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P82B96: Issue with long distance communication

Yakov Heifetz
Prodigy 10 points

Part Number: P82B96

I am using the P82B96PWR device.

The connection between the master side and the slave side is made using a 10-meter cable, as shown in the diagram.

When the cable is 5 meters long, everything works perfectly. But when I add more length to the cable, communication stops working.

I used the TI Application Note to calculate the pull-ups, and my questions are:

  1. For calculating VOL in case of VCC = 12V, there is no specification in the buffer’s datasheet for the VOL of the TX/RX lines — only a current specification of 30 mA.
  2. Do I need to place pull-up resistors on the TX/RX lines on both sides — both on the master side and on the slave side?
  3. If pull-ups are indeed required on both sides, and assuming the calculation results in 1 k, should I place 1 k on each side, or 2 k on each side, or does each side individually need 1 k?



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  • 0
    TI__Guru 55728 points

    Hi Yakov,

    Yakov Heifetz said:
    For calculating VOL in case of VCC = 12V, there is no specification in the buffer’s datasheet for the VOL of the TX/RX lines — only a current specification of 30 mA.

    For when IOL = 30 mA, the voltage output of VTx, VTy = 0.4 V. We can use these numbers for the PU resistor calculation. 

    Yakov Heifetz said:
    Do I need to place pull-up resistors on the TX/RX lines on both sides — both on the master side and on the slave side?

    It shouldn't matter which side.

    In the case that you would unplug the master from the slave, it might be best to have some PU resistor on the transmission side so that it is pulled up to 12V when disconnected, otherwise it would be left floating. If unplugging is not of concern, PU resistor on either master side or slave side is ok. 

    Yakov Heifetz said:
    1. If pull-ups are indeed required on both sides, and assuming the calculation results in 1 k, should I place 1 k on each side, or 2 k on each side, or does each side individually need 1 k?

    If you end up calculating 1k PU resistor requirement, and you would like to place PU resistors on both master side and slave side, you would need to have two 2k PU resistors. 

    One 2k PU resistor on master, and one 2k PU resistor on slave boards. The equivalent resistance of the 2k || 2k = 1k. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 10 points

    Thanks 
    So, for calculating the PU on the TX/Rx Side I need to use VOL = 0.4V, IOL =30mA, and for Vx/Vy VOL=1V IOL=3mA? 


  • 0 in reply to Yakov Heifetz
    TI__Guru 55728 points

    Hi Yakov,

    I will respond tomorrow by noon CST.

    Regards,

    Tyler

  • 0 in reply to Yakov Heifetz
    TI__Guru 55728 points

    Hi Yakov,

    I agree with your statement. Would you know how to quantify the capacitance of your cabling? 

    Also what I2C speed are you attempting to communicate at ? 

    I attached this app note for reference. 

    I2C Bus Pull-Up Resistor Calculation

    Regards,

    Tyler