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SN65LBC176-Q1: Impedance for SN65LBC176-Q1 when the device is powered off

Ryuuichi machida
Guru 11655 points
Part Number: SN65LBC176-Q1
Other Parts Discussed in Thread: SN65LBC173

Hi,

I would like you to confirm impedance value of "D", "R", "A" and "B" pins in case of following condition.

* SN65LBC176-Q1 is powered off (Vcc is not supplied.)

Could you please send me your comment about above condition ?

Best Regards,

  • 0
    TI__Guru 62910 points
    Hello Machida-san,

    For the A and B pins, the minimum (worst-case) input resistance is 12 kOhms. This is present whether the transceiver is powered or not.

    For the D pin, the input leakage current (which correlated with input impedance) is no greater than +/- 100 uA (see IIH/IIL specifications). For the receiver this is +/- 20 uA (see IOZ specification).

    Regards,
    Max
  • 0 in reply to Max Robertson
    Guru 11655 points
    Hello Max-san,

    Thank you for your reply.
    Let me confirm about below.

    >For the A and B pins, the minimum (worst-case) input resistance is 12 kOhms. This is present whether the transceiver is powered or not.
    I also confirmed other RS-485 device to get the answer in my side, then I found following description in SN65LBC173.

    When disabled or powered off, the receiver inputs present a high-impedance to the bus for reduced system loading.

    SN65LBC173 also have 12 kohm at receiver input resistance.
    However, it seems that input impedance become higher than 12k ohm in case of powered off in this device.
    Can we say same comment for SN65LBC176-Q1 as well ?

    Best Regards,
  • 0 in reply to Ryuuichi machida
    TI__Guru 62910 points
    Machida-san,

    The maximum bus input current ("II" spec) is the same for both VCC = 5 V and VCC = 0 V conditions for LBC173, so there is not a substantial reduction in input resistance when the device is powered off. I believe "high impedance" in the statement that you mentioned means that the impedance does not go beneath what is required of a single "unit load" per the TIA/EIA-485 specification. (You can see the I/V characteristic in Figure 8 of the LBC183 datasheet on page 7.)

    Regards,
    Max
  • 0 in reply to Max Robertson
    Guru 11655 points

    Hi Max-san,

    Thank you for your reply.

    Understood.

    Best Regards,