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SN65LBC180 - About a change of the threshold

Dice-K
Mastermind 7480 points
Other Parts Discussed in Thread: SN65LBC180

Hello,
I have a question about SN65LBC180.

The standard of RS-485 defines the differential signal in +200mV or more as High and defines it in -200mV or less as Low.
SLLS174G describes "FUNCTION TABLES" in the page-2.

I increased connection number and lengthened a communication wiring.
I observed the wave pattern of the differential signal (A-B) and the output signal (R) of RECEIVER of the IC at the same time.
Output R changes to Low at the differential signal = -300mV.
Output R changes to High at the differential signal = -200mV.

Because the differential signal has a long communication wiring, it oscillates at +1V, -1V.
Because there is much connection number, it becomes the wave pattern which became dull under the influence of the external capacitors of each apparatus.

What is the reason why the voltage of the threshold is changed to?
Is it changed by the conditions such as connection number, the length of the communication wiring, the external circuit of the IC?

Regards,
Dice-K

  • 0
    TI__Guru 62920 points
    Hi Dice-K,

    I actually would not expect the threshold to depend on any of the other factors that you mentioned. To make sure I understand the behavior you describe, though, can you please share the data (e.g., oscilloscope waveform captures) that shows the shift in threshold voltage?

    Thank you,
    Max
  • 0 in reply to Miguel Robertson
    Mastermind 7480 points

    Hello Max,
    Thank you for your reply.

    By your answer, I understand that the threshold does not change.
    I review an examination condition and measurement method.
    If I do new discovery, I contact you.

    Best regards,
    Dice-K