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clock, data, and power on same line?

Discussion in 'Electronic Design' started by Jon Slaughter, Aug 22, 2007.

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  1. Is there any theorical reason why this can't be done? and if not then why is
    it not practical?

    I'm just curious... but I know the egomanics like Eyeore are going to chime
    in with "Your a moron" bs to feed there ego's but luckily I have learned how
    to use the ignore feature of my usenet app.


  2. What I mean by this is that I have never seen any mention of something like
    this(although I think I do recall a 1-wire protocol somewhere but don't
    remember much). I do know that you can encode a clock with data(I think its
    manchester encoding?) but wonder why not move to the next "logical" step and
    add power. (I know its possible but it seems rarely used)

    I imagine, say, if I have some speakers that need power for something(say
    some lights or maybe an active filter) but instead of running an extra wire
    for power(and I'll probably need one for ground) why I couldn't just run the
    power over the same analog audio lines.

    Since the power is DC I could just use a cap to drain the power into and
    another to block DC for the audio part? (of course this is for analog stuff
    but I figure the same could be applied to digital)

    I do realize that noise would probably become a much larger issue but I'm
    sure there are other things and such... which is why I ask.
  3. DJ Delorie

    DJ Delorie Guest

    It has been done. The Dallas/Maxim 1-wire protocol has power, clock,
    and data all on one wire.


    The line is normally at +5v (providing power, devices have a diode and
    cap to maintain it during signalling).

    Each falling edge is the "clock" signal.

    Each rising edge is the "data" signal - short duration low pulse =
    binary 1, long duration = binary 0.
  4. Thanks guys. I'll look into it some more later now that I know it works and
    its actually used.

  5. Jamie

    Jamie Guest

    yes. just modulate the line and keep the level up
    to where the device will not suffer for it's rail
    supply. isolation with something like a diode and
    cap will help maintain the rail line to the device
    while the device can modulate it.
  6. Eeyore

    Eeyore Guest

    There's no shortage of 'self-clocking' interfaces.

    Look at the AES/EBU interface for example.

    Power is low Z though. Do you really want to drive a low impedance line with
    high speed data ?



    Of cours just because it *can* be done doesn't mean it *should* be
    done. A few more connections can make things a lot better.

    Best regards,
    Spehro Pefhany
  8. Not so in the case of the Dallas/Maxim 1-Wire iButton Products.
    Perfect example of how a 1-Wire interface can be used to make a unique
    and very effective product.

    I just put a 1-Wire ID chip onto a whole bunch of boards that go into
    making up a product, and the ability to program the chip with info
    though a single wire interface while the board is un-powered is very
    flexible indeed.
    Then when the product powers up it can interrogate each board and know
    what boards are plugged in, what revision they are etc, and can then
    auto-configure itself accordingly. And it only takes a single wire
    which is great for retrofitting into existing designs.

  9. Terry Given

    Terry Given Guest

    And I just yanked a DS1821 digital thermometer from a design cos it
    costs $12! Ive replaced it with a smt thermistor, a resistor and a
    capacitor, total cost $0.20. Funnily enough the IO pin it was connected
    to was also an ADC input.....

  10. Welcome to the principle of horses for courses.

    My $30 ThermoChron iButton is worth every cent when I can attach it to
    my shoelaces or harness to measure water and air temp when I go
    canyoning, or put them throughout my house or inside a product cabinet
    to log thermal profiles.

    Can't use your 20 cent solution on a sole remaining I/O pin that
    doesn't happen to have an ADC input.
    And of course the DS1821 can also be used standalone when your product
    is too busy doing other things or went to sleep or something. Want to
    add some more sensors to the your only remaining I/O pin? - oops.
    You could of course add a $.50 PIC in that case if you were counting
    your pennies.

    That's why there is a product for every purpose.

    BTW, you were getting ripped off, the DS1821 is only $6.31 at Digikey.

  11. Terry Given

    Terry Given Guest

    indeed. the original decision to use this part (not, of course, mine)
    was downright daft.
    yes, you just need to think harder (and maybe spend another $0.10)
    I suspect the "purpose" of the DS1821 is to allow softweare people to
    hook temperature sensors to micros without having to do any electrical
    engineering. Hell, I could use a 100nF Z5U cap to sense temperature
    here, I have a micro......
    I built 5 prototypes of the rev 3 circuit, so didnt really care too much
    about the pricing; my BOM is being quoted as we speak (?!) but US$6.31
    is about NZ$10, so its still more expensive than my large SCRs.
  12. DJ Delorie

    DJ Delorie Guest

    AH, but if you don't *have* those connections? My home furnace
    control system uses 1wire because I have four conductors to each
    thermostat, and can't easily run more. Power, ground, LCD serial, and
    1wire. The 1wire does temperature, humidity, and four pushbuttons.
    Outdoor weather stations use 1wire to report temp, humidity,
    precipitation, air pressure, wind speed, and wind direction - all on a
    single twisted pair.
  13. Paul Burke

    Paul Burke Guest

    It is done. Model train buffs have a system called DCC, which uses a
    switched clock, the edge separation defining 1s and 0s. Communication in
    the reverse direction is by current loading. It's a bit slow and clunky,
    but it works.
    Why the gratuitous razz?

    Paul Burke
  14. Guy Macon

    Guy Macon Guest

    Look here:
    [ ]
    A variation of the basic idea is here:
    [ ]
  15. Sure it can be done, using different frequencies is one way.
    But the extra circuits needed are more complex then 2 extra copper wires.
  16. Winfield

    Winfield Guest

    The new PoE ethernet-cable-powered network
    devices are going to be pretty convenient.
  17. Winfield

    Winfield Guest

    The new PoE ethernet-cable-powered network
    devices are going to be pretty convenient.
  18. Winfield

    Winfield Guest

    There are ocean-going systems that only use
    one wire. Plus the ocean, of course.
  19. Tim Shoppa

    Tim Shoppa Guest

    Devil is always in the details. There are many examples of it being
    done, and being done for at least 70 or 80 years.

    I actually like the old Westinghouse SCADA equipment that used 807's
    to put the carrier onto the power lines. I saw it being deinstalled as
    recently as ten years ago.

  20. PoE = Two pairs, minimum, right? = 4 conductors.
    USB = 4 conductors
    IEE1394 = 6 conductors (if power provided)
    POTS = 2 conductors minimum
    1-wire = 2 conductors minimum, 3 used at times

    Best regards,
    Spehro Pefhany
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