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

5 Theoretical Glitch Case for RS-485 Transceivers

For a RS-485 transceiver to exhibit this RXD error, several setup conditions must be present in conjunction:

  • The TXD voltage is less than or equal to VIL,max .
  • The RXD line is pulled high to VCC.
  • nRE and DE pins are shorted together.
  • High relative levels of capacitance are on the bus.
  • The transceiver is recently switched from transmitting to receiving on the bus.

Due to the change in modes, the RXD pin switches from high impedance to actively outputting the bus differential. With little capacitance on the bus line, the RXD line stay at a high voltage because of the pull-up resistor. But due to various sources of capacitance, the RC constant effectively resists the RXD voltage pulling to a high voltage value. If the RC constant is large enough, the voltage discharge on the bus can reflect on the RXD line as a drop in voltage as Figure 4-1 shows.

The THVD1400 can be tested for this case and is representative of standard half-duplex RS-485 devices. Similar results can apply to other RS-485 half-duplex transceivers with shorted enable pins.