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Latchup? RS-485 receivers

Discussion in 'Electronic Basics' started by Grant Stockly, Oct 11, 2008.

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  1. I have a project where the input of an RS-485 receiver can have
    voltage on it before the main power supply comes on.

    My project has worked well in the past when my cable's bus voltage was
    12v and I used some 12v to 5vDC Astrodyne DC-DC converters.

    I am now using a cable bus voltage of 48vDC and some MeanWell DC-DC
    converters.

    I have experienced strange part failures since switching over to the
    48vDC supplies. I think the source of the problem may either be
    latchup or maybe the regulated output of the DC-DC converter rising
    too slowly. It takes about 200us to go from 0v to 6v and within 50us
    has settled to 5v. None of the parts should be damaged from 6v, even
    continuously. The parts rarely survive 5-10 power cycles.

    I have two signals, one that is about 50% duty cycle at 1MHz, and the
    other that is "on" for about 2us and "off" for about 5ms. So far I
    have not experienced a problem with the RS-485 receiver connected to
    the 2us/5ms signal, just the 1MHz signal. The duty of the signal
    shouldn't matter with RS-485, right? Considering A will be what B is
    not...

    The part failure seems to be where one of the RS-485 inputs either
    sinks or sources current into the transmission line. I one part I
    observed a 10 ohm resistance from an input pin to ground. This part
    does seem to still have some functionality since if both inputs of the
    receiver have the same signal the "R" output of the receiver will
    mirror its inputs, however there is still current being either sinked
    or sourced depending on the specific chip and how it decided to die, I
    guess...

    The RS-485 devices that I have observed this with are the 75ALS180 and
    the MAX3467. Neither are specifically "Hot Swappable".

    From reading the Maxim IC data sheet

    http://datasheets.maxim-ic.com/en/ds/MAX3465-MAX3469.pdf

    I don't understand what makes the MAX3467 not hot swappable and the
    3468 hot swappable. As far as I can tell, the feature only makes sure
    the transmit stage doesn't activate during processor initialization.
    This isn't my concern since I only receive a signal.

    I've read about solutions to protect the ICs against latchup. One of
    them involved series resistors. I may try to place 10k resistors in
    series with the A+B signals after the termination resistor. I've
    looked at other schematics and seen 2.2k to 330 ohm resistors used to
    pull up A to 5v and pull down B to GND. I've seen situations where
    people have used 10 ohm series resistors and when termination was
    present, use 1.2k pullup/down resistors.

    Can anyone suggest what may be causing my problems?.Which one of these
    ideas should I use? Pull up/down resistors? The RS-485 standard
    already allows for voltages above VCC and below GND, so if I'm not
    crazy I shouldn't be having issues with these parts! The only part I
    have left with enough quantity to work with is the LTC485.

    Sorry for the long unorganized message. Its been a very long day!
    I'll be working on this project all weekend it looks like...

    Thanks,
    Grant
     
  2. neon

    neon

    1,325
    0
    Oct 21, 2006
    at power up the power supply if not designed well can exceed the specs and at power up when i design something I hold everything into reset until everything is stabilized for a few ms. with a scope you could find your trouble in minutes.
     
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