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P82B96: Design help and clarification

Genius 10282 points
Part Number: P82B96

Hi Experts,

Good day. Posting this query on behalf of my Cx, seeking your advise:

I would like to use the P82B96 IC to extend the I2C communication distance.
It will be used to connect to two temperature/humidity sensors (BME280), 3 soil moisture sensors (I2C Soil moisture sensor from Catnip Electronics) and 1 light sensor (TSL2591) with a Raspberry Pi.
The cable that will be used will be a shielded twisted pair Cat 6 cable connected in pairs as follows:
1 pair SDA and GND
1 pair SCL and GND
1 pair Vcc and GND
1 pair not connected (or if better, both pairs connected to ground?)

I would need help with following questions:

  1. Can I safely use only one resistor pair (SDA and SCL) of 750 ohms on the buffered side of the P82B96?
  2. If 750 ohms is not the right value, could you please suggest a more appropriate value?
  3. There is only one resistor pair needed for the whole buffered side, correct?
  4. Not shown in the diagram, but which shottky diode is better to be used, BAT54A or BAT54C?
  5. The shottky diode needs to be installed on each P82B96 buffered side, correct?
  6. Would it make sense to install also a 12v zener clamp?

Thank you.

Regards,
Archie A.

  • It might make sense to make the fourth pair also VCC/GND.

    You need only one resistor pair. Its value depends on the bus capacitance. Check the waveform with an oscilloscope.

    The difference between BAT54A/C does not matter here.

    Schottkys and Zeners can protect against over/undershoots caused by ringing at the signal edges.

  • Hi Team,

    Thanks Clemens, for your response.

    Currently we don't have scope.
    We took the 750 ohm resistor value from the example in the datasheet.
    For 100 and 250 meter the cable capacitance is shown there as n/a (delay based). Wouldn't this be also in my case? (cable length ~26 meters)
    The shottky diode needs to be installed on each P82B96 buffered side, correct?
    Could you recommend a clamping diode ? (will be using 12v power supply)

    Thank you.

    Regards,
    Archie A.

  • The capacitance of a 25 m cable is estimated as 1 nF, requiring about 330 Ω. Try that.

    Ringing can happen everywhere; the Schottky diodes protect only devices near them.

    The buffered I/Os are overvoltage tolerant up to 18 V, so you probably do not need Zeners.