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Thread: New passive tap design

  1. #1
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    Default New passive tap design

    Yo!
    Long time no read, sorry few things took priority.

    Aaaanyways, got around to something I thought of a couple of months back, its a passive tap but with a difference.

    Simple version:
    I detached the socket from a network card, and chopped the end off of a cat 5 cable. Soldered them together and had in effect a 1/2m cat 5 extension cable. Plugged it in and no problems, network fine, therefore all soldered up ok.

    I then took a sprout off the TX+ and RX+ cables, each through a diode, joined them, then put this down the RX+ of another cable. Then soldered together the TX- and RX- down the R- of the other cable.

    So what I have is is in effect... a cable coming off another cable (like a T junction), however it only receives as these are the only wires wired up. This is an attempt to improve on the existing tap that can only hear traffic one way at a time (the one that has been linked to forums here once or twice).

    On the laptop network socket the green and amber light are on (however its the green flickering not the orange one). I put eth0 in promisc mode (confirmed by doing ifconfig -a), go into wireshark and click capture.... no packets found. I even go back to console and do ifconfig -a, rx = 0, tx = 6 (however this is not chaning, so dont believe its related to my cable and traffic).

    Basically it doesn't work (the sprout cable bit) and I have few ideas why but would appreciate input.

    1) These are normal diode's, therefore iirc the drop is 0.7v, which would take the 2.5v of cat5 down to 1.8v which iirc is about the limit of what it can take (read something about vendors not dropping the V to lower than 2 as it wasnt stable in all devices).

    2) The diode's aren't quick enough and therefore don't recover, meaning some data will make its way down the cable but not in enough of the right order to constitute a proper packet

    3) Joining together the TX- RX- together is buggering it up. Bearing in mind the PC that I have tapped is on the network without any problems and wireshark isn't going mental with random stuff, so to me this indicates that traffic is ok, just not sure how the laptop would recieve data with a shared earth.


    1) I can replace the diode's with germanium(?), these have a 0.2v drop, which is much better, but have to do lots of shopping around as I am only after 2.

    2) Can't think of anyway around this

    3) Can't think of anyway around this


    Any thoughts appreciated

    PS - on close inspection there is one semi-iffy joint (on the problematic side), I'm SURE its ok but cant be sure until I get hands on a multi-meter, so I reserve the right to blame poor workmanship lol
    wtf?

  2. #2
    Member PeppersGhost's Avatar
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    Post up a schematic please.
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  3. #3
    Junior Member MrWrong's Avatar
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    It sounds to me like youre trying to build a passive network hub.

    Check out http://www.zen22142.zen.co.uk/Circui...ce/pethhub.htm for the schematics on one.
    Wrong place
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  4. #4
    Senior Member streaker69's Avatar
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    I believe this is the part you're looking for.

    http://www.allelectronics.com/cgi-bi...NG_DIODE_.html

    Unless that's what you're already using, you didn't specify part#.
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  5. #5
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    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARG !!!!!!!!

    Thanks for link streaker, got around to finding and buying some switching diodes, guess how many I bought? thought I needed 12 or so, so bought 20.

    Nope... need 22.

    grrrrrrrr... time to place an order for 2x £0.05 diode's lol

    (I will complete this if it kills me)

    edit - thanks for link MrWrong
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  6. #6
    Senior Member streaker69's Avatar
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    Quote Originally Posted by Andy90 View Post
    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARG !!!!!!!!

    Thanks for link streaker, got around to finding and buying some switching diodes, guess how many I bought? thought I needed 12 or so, so bought 20.

    Nope... need 22.

    grrrrrrrr... time to place an order for 2x £0.05 diode's lol

    (I will complete this if it kills me)

    edit - thanks for link MrWrong
    I always go through that with ordering parts. I always forget something that I need. But I do happen to have a couple dozen 1n914's sitting in a drawer in my shop.
    A third party security audit is the IT equivalent of a colonoscopy. It's long, intrusive, very uncomfortable, and when it's done, you'll have seen things you really didn't want to see, and you'll never forget that you've had one.

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