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

11 Summary

Shorting the driver and receiver enable pins for any half-duplex RS-485 transceiver can potentially create R pin voltage drops with high capacitance after switching into the receiver mode. Testing the application scenario with the THVD1400 and THVD2410 with bus capacitance ranging from 50pF to 1000pF showed the glitch occurring during the TX to RX mode switch. The THVD1400 glitch was a result of the time RC time constant on the differential bus, while the THVD2410 glitch was a combination of the fail-safe delay and the RC time constant on the differential bus. Sending a logic high on the TXD or D pin before entering receiver mode of the transceiver successfully removes any voltage drop caused by bus capacitance. All tested capacitance showed no voltage drops when the TXD pin was held high for both THVD1400 and THVD2410.