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TXB0108: Active at the same time

?? ??
Prodigy 10 points
Part Number: TXB0108
Other Parts Discussed in Thread: TXS0108E, , SN74LS640

Hi

If the input of port A becomes high active and the input of B port becomes low active at the same time, is there any problem?

Also, when inserting pull-up resistors on both sides, 50kΩ is used, and is there anything else to be aware of other than the drive strength ± 2mA?

  • 0
    Guru 318130 points

    The edge accelerators will try to drive the lines to the other value, but they will time out quickly. The output drivers on both sides are always active, but they are very weak to allow the external devices to override their voltage.

    The TXB outputs are stronger than the pull-up resistors. So the resistors will have an effect only when the TXB's outputs are disabled. If you need pull-ups, consider using the TXS0108E instead.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    thank you for your reply.

    I have a new question, so I will ask you a question.

    I am trying to connect an LS640-1 to port B and use a pull-up resistor.

    Since the high level output voltage of the LS640-1 is 2.4V, it does not reach the high level input voltage of the TXB0108.

    Therefore, when the output voltage of LS640-1 goes from low level to high level or becomes invalid, I think that it may be detected as high while the voltage is slowly rising.

    Even in such a case, is it possible to stably output the high level without the output voltage of TXB0108 fluctuating significantly?

    Only TXB0108 can be used.

    As a supplement, the Vcc of the A port is 3.3V, and the Vcc of the B port is 5.0V.

    Also, I have another question. However, I am sorry because the question has nothing to do with TXB0108.

    If there is noise and the slowly changing waveform is an input signal, isn't a current like a through current flowing even if a TTL level IC is used? How long is the pull-up time allowed?

  • 0 in reply to ?? ??
    Guru 318130 points

    The only way to make a TTL output work with a TXB input is to use a supply of about 2.4 V, but then the output voltage might be too low. You should use a TXS translator instead.

    What happens with slowly changing input voltages is shown in figure 2 of the SN74LS640 datasheet.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you for your reply.
    I'm sorry. My question was bad.
    Port B is going to use LS640 and pull-up resistor of 50kΩ or more with power supply voltage 5V, and port A is going to use CMOS logic IC with power supply voltage 3.3V.
    Is it possible to output a stable high level without significantly changing the output voltage of the TXB0108 even when the LS640 is disabled and the signal slowly rises due to the pull-up resistor?

  • 0 in reply to ?? ??
    Guru 318130 points

    The 50 kΩ resistor is weaker than the TXB output, so if that signal was low previously, it will never rise.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you for your reply.
    It was understood that the pull-up resistor had no effect when the TXB was outputting.
    When the high level output voltage of LS640 is min 2.4V, it has not reached the high level input voltage (3.25V) of TXB, so I think that it will be a signal that rises slowly due to the pull-up resistor.
    I would like to ask if it is possible to output a stable high level without significantly changing the output voltage of the TXB even with a slowly rising signal.

    A question arose from your answer.
    Is the TXB outputting the low level voltage from the A port while the low level voltage is input to the A port of the TXB and the low level voltage is output from the B port?
    Also, does the TXB continue to output low-level voltage even if the ICs on both sides are disabled?

  • +1 in reply to ?? ??
    Guru 318130 points

    What is most dangerous about the LS640 output signal is not that it is slow, but that the pull-up resistor alone is not strong enough to pull the voltage past the switching threshold (see section 8.3.2).

    The output drivers on both sides of the TXB are always active; this is how it implements bidirectionality.

    The TXB is not compatible with TTL signals. You should use a TXS translator instead.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I was able to understand. I'm really thankful to you.