Adding A Serial Port Through An RJ45 Connector

[Mike Lu] likes to add serial ports to his routers to use for debugging but he didn’t want to drill holes in his new RT-N12. After a bit of head-scratching he thought about repurposing the four unused wires on one of the RJ45 Ethernet connectors. This would allow him to interface with the necessary signals and still have the option of using that port for a network connection. The first step was to build the circuit to output the correct serial levels and connect it to the unused pins on the jack. Next, to separate serial and Ethernet on the outside of the router he build a short adapter cable.

This is an elegant solution if you’re looking for zero case modifications. But if you don’t mind a few inconspicuous holes we love the serial port used on this Dockstar.

21 thoughts on “Adding A Serial Port Through An RJ45 Connector

  1. I’m not giving away any coprorate secrets here,
    but the Cray XT3, XT4, XT5 and XT6 all used this
    technique to sneak a differential serial bus from
    the rack controller up to each chassis along with
    a 100base-tx Ethernet signal.

    It was interesting when people in the field
    couldn’t power up a chassis and we said “Change
    the Ethernet cable” and it would then power-up.

  2. In your emergency Michael, apparently you haven’t been getting true GigE.

    “If two gigabit devices are connected through a cable with two pairs only, negotiation takes place on two pairs only, so the devices successfully choose ‘gigabit’ as the highest common denominator (HCD), but the link never comes up. Most gigabit physical devices have a specific register to diagnose this behaviour. Some drivers offer an “Ethernet@Wirespeed” option where this situation leads to a slower yet functional connection”

  3. A very good friend has a saying from his engineering days, “If it works, leave it alone!”.

    So there you go folks. I say the fellow came up with an excellent solution. It is the fault of the router designers to not make it easier on the users for the fact that these routers do have serial console points.

  4. Running stuff over the two spare pairs is nothing new really. Neat idea, I personally hate having to have adapters etc though. Like Cisco stuff.. Why couldn’t they have just fitted a normal DB9 serial port? Nope, because people loose the one they had and nick one of off a random switch you always you have to dig around in your junk box for the fecking adapter when you come to work on that switch. grrr.

    Why not fit a bluetooth UART module?

  5. @detn8r Note what he’s running on the router in place of what it was running before.

    Sometimes it requires extensive debugging to work properly. The only way to accomplish that is to watch the router startup via a serial connection to the router.

  6. @bill, wikipedia is not always correct, now 1000base t4 may use it, however, the switches and my pc, do not. I had to split pairs in a remote office (150ft away) due to the fact there was not enough copper running into the building, and I did POE ethernet to a VoIP phone on the 2nd set of wires (blue,brown) and I did GigE over first 2 (orange,green) and GigE rated 1000 on my test equipment, and I run PCoIP to those desks.

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