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TIDA-00333: TIDA-00333

Alexei Kamaev Victorovich
Prodigy 50 points
Part Number: TIDA-00333

Hello!
I use same one shot autodirection control schematic in my device as in TIDA-00333. According to this method RS485 line reaches full -1.5 V (driver output on A and B lines) durind 0 transmission, but reaches only litle bit more than 200 mV (failsafe bias level) during 1 transmitssion instead of minimum 1.5 V (min driver output) according RS485 standart. Does it decrease robustness of such RS485 design on long distances compared to standart RS485 implementation with DE (driver enable) controled by PC or MCU during entire 8 bit transmissin?
Best regards!

  • 0
    TI__Genius 17100 points
    Hi Alexei,

    Thank you for posting this question on E2E! Please allow us some time to review the TI Design and respond thoroughly.


    Respectfully,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Expert 6955 points

    Alexei,

                          

    Are you implementing the R(FS) resistors and the R(T) resistor as shown in the images above?

    Respectfully,

    Lucas

  • 0 in reply to Alexei Kamaev Victorovich
    TI__Expert 6955 points
    Alexei,

    Are you able to provide scope shots of the waveforms and a schematic of your system? This will help me greatly in finding the issue.

    Respectfully,
    Lucas
  • 0 in reply to Lucas Schulte9
    Prodigy 50 points

    Lucas,

    There is no any issues in my design.

    It was just question about signal levels and its influence to bus robustness.

    This is like standart signal amplitude for RS485 looks. We see full signal amplitude +/-1.5V during data transmit/receive.

    In case of TI design with auto direction control we see that during bit 0 transmition we have full signal amplitude but  during bit 1 transmition signal level is the same level as in bus idle state (lsignificantly ess than standart 1.5V).

    Its clear that for long distance communication bigger signal amplitude gives more robustness. If signal amplitude decreases the robustness is decreasing too.

    My question is - does this difference in signal amplitude decrease robustness of bus with autodirection control compared to bus without autodirection control with full amplitude signals?

    Respectfully,
    Alexei

  • 0 in reply to Alexei Kamaev Victorovich
    TI__Expert 6955 points

    Alexei,

    I apologize for the misunderstanding here. Yes it is true that higher differential voltage (Vod) offers better noise immunity because it has a higher SNR (signal to noise ratio). But one can increase Vod to say 1V  by decreasing the failsafe resistors from 590ohm to roughly half that at 300ohm. The only disadvantage here will be slightly higher current consumption from Vcc to this resistor network. Note that the reduced Vod is due to the failsafe resistors and not the auto-direction portion of the circuit. The TIDA-00333 actually allows the user to disconnect the failsafe resistors through J6 and J7 if they are not needed.

    The failsafe resistors are also needed for older generation transceivers as explained in this document. Sometimes older transceivers can be on the same network with newer generation transceiver and the failsafe resistors are added for compatibility. 

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Prodigy 50 points

    Lucas,

    Normaly (without autodirection control) Vod decreases by fail safe resistors during bus idle stay. With autodirection control, Vod decreases during data transmit (but 1) because autodirection circuit disable transmitter during bit 1 transmission. It means that such solution decrease robustness comparing to MCU or PC direction control? And the only way to improve SNR is to decrease resistors value?

  • 0 in reply to Alexei Kamaev Victorovich
    TI__Expert 6955 points

    Alexei,

    I apologize for the late response here. Yes it seems that using the autodirection control decreases robustness to a certain point. Decreasing the failsafe resistor values should improve SNR. Unfortunately, I have to close this thread but please feel free to start a new one if you still more answers or would to discuss further.

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Prodigy 50 points

    Lucas,


    OK, we can reduce the value of the fail-safe resistors, but how will this affect the limitation on the number of nodes (32 normaly) in the network?

    Respectfully,

    Alexei

  • 0 in reply to Alexei Kamaev Victorovich
    TI__Expert 6955 points

    Alexei,

    Reducing the resistor value will most likely reduce the amount nodes able to be driven due to the increased load of lower resistor values. The effect of this will increase with lower resistor values.

    Respectfully,

    Lucas