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P82B96: Help to calculate the I2C communication cable length

Eric Shen42
Expert 2750 points
Part Number: P82B96


Dear TIer:

Base on the  schematic of below. If I use VCC1=VCC2=+V Cable driver = 5V, how long can the P82B96 support?

How to  calculate the  I2C communication cable length?

Table 1 do not show the cable length with condition of Vcc1=Vcc2=+V Cable=5V.

  • 0
    TI__Mastermind 23636 points
    Hello Eric,
    It isn't about the cable length directly. It is about the amount of capacitance on the bus it can support. The P82B96 can support up to 4,000pF on the bus (cables). You need to find out what the pF/length specification is for cable you plan on using than multiply that by the length you plan on using. That gives you an estimate on the capacitive load on the bus. Once you know the capacitive load you can select the correct pull up resistance. The datasheet goes through an example. Can you explain what specifically you don't understand?
    -Francis Houde
  • 0 in reply to fhoude
    Expert 2750 points
    Hello Francis:
    Thanks!
    You mean the "+V cable" is not the key parameter? The key parameter is pull up resistance and cable capacitance?
    If I want the cable is 50M or 100M, but i don't know the capacitance for the cable. How to set the pull up resistance?
  • 0 in reply to Eric Shen42
    TI__Mastermind 23636 points

    Hello Eric,

    Most cable manufacturer's provide specifications for the cable that define the capacitance per unit length.  If you don't have that than you may have to measure it.  

    Here is example of what you will see from a manufacturer.  If it isn't in the datasheet you may have to ask the vendor for the information.

    -Francis Houde

  • 0 in reply to fhoude
    Expert 2750 points
    Hello Francis:
    Thanks a lot!
    Sorry, i need ask again. If the total capacitance of my calbe(50M or 100M) is no bigger than 4000pF, and +V cable =5V, than the pull up resistance can set to same normal value like 4.7K or 10K?
  • 0 in reply to Eric Shen42
    TI__Guru 74322 points

    Eric,

    Like Francis said, the limitation is the amount of capacitance on the bus, which is heavily affected by the cable used, but also by the capacitance added by each slave device. This limitation is based on the RC time constant, which affects the rise time of the message waveform (this is why V+ voltage, VCC voltage, pull up resistance, and bus capacitance are important parameters), and thus the signal integrity of the bus waveforms.

    This paper explains this concept in more detail, and explains how to calculate the pull up resistances based on different system parameters. 

    To answer your question, that is going to depend on how many slave devices are present, your cable parameters, and what rate your message is being transmitted.

    If you have any other questions, please don't hesitate to post them here.

    Regards,

    Eric Hackett