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MAX232, Sharing RxD lines

Discussion in 'Electronic Basics' started by Guest, Oct 14, 2003.

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  1. Guest

    Guest Guest

    I have a system where I have one device talking to two others via RS232. I
    have the two receivers RxD lines tied together and to the transmitters TxD.
    Is this OK? The MAX232 spec shows an input resistance of ~5K ohms which
    seems low.....

    Richard
     
  2. CFoley1064

    CFoley1064 Guest

    I have a system where I have one device talking to two others via RS232. I

    The classic RS-232 receiver chip, the DS1489, has an input impedance of 4K
    ohms. It's pretty much standard to assume a 3K load. If you look at the
    MAX232 data sheet, you'll see that the output voltage will bog down about a
    volt if you double up (going from a current draw of 3.3 mA to 6.6 mA). Still
    got plenty of headroom there.

    Short answer -- you've got nothing to worry about as long as you have a
    "standard" RS-232 output, or even anything close to it. It's not common
    practice, but it's been done before, many times. Go for it.

    http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf

    Good luck.
    Chris
     
  3. budgie

    budgie Guest

    Yep, done often.
    Most RS232 devices have low input resistance.
     
  4. Yes - one RS-232 transmitter should be able to drive a few receivers.
    However, you cannot have multiple transmitters talking on the same
    wire.
     
  5. CWatters

    CWatters Guest

    The short answer is that it should be OK but might be a problem if you have
    very long cables and low power drivers. If in doubt, load up your
    transmitter (include any cable capacitance) and measure the peak to peak
    wave form (at the max bit rate) at the input to one of the receivers with an
    oscilloscope

    I believe that the minimum RS232 logic levels (eg with transmitter loaded)
    is +/- 5V and that receivers are specified to operate with input levels as
    low as +/- 3V (the difference between +/- 5 and +/- 3 is to provide a noise
    margin).
     
  6. Guest

    Guest Guest

    OK, this is good news. Thanks to all for the comments.

    Richard


     
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