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Ethernet over video cable?

Discussion in 'Electronic Design' started by GrahamH, Jan 8, 2004.

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  1. GrahamH

    GrahamH Guest

    Any thoughts on using 75 Ohm video co-ax for 10 or 100 Mbit (strongly
    preferred) Ethernet network?
    10Base2 provides 10 Mbit over 50 Ohm cable
    10Broad36 is supposed to work over 75 Ohm cable but I can find no products
    that support it. Is it a dead technology?

    I only want point to point connections rather than 10Base2 bus method. An
    alternative to Cat5 because the Co-ax already exists. Twisted pair to co-ax
    baluns exist but they require one co-ax per pair.

    Any suggestions?

    Graham
     
  2. Ian Stirling

    Ian Stirling Guest

    What happens if you connect a 25 ohm resistor in series with a 10base2
    card?
    Will 10base2 work at all in this case?
     
  3. Joel Kolstad

    Joel Kolstad Guest

    ....hence you'll just need two coax cables per 100Mbps connection. Still
    seems like quite an improvement over one coax for 10Mbps, no?

    ---Joel Kolstad
     
  4. I read in sci.electronics.design that GrahamH <>
    If your cable runs are reasonably short, the 75 ohm cable will probably
    work. Try it.
     
  5. Mac

    Mac Guest

    I've never heard of it, but that doesn't necessarily mean anything.
    Well of course. How else would you expect them to work?
    In 100 Mbit ethernet over cat5 cable, there actually is a dedicated
    transmit and receive. So you can't very well replace both with a single
    piece of coax. I think you are going to need to run two pieces of coax.
    This coupled with the balun you mentioned should do the trick. The other
    two pairs are not used in 100 Mbit ethernet.

    If this means you will have to run new coax, then you might as well just
    run cat 5 (or better) instead. Make sure you use cable and a pinout that
    is compatible with gigabit ethernet, which uses all four pairs of wires.

    Mac
     
  6. GrahamH

    GrahamH Guest

    Thanks for the comments so far, generally in-line with my statements of the
    obvious. Perhaps I should reiterate that the co-ax is already installed and,
    because of its location, is **very** expensive to replace.

    So I am hoping for a solution that will provide best speed point to point
    tcp/ip (not necessarily Ethernet but that seems the obvious choice).
    10Base2/5 achieves this using collision detection and half-duplex. This may
    suggest a solution. If I can configure a 10/100BaseT interface to operate
    half-duplex and use a balun device to merge rx & tx onto a single co-ax it
    might appear to the interface that it effectively had a 10Base2 co-ax
    connection. Far fetched or possible?

    I would require 100's of transceivers so custom hardware is OK.

    Graham


    Graham
     
  7. Tim Shoppa

    Tim Shoppa Guest

    Short distances of 75-ohm cable will work on 10Base2, but not over
    a few dozen feet. I don't think there's any reasonable hope of getting
    100 Mbits.

    10Base2 uses DC levels for collision detection, so you need to use
    50 ohm terminators, and above a few dozen feet the reflections from
    the 50/75 mismatch does you in.

    50-to-75-ohm baluns won't work with 10Base2 because they don't
    preserve the DC levels.

    It's vaguely possible that pull-up networks instead of terminators will
    get you the 10Base2 DC voltages with 75 ohm terminations. Hardly
    seems worth the effort, but then again I don't know the limitations
    on running new cable in your plant. You gotta be pretty desperate
    to get to the question you're asking, and I don't want to push you over
    the edge :)

    Tim.
     
  8. I read in sci.electronics.design that Tim Shoppa <[email protected]
    Use 75 ohm terminators! The fact that terminators are needed shows that
    the device input impedances are high, so the terminators have to match
    only the cable impedance.
     
  9. Mac

    Mac Guest

    Sounds pretty far-fetched, but I don't know that much about the signalling
    scheme of 10base2.

    However, I think John Woodgate's idea will work just fine: Terminate the
    line in 75 Ohms instead of 50, and use stock 10Base2. You can literally
    put a 75 Ohm resistor across the end of the cable. Don't use any balun or
    transformer. As a terminology note, 100baseT2 has nothing to do with
    10Base2. The former uses two twisted-pairs, and can span 200 Meters (hence
    the 2) while the later uses a single piece of coax. The only similarity is
    the 2, which designates 200 Meters in both cases.

