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SN65LBC180: RS485 related issue

DHIRAJ PUTHRAN
Prodigy 20 points
Part Number: SN65LBC180
Other Parts Discussed in Thread: THVD1512, THVD1552

I still have some questions about RS485 (SN65LBC180DR),

  • Whether termination is required ? If yes, What is the value and whether it is required for both the ends ?
  •  Since we observe different state of DE and RE pin ?What should be the standard state of DE and RE pin to work in half and Full Duplex ?
  Full Duplex  Half  Duplex 
  DE RE DE RE
Transmit  High  Low  High  Low
Receive  High  Low  Low  Low 
  • +1
    Guru 269890 points

    Both ends of the bus should have termination. But if you are using unidirectional point-to-point connections, then you need termination only at the receiver.

    The termination resistor should be equal to the characteristic impedance of the cable. RS-485 cables usually have 120 Ω.

    In a half-duplex system, DE must be enabled only when you are transmitting. You can connect DE and /RE if you do not want to receive your own data.
    In a full-duplex system, you can leave both driver and receiver enabled at all times. You might want to disable them sometimes to save power.

    (The LBC180 is a very old device; the THVD1552/THVD1512 are cheaper and better.)