturers (typically international products and not UK specific) will be expecting the line on the middle pins (3/4) of the RJ11 socket on the device and to make them fit UK sockets simply provided a pre-wired crossed cable so the pins 2-3 are crossed and 4-5 are crossed.

Hope this clears it up.

Paul

2 REPLIES

Re: Definitive Answer to UK FXS Pinouts

OK the FXS ports on Cisco routers will present the line on pins 3/4 of an RJ11, as you say the middle 2 pins.

If you then pass this through to a patch panel for RJ-45 then connect it to 4/5 again the middle 2 pins.

These pins should terminate on pins 2/5 in a BT socket (not the middle pins).

As for the cable that works for a phone and not a fax, this is probably down to the manufacturer of the phone/fax putting a crossover in the cable. Some manufacturers (typically UK specific products) use straight cables RJ11 1-6 (or 2-5 if they only use 4 wire cable/plugs) to BT 1-6 (2-5 with 4 wire cable), and then pickup the pins within the device to connect the line internally, whereas other manufacturers (typically international products and not UK specific) will be expecting the line on the middle pins (3/4) of the RJ11 socket on the device and to make them fit UK sockets simply provided a pre-wired crossed cable so the pins 2-3 are crossed and 4-5 are crossed.

Hope this clears it up.

Paul

New Member

Re: Definitive Answer to UK FXS Pinouts

Indeed it does... thanks.

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