    It is not physically impossible to send data over a single cable in both
    directions at the same time. It's just that you need to somehow subtract
    the local TX contribution to get the RX part. And you have to make sure
    the TX contribution doesn't get fouled up by the RX contribution. The
    problem is that this all has to work with, in the case of 100BaseT, pretty
    fast rise times. I think the rise time is one or two hundred picoseconds
    or something. Oh, and you need to maintain 75-Ohm termination at both ends.

    Still, you might be able to design a multiplexter that had a 100baseT to
    coax conversion system. I'm just not sure it can be done with passive
    components only.

    I can almost envision how to do this with op-amps, but I don't know if it
    would work in practice. If you want me to draw the circuit, let me know.

    And now for the most far-fetched idea: Get wireless cards that can be
    connected to external antennas. But don't use an antenna, just connect the
    two cards to opposite ends of the cable. Please don't get mad at me if it
    doesn't work, or if it ruins the cards. And do use a 50-Ohm to 75-Ohm
    transformer if you can. Make sure it is rated for the frequency you are
    using.

    This actually might work, but if it doesn't, it will be because the signal
    is too strong or too weak. If it is too weak, you can add an RF amp at the
    feedpoint.

    If it is too strong, you can put attenuators in line. It might be a good
    idea to start with some attenuators in place and then gradually remove
    them if you don't get a strong link.

    Good luck!

    Mac
     
  10. Mac

    Mac Guest

    I would have agreed with you if I hadn't read John Woodgate's reply about
    using 75-Ohm terminators.
    Right, since it is a 10 Mbit protocol...
    Mac
     
  11. Tim Shoppa

    Tim Shoppa Guest

    No, won't work. 10BaseT transmitters inject current; receivers sense
    voltage. With 50% more resistance than you're supposed to have, the
    receivers will see 50% more voltage than they were designed for. Since
    collision detection is based on seeing more voltage than a single
    transmitter's current would imply, you end up with a collision whenever
    you try to talk.

    I know from experience that 75 ohm cable with 50 ohm terminators works
    if the cabling is short. But 75 ohm terminators is definitely a no-no.

    Tim.
     
  12. I read in sci.electronics.design that Tim Shoppa <[email protected]
    You mean that this digital communications system uses an *analogue*
    method of collision detection?

    HERESY!

    ANATHEMA!

    Prepare the racks and the bonfires immediately! (;-)
    Do you know THAT from experience?
     
  13. Mac

    Mac Guest

    Who is talking about 10BaseT? I was talking about 10Base2. For all I know
    your objection is correct in both cases, but I'd like to see you say so
    explicitly. Anyway, your argument sounds convincing. Maybe one of my other
    ideas will work.

    regards,
    Mac
     
  14. Tim Shoppa

    Tim Shoppa Guest

    I will be the first to point out that every digital component is made with
    analog parts!
    It's trinary, but still digital :).
    Yep. Every mistake possible to make with Thinnet, I made it or saw
    it made. Plugging the cable into a video out jack was extremely common!
    I won't mention the illegal SHV to BNC adapters and the resulting
    fireworks! I still deal with Thinnet and Thicknet on a daily basis, but
    most of the disasters were over a decade ago.

    Actually, the more I think about the way that 10BaseT transceivers work
    internally, it may not be unreasonable to get a bunch (heck, six years ago
    they were a buck each on Ebay, the hard part will be finding any at all
    today!) of the older lesser-integrated models, modify the resistor
    networks appropriately, and use them at 75 ohms. Then you're left with
    the problem of finding NIC's with AUI connectors so you can plug them
    in.

    ARCnet, if I'm not mistaken, used 75 ohm cable. A decade ago, ARCnet
    NIC's were being dumped on the surplus market in massive quantities,
    but I doubt there's any left today. That ignores the issue of software
    support for ARCnet drivers.

    Tim.
     
  15. ARCnet uses/used 93ohm cable. I can remember very thin centre
    conductors and hard-to-get BNC connectors.

    Regards,
    Allan.
     
  16. RG-62, the same as used on old car radio antennas.
    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  17. Octa Ex

    Octa Ex Guest

    The 100's of transceivers and connection points is potentially a
    problem.
    The could be far too many collisions with a large number of
    connections to the ehternet..
    It would be better if youc could break that down into several
    different collision domains.

    X X
    X
    X X
     
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