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Power boost from a UART

Discussion in 'Electronic Basics' started by Conrad, Robert, Dec 14, 2003.

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  1. I had a PIC (16F627) that is going to be transmitting to another PIC over a
    4 wire setup using both PIC's UART's. The wire distance will be about 100
    feet at most. What do I need to do in order to power the signal between the
    two? Will the chip provide enough power or do I need a transistor?

    I would prefer to not add more microchips if posible.
     
  2. The question is more of frequency handling ability of the wire,
    than power loss. What baud rate do you plan to use?

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  3. Likely 9600 or 19200. Baud isn't a huge issue as I will only be sending a
    single byte at a time on both directions. I am just worried about the chip
    being unable to drive against the resistance of the wire over the given
    distances.
     
  4. Robert,

    Tose PIC output pins are not designed to drive a line. Nevertheless they're
    often used to do so and often there's no harm done. But a 10ft is quite a
    distance and will build up a relative high - mainly capacitive - load.
    Although I do not expect this load to blow the PICs, I do not expect this to
    be a reliable connection. Other problems may arise like noise- and ESD
    susceptibility. The latter being able to blow the PICs very easily.
    Lightning for instance may blow all electronics but chips that have pins
    connected to lines are prime victims.

    So to stay on the safe side I advise to use line drivers/receivers. For the
    distance you need a MAX232 type will do perfectly but will require some
    extra components but the chips. Differential drivers/receivers like MAX3462
    are also very good or even better.

    petrus.
     
  5. als

    als Guest

    A simple 30mA interface should be easy, cheap, and sufficient.
    <als>
     
  6. Paul Blitz

    Paul Blitz Guest

    Yes, you DO need something to drive the line: 100ft of cable is qute a
    capacitance, and could easily blow-up your PIC outputs.

    There are many types of "proper" line-driver chips available (both
    single-ended and balanced.... balanced is better, but will need 2 pairs of
    wire, rather than 3 wires).

    Another cheap option is to drive the line with something like an
    open-collector comparator (with a pull-up resistor at the remote end).... Or
    even just use a TL072 op-amp... you could set that to be either single-ended
    or balanced.


    Paul
     
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