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TXB0102: TXB0102 for UART Application

Charles Lai
Prodigy 50 points
Part Number: TXB0102
Other Parts Discussed in Thread: SN74AXC2T245

TXB0102 for UART Application

TXB0102 is used for UART translation usually. Will following scenario cause some problem?

  1. Power on the target board first, VCCA and VCCB are supplied.
  2. Then connect UART cable to the target board.

What we worry about is that, UART RXD could be Hi-Z for both sides. (One side is input and the other side is floating.)

Perhaps, TXB0102 is configured in wrong direction, since both sides are Hi-Z. Will it make any trouble when we connect UART cable at this moment?

Thanks,

- Charles Lai

  • 0
    TI__Mastermind 46355 points

    Hey Charles,

    If the TXB device is disabled, all of the I/O will be disabled (meaning both sides will be hi-z). I don't see any issues with doing this as long as there is adequate system level ESD protection if that cable is being physically connected.

    For UART, it will likely be better for you to use the SN74AXC2T245 device.

  • 0
    Prodigy 50 points

    Dear sir:

    As you know, most designs do not control the OE pins. They just connect OE to VCCA directly. The scenario I mentioned, hosts and the peripherals are Hi-Z. TXB does not know which direction is right. That's the issue we are concerned about.

    The same issue for MCU bootup, we supply both powers and OE firrst. But MCU takes time to reset itself. What's the side effect, if we change the directions without pulling down OE pins?

    So far, we did not get any trouble. That's the reason we need your opinion. According to your answer, it seems to work, but not a good design.

    I had ever used SN74LVC1T245 for UART application. Also saw many peolpe use TXB series. I thik, no direction confusion and high driving capibility would be the benefit of xT245.

    Thanks for your reply,

    - Charles Lai

  • 0 in reply to Charles Lai
    Guru 318050 points

    The TXB is never configured for a single direction; when OE is high, the output drivers on both sides are always active. The internal 4 kΩ resistors allow other (stronger) devices to override the voltage.

    When neither side is driven by other devices, the TXB will stay in the last state (like a bus-hold circuit); after power-up, the initial state is random, either high or low.

  • 0 in reply to Clemens Ladisch
    Prodigy 50 points

    Dear Clemens:

    Got it!

    Thanks,

    - Charles