ADM1485 fail-safe feature


According to the ADM1485 datasheet page 9/12 Reciever Open-Circuit Fail-Safe section, this product guarantees a logic high on the reciever. How does it guarantee a logic high?  I suppose that it can not guarantee it from its differential input threshold spec.



  • 0
    •  Analog Employees 
    on Feb 26, 2018 10:04 PM

    Open circuit fail-safe ensures that the ADM1485 receiver output
    is high when the RS-485 A pin and B pin are disconnected, with no
    termination resistor or other nodes present. This feature is present
    on all Analog Devices, Inc., RS-485 transceivers. There is an
    internal pull-up resistor on the ADM1485 A pin. If the A pin is
    disconnected and floating, then this pull-up resistor pulls the A pin
    to greater than +200 mV. There is a pull-down resistor on the
    ADM1485 B pin. If the B pin is disconnected and floating, then
    this pull-down resistor pulls the B pin to less than −200 mV. In this
    scenario, the A pin voltage is greater than the B pin voltage, which
    creates a bus differential voltage high, and the receiver output logic
    is a constant high.