SLLA660 December   2024 THVD1400 , THVD2410

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Introduction
  5. 2 Overview of RS-485
  6. 3 Shorting DE and nRE Pins
  7. 4 General R Pin Glitch Background
  8. 5 Theoretical Glitch Case for RS-485 Transceivers
  9. 6 Theoretical THVD24XX Idle Fail-safe Case
  10. 7 RS-485 Testing Setup
  11. 8 THVD1400 Capacitance Results
  12. 9 THVD2410 Capacitance Results
  13. 10Voltage Drop Workarounds
  14. 11Summary
  15. 12References

3 Shorting DE and nRE Pins

Focusing on the nRE (denoted as not RE) and DE pins of the transceiver, we notice a single MCU pin—labeled DIR—controls whether the device receives or drives data on the A and B bus lines. When nRE and DE are a logic high voltage, the transceiver is only driving signals on the bus. Likewise when nRE and DE are low, the transceiver is only receiving signals. From this setup, switching from driver to receiver modes when the RC time constant is long enough can cause the receiver to output invalid voltage levels.