need help with ADM2484E

Have received a problem at use ADM2484E - the starting bit turns out with  "step". The circuit drawing and signal photo in the appendix. In the photo:

- blue signal (3) - transmission enable - pin 5 DE ADM2484E

- green (2) - TxD signal on 6 pin of ADM2484E

- yellow (1) - RS485 line signal (DATA+ pin 14 ADM2485E)

Thanks!

attachments.zip
  • Hello Conal,

     

    excuse for a delay with the answer. In the attachment - diagram of DATA+

    (yellow) DATA- (green) and DE (blue).

     

    Regards,

    Yuri Kolbasin

  • 0
    •  Analog Employees 
    on Nov 21, 2011 5:48 PM

    Hello Yuri,

    Thank you for the additional oscilloscope plot. This does look like unexpected bus contention for a short interval. That is to say, one of the other nodes is attempting to transmit at the initial interval where the ADM2484E is enabled. This would explain the initial undershoot on Data- in this plot (both ADM2484E and other node transmitting "1"), as well as the subsequent unintended high level on Data+ (also at a higher voltage level than the normal ADM2484E non-inverted driver output level for logic "1").

    Regards,

    Conal

  • 0
    •  Analog Employees 
    on Aug 25, 2015 4:55 AM

    Hi Conal,

    I have got a similar request.

    ADM2484EBRWZ is used in an equipment. Speed is 115200 bit/s. Line length is 30 cm. When terminator is not used a following picture is observed on the Data Line.

    c196f877e9252934538890d5395b6a63.html

    Why should terminator be used in the equipment where transmission line is very short (30 cm only)?

    A following block diagram is used: PC <-> I-7561U USB/RS485 converter <-> ADM2484E <-> Equipment.

    When I-7561U is replaced by MOXA USB/RS485 converter for example all works fine. I-7561U has 5 V voltage difference between A and B lines while MOXA has 3 V difference.

    But RS-485 transceiver from another vendor works fine even with I-7561U without any terminators.

    Could you please recommend a solution when I-7561U will work with ADM2484E (or any other ADI isolated RS-485 transceivers) without any terminators?

    Regards,

    Ivan

  • 0
    •  Analog Employees 
    on Sep 3, 2015 12:46 PM

    Hi Conal,

    Several tests were made with ADM2484E.

    ADM2484E has +3.3 V at Vdd2. Another node has +5 V power supply.

    If voltage difference between A and B is decreased to 3 V by placing 3k Ohm resistor between A and B all works fine. But if A and B voltage difference become higher than 3.3 V the 1st bit error appears.

    Placing 10k Ohm resistor between B and isolated GND pins (between 12,13 and 9, 15 pins) solves the issue.

    Is it correct to place 10k Ohm resistors between 12,13 and 9, 15 pins of ADE2484E?

    Do you have a recommendations how to provide interoperability between +5V and +3.3V RS-485 drivers? Should we use terminators? If yes, why another RS-485 vendors do not use it?

    Regards,

    Ivan

  • 0
    •  Analog Employees 
    on Sep 3, 2015 8:07 PM

    Hi Ivan,

    More recent investigation of similar issues shows that this can occur only at initial communication when not only is the bus unterminated, the cable is short, and the other node powered by 5V, but there is no common wire connecting ADM2484E GND2 and the ground of the other node, and finally the scenario also includes the 5V node having pull-up and pull-down resistors.

    This is simply a scenario where there is a delay in the ADM2484E driver asserting the bus state. There are several options for avoiding this:

         a) termination

         b) larger resistor between A and B

         c) add a common wire between GND2 on ADM2484E and the other node

    We haven't tested 10k from B to GND2.

    This isn't a general 3.3V to 5V issue, it's only at initial comms, when all 4 other conditions are present (unterminated bus, short cable, no common, pull-up and pull-down at the other node).