Hacking Old Ethernet Gear

Have you ever wanted a pocket-sized device that could tell you if a network jack was live or not? [TanzerGuy] did and he hacked a piece of old networking gear to do the job.

Today when you think of Ethernet, you probably think of CAT-5 cable or something similar. But it hasn’t always been like that. In the early days of Ethernet networking, an Ethernet cable was a big piece of coax. A media attachment unit (MAU) clamped to the cable and then connected to an attachment unit interface (AUI) that resided in the actual network card. Later standards used thinner coax that attached to the card using a Tee connector, but even these are rare today.

However, there has been a significant demand over time for compatibility between new network gear and old network gear. That means you can find inexpensive surplus MAUs that convert from AUI to modern network cable in the usual places you find surplus (like eBay, for example). These cheap adapters have a network link light on them and all the logic required to drive it.

All you need is a 12V source. [TanzerGuy] had seen an earlier project that used an MAU in a plastic case and did some serious surgery on it that involved drilling and soldering. His MAU had a metal case, so he opted to get a DA-15 connector that fits an AUI connector (despite having 15 pins, these aren’t the same as a VGA connector, which is a DE-15; both are often erroneously called DB-15s). A 12V battery and clip soldered to the DA-15 socket allow [TanzerGuy] to plug a cable into the MAU and determine if the cable is live or not. Including the batteries, he spent less than $20.

If you are interested in learning more about old style Ethernet, [MattMillman] did a 10Base5 project “just because.” Of course, you could carry around a little travel router (or a dedicated tester) to test cables, but what fun is that? If you think CAT5 or CAT6 cable is as good as coax, you might need to consider the application.

22 thoughts on “Hacking Old Ethernet Gear

    1. This is a small uC and an Ethernet port, it just happens to be reusing some old kit. I did exactly this with an old fibre converter, it had a convenient 9V barrel jack and captive RJ45 cable. I should dig that back out, we have an office move shortly.

      As they are transparent bridges, they work at (almost) the lowest common layer of the network stack. This ensures a good level of compatibility (we wont talk about picky fibre SFPs) so you have a high degree of confidence in the status of the connection.

      What most of these wont do is tell you is the speed they are connected at, but if you need that level of detail then absolutely something a bit more engineered would be needed.

  1. An ordinary 9V battery works just fine for the MAU’s that I have used. One of the IT guys that I worked with back in the 1990’s had a setup like this and later, when I started doing a lot of IT support, I built one of my own. In response to the question of why not use a micro-controller and monitor network traffic, many times just knowing that the end of the net cable is live is enough information to move on to the next step in troubleshooting the connectivity problem. Why peel an orange with a machette, when the pen knife in your pocket will do?

  2. A high brightness LED across the TX pins will light up a little bit and let you know if there is some signal. However, this does not work for newer ethernet ports which shut down the TX is no incoming signal is detected, and then enable only from time to time to check.

  3. This is fine for testing the 2 pairs of wires for 10BaseT or 100BaseT signals. Gigabit Ethernet uses 4 pairs and this won’t fully test the cable. Gigabit can fall back to 100BaseT, so at least you know you’ll get some connectivity.

  4. Id love to find out how to use all these old parts in my junkroom; especially connecting Mcu’s to ISA cards! Many full-length ISA are 8 serial ports would as great serial fan-outs. Lots weird stuff too;. Old print servers, scsi to ether net Mac stuff appletalk galore, etc.. How shall I explore the repurpose/hackability….? Suggestions?

  5. But those old AUI adapters were all 10 megabit Ethernet or Thin Net or ARCNet. A hack like this can’t tell you if the line is 100 megabit or gigabit.

    What I wanted to do, but never had enough of them to do it, was to shoot a stop motion video of a robot built of Thin Net and ARCNet BNC coax terminators and connectors. With T, F, right angle and gender changer BNC adapters available, one could construct a fairly complex thing. Some custom made short pieces of cable would go nicely on the project.

    1. Allied Telesis made converters that were meant for this — I’ve got a few pairs of 10baseT -> 10baseFL. They also made a 10baseT -> 10base2 media converter in the same little metal box as their other various converters. I finally bought a few of those and got rid of my 16-24 port 10baseT hubs (most/all of which had many dead ports) for 10base2 conversion.

  6. I’ve found a USB 3 / Gigabit ethernet adapter (with link/activity LEDs) hooked up to a USB battery pack works pretty well for link testing. You don’t know if it’s gig or 100mbit, but it’s a pretty alright “something is broken” or “this should at least work” check.

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