Re: half duplex SLIP

From: Randy Dawson (rdawson12_at_houston.rr.com)
Date: 01/26/05

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    Date: Wed, 26 Jan 2005 07:03:42 -0600

    On Wed, 26 Jan 2005 11:52:53 +0100, Trygve Selmer <trselmer@start.no>
    wrote:

    >Randy Dawson wrote:
    >> On Wed, 26 Jan 2005 02:23:50 +0100, Trygve Selmer <trselmer@start.no>
    >> wrote:
    >>
    >>>Randy Dawson wrote:
    >>>
    >>>>I need to share a single conductor for communications.
    >>>>
    >>>>It is a long coax cable and I am looking for ideas and inspiration to
    >>>>share power and data on this wire.
    >>>>
    >>>>Can I force SLIP to half duplex, make the TX RX R-S232 share a single
    >>>>wire?
    >>
    >>>Please make up your mind. Do you use a coax cable or a RS-232 serial cable ?
    >>
    >> Nothing is standard in this design.
    >
    >So it seems :-)
    >Difficult to say anything without knowing the exact layout of your project.
    >
    >> My communicatons is over 2-3 miles of wireline cable for an imbedded
    >> linux system going into an oil well. I will use the TTL levels from
    >> the PC's 16550 UART serial chanel and custom signal conditioning to
    >> get the data on and off the wire.
    >
    >2-3 miles are way too long for standard TTL levels. In the "old and
    >golden days" we used 20ma current loops for distances like that.
    >
    >> This is not ethernet, but baseband serial. the attenuation of an
    >> ethernet signal is too high over this distance.
    >
    >So is the attenuation of TTL signals.
    >
    >> The cable is coax, and I hope to get a megabaud to send data from a
    >> digital camera.
    >
    >I'm not confident you will reach this speed on this long line.
    >
    >> So, I must have RX TX on the same conductor and my worry is how to
    >> force half duplex in the protocol.
    >
    >If you want to send TX and RX on the same wire, you must make some sort
    >of interface to sense the switching (like ethernet have done), and still
    >you have the problem with power. I would go for a cable with 3
    >conductors pairs and a "coax" shield running 20mA current loop.

    Thanks for you input Trygve.

    I cant change the wire, this is a standard wireline for well logging.
    I have succeded in sending a 4Mhz baseband video signal over this
    line, and a 250 K baud serial signal in another test. I used a high
    current driver transistor, and a choke on the oposite end to decouple
    the power supply from the data. So I know I can drive the wire, my
    question is really what I will need to do to the Linux SLIP code to
    force it half duplex.

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