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DS16F95QML-SP: Damage Risk of an Unpowered Node in RS-485 bus

Zafer Caliskan
Prodigy 50 points
Part Number: DS16F95QML-SP

Hello,

In RS-485 bus architecture if a node (DS16F95QML-SP) is powered off, is there any risk about the component whether it can be damaged or not due to other powered nodes in the bus? And also, is there any possibility that an unpowered DS16F95QML-SP in RS-485 bus can cause any abnormality on an unpowered board such as voltage increase on its VCC pin (normally should be at ~0V)? If possible, could you please share the output stage of DS16F95QML-SP to be able to check the protection options?

Regards,

Zafer Çalışkan

  • 0
    Guru 264040 points

    The absolute maximum ratings allow bus voltages between −10 V and +15 V in all circumstances, even when powered off, so there will be no unspecified leakage.

    The specified input current (II) shows that the receiver's input impedance (at least 12 kΩ) does not change when powered off. I do not know if this current flows into GND or VCC.

  • 0
    TI__Mastermind 45877 points

    Hi Zafer,

    For your first question - Clemens is correct the bus pins can handle -10V to 15V independent of VCC - so an unpowered node won't be damaged by an active bus. The unpowered node still loads the bus - so you can't just shut off nodes to increase number of nodes on line just as a note. 

    For your second question - RS-485 devices will not feed back signals to VCC if you are using the device within the specified operational range. With the large common mode input range required by RS-485 + the fact it is a multi-point bus RS-485 has to prevent back-powering or else RS-485 would be an awful device to use as an ground potential difference could destroy the power system controlling the device. So no there shouldn't be risk of increasing VCC - the current into an unpowered device should be shunted to ground. We don't share the internal schematics of devices - but back-powering isn't a concern when you follow specified operational ranges in datasheet. 

    Best,

    Parker Dodson. 

  • 0 in reply to Parker Dodson
    Prodigy 50 points

    Hi Parker,

    Thank you for your answer. What about R and D pins? Is there any leakage current possibility while DS16F95QML-SP is unpowered and other nodes are actively communicating to each other?  

    Regards,

    Zafer Çalışkan

  • +1 in reply to Zafer Caliskan
    TI__Mastermind 45877 points

    Hi Zafer,

    I apologize for the delay - I was out sick yesterday. 

    For the D (and DE , /RE) pin - the abs max voltage is independent of VCC which means that the D pin will be high impedance during VCC = 0V and it shouldn't back power. If there was a diode here its max rating would include "VCC + x (usually 0.3V)". 

    For the "R" pin it isn't specified and we don't specify it (read this as we don't guarantee it) ; it shouldn't back-power based on its design but it isn't specified. Generally why we don't specify this is because if the "R" pin is pulled up - it's generally pulled up with its own supply and if it isn't we'd suggest it should be (or just not have it pulled up at all - the device doesn't require a pull-up here, but a lot of controllers will) 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson