ADM1485, fail-safe feature

Hi,

ADM1485 datasheet p.3 Table II specify when differential input A-B is <Inputs Open>, RO output becomes to logic High.

Here, are you talking Inputs Open by defining A-B actual differential voltage is from -0.2V to 0.2V, isn't it ?

By another word, when it knows differential voltage is between -0.2 and +0.2, ADM1485 recognizes it is open and put RO output to logic high ?

Could you please confirm.

Another confirmation and maybe double check same as above, that even when it is terminating the extreme ends of line using resistors, and opposite node becomes to be high-impedance, the feature of fail-safe still be working good ?

Best Regards,

  • 0
    •  Analog Employees 
    on May 15, 2013 7:24 PM

    Hi usaghi,

    The open circuit fail-safe refers to a bus that is left disconnected without termination. For this condition the ADM1485 will bias the bus to a condition where the RO = 1. The condition you are describing is short circuit fail-safe in which case a pull-up and pull-down resistor is required on the bus as the ADM1485 does not contain short circuit fail-safe. This is described in application note AN-960.

    Regards,

    Hein

  • Hi Hein,

    Thank you very much for your reply.

    I read AN-960 and it is really good reference to us help understanding RS-485. However, the datasheet does not show clearly this fail-safe feature of ADM1485 is not for the case with termination.

    If we use ADM1485 as receiver in network with termination, how RO could be when another node becomes to be high-Z ?

    Best Regards,

  • 0
    •  Analog Employees 
    on May 16, 2013 6:04 PM

    Hi usaghi,

    If the ADM1485 receiver is in a network where there is termination without biasing resistors, when all other nodes becomes high-Z the RO will be undefined as the bus voltage will be somewhere in between -0.2V to +0.2V. This can be determined by looking at the receiver differential input threshold specification on page 2 of the datasheet.

    Regards,

    Hein

  • Hi Hein,

    I understand your point and somehow these are described in datasheet.

    However I still have a question, sorry that it may be basic.

    Datasheet defines RO=1 when Inputs Open, and Inputs Open means 0V voltage in this case, I assume.

    Here 0V is also in the range between -0.2V to +0.2V which is the case with termination you described.

    If so, how does ADM1485 distinguish the case of OPEN need to put RO=H, and case of with termination which RO is indeterminate ?

     

    Best Regards,

  • 0
    •  Analog Employees 
    on May 21, 2013 3:17 PM

    Hi usaghi,

    Inputs open described in the datasheet for the ADM1485 was assumed to be no termination(not 0V).

    To solve open circuit and short circuit fail-safe for the ADM1485 you will need to add biasing resistors onto the bus.

    Regards,

    Hein