LTM2881 abnormal output signal from receiver output during open input line

Hello all,

I have question about LTM2881. When I open the input signal line of receiver, the output signal from receiver output doesn't fixed High level. The schematic is following in this case.

The normal output signal is following.

And the abnormal output signal is following.

Does not Failsafe function work like this case?  And why does the abnormal signal seem to high speed data rate as the normal signal?

Best regards,

Akira

  • Hello Akira,

      The fail safe feature is intended to address a properly connected idle bus as well as the abnormal cases of a completely open; shorted; or terminated, but not driven bus.  Opening only one of the signal lines can result in conditions where the receiver will see a sufficiently large enough differential signal that the receiver reacts accordingly.  

    In terms of determining the cause of the apparent higher data rate signal image, I have some questions:

      Is the input data sequence the same for both image?

      What is the time scale in your scope images?

      Is the LTM2881's terminating resistor enabled or disabled?

      What is the physical configuration of the RS-485 bus?  Length, number of nodes, relative position of the LTM2881, location of terminations, etc.?

      Is the physical open circuit right at the LTM2881 or is it someplace else (e.g. point-point configuration and open is at driving end)?

    Eric