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P82B715: Power Supply

Paul Van den Bossche6
Prodigy 50 points
Part Number: P82B715

Dear forum,

I need to connect 2 I2C slaves [A]  and [B] close the master, and one [C]  far (+/- 6m) away. The one far away will be interfaced with a set of P82B715P's. (I can't afford loss of data !)

1. Can I use the same power supply (5V) for the master + the two close-by + one P82B715 and another PSU (also 5V) for the 2nd P82B715 + the [C] slave ? (of course all grounds will be connected together)
Here is a schematic of the "chain" :

MASTER <-> A <-> B <-> P82B715 <-------------------------------> P82B715 - C
PSU 1                                                                            PSU 2

2. Since the distance is "only" 6m, can I use a "normal" cable ? (not Cat5e or the like)

Any info concerning this matter will be highly appreciated !
Thanks in advance !

  • 0
    TI__Mastermind 23326 points
    Hello Paul,
    You can, you just need to be careful with drops in cable and proper decoupling given the load at the remote devices. I would make sure you don't have too high a load, the P82B715 alone isn't too bad, but if you have a bunch of other devices there then you need to account for that.
    -Francis Houde
  • 0 in reply to fhoude
    Prodigy 50 points

    Thanks for your fast reply to my second question? Francis. but I'm still looking for the answer to my first question.  When I go through the datasheets I see that the Power supply for both P82"s is the same.  My first question was if each P82 can have it's own power supply  (GND's connected).

    Again : thanks for your concern.

  • 0 in reply to Paul Van den Bossche6
    TI__Mastermind 23326 points
    Hello Paul,
    Sorry I missed that first question. The answer is yes you can have two separate supplies assuming they are the same voltage. Make sure you have a solid ground connection between the two boards to negate any significant ground offsets. Most people use the same supply because it is more cost effective that way.
    -Francis Houde
  • 0 in reply to fhoude
    Prodigy 50 points
    Thanks again, Francis.
    The reason why I do this is because the main power supply for the whole system is 12V. I convert it to 5 V for the master etc, and run this 12 V to my "far away" I2C. There I take it down to 5V (same as Master + I2C[A] + I2C[B] + P82B715 to avoid disturbances, varying line drops etc. I'll have a few extra wires in my multi cable, I'll use those to carry the GND. I'll connect the screen of the cable ONLY to GND of the master unit.

    Now just one more question (witch is irrelevant for this project) : Is it possible to tap between the two P82B715's without using a third one ?
    Again : thanks for your concern !!!
  • 0 in reply to Paul Van den Bossche6
    TI__Mastermind 23326 points
    Hello Paul,
    I am not sure what you mean by "tap between two P82B715's without using a third one?" Can you clarify?

    Also, what do you mean "screen of cable"? are you talking about shielding of cable?

    -Francis Houde
  • 0 in reply to fhoude
    Prodigy 50 points

    Thanks for your reply, fhoude. But I saw that my question was stupid. I think I can't mix SDA/SCL lines with LX/LY ones. I saw in the datasheet that when you tap into the LX/LY line, you GOT to have a P82B725 to "translate" the signal back to SDA/SCL.

    Nevertheless, a picture says more than 1K words ... :)

    Do you see any problems with the setup you find on the diagram ? (any I²C device that should be removed or re-placed ?)

    thanks in advance !

  • 0 in reply to fhoude
    Prodigy 50 points
    ... and yes, I'm talking about the shielding of the cable.
    - Shall I use the shield as GND and connect to both ends of the LX/LY cable ?
    - Or do I use a separate wire in the cable to connect both GND's, and connect the shield only on the MASTER side ?

    1K Thanks !! :)
  • 0 in reply to Paul Van den Bossche6
    TI__Mastermind 23326 points
    Hello Paul,
    Use the shield as ground, this will be a nice low impedance path. Depending on the connectors you may want to have one wire to be ground for the sake of redundancy. In my experience, when connectors are moved there can be short intermittent ground bounces that could interfere with coms but depends on how solid the connection between cable shield and board.
    -Francis Houde