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SN65LBC173: the definition of determinate state

Noriyuki Takahashi
Genius 11910 points

Part Number: SN65LBC173

Hi team,

Customer has following question.

When the differential inputs are −0.2 V < VID < 0.2 V, the output is indeterminate. What is the definition of indeterminate state? The state is one of L, H and Z?

Regards,

Noriyuki Takahashi

  • 0
    TI__Genius 11910 points

    Hi team,

    I forgot to ask one more.

    Is it OK that one channel of the device can't affect other channels?

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Guru 53535 points

    Hi Noriyuki-san,

    The term "inderterminanant" in this case indicates that the state of the device cannot be determined based solely on the current state of the bus. Because of the hysteresis introduced by the input-low threshold being -200mV and input-high threshold being 200mV, the state of the receiver when the bus rests between these two values will depend on previous conditions (weather the input was more recently below -200mV or above 200mV). In short-hand, this is represented by the '?' because the value is still unknown given the other information in the table. The device will not be high-Z in this state (as the enables keep the device active). 

    Each channel operates independently and the bus inputs of one channel will not affect the outputs of another. All channels do share the same enable inputs and supply rails so changes to these pins will effect all channels. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Genius 11910 points

    Eric,

    Thanks!

    Please let me confirm again. Regarding the 1st question, can we say that the output can be fixed on either of L and H? Customer wants to know the output can be fixed on one of L, H or intermediate voltage during indeterminate state, or the output  can always change the state and we can't know the state.

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Guru 53535 points

    Hi Noriyuki-san,

    Yes, the digital output of the device will only change when an input threshold is crossed. The driven state of the output pin will either be a digital high, or digital low - no intermediate voltage values. The output will not sporadically switch while the differential lies between the -200mV and 200mV range. This is in accordance with the RS-485 standard to ensure every transceiver has a  minimum noise immunity. 

    Modern transceivers will also be able to detect an idle or disconnected bus and keep the output at a known high state when the differential is close to 0V. 

    Regards,
    Eric Schott