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

6 Theoretical THVD24XX Idle Fail-safe Case

The THVD24XX series is another RS-485 transceiver line that adds additional fault protection and other protection features. Namely, THVD24XX devices have a fail-safe feature that can detect if there is open, short or idle circuit on the differential bus lines. All three of these conditions can result in the bus having near 0 voltage. To detect these conditions, if the bus voltage is inside the VTH_FSH range for longer than time of tD(OFS) , the device can output a logic high state. Equation 2 shows this:

Equation 2. If     V TH F SH - < V Bus <     V TH F SH +   For     t D OFS Then     V RXD = Logic   High

Similar to the general RS-485 glitch case, the half-duplex THVD24XX devices can experience a voltage shift on the RXD line with high enough capacitance present. The same conditions mentioned previously must be met for the THVD24XX device to have a change in voltage: TXD is at less than VIL,max, RXD is pulled high with a resistor, and the shorted nRE/DE pins recently switched the device into the receiver mode.

However, the key difference for the THVD24XX fail-safe devices is that the devices can enter fail-safe mode with enough capacitance on the bus when entering a receiver mode. This is different from general RS-485 devices as the devices can show voltage drops for various times.

Since the voltage on the A-B bus differential is changing slowly due to capacitance, the R pin can follow the last state of the bus. If this state is a voltage low around 0V, the state can trigger the fail-safe feature if the voltage is inside the threshold range. Once triggered, the R pin can output low until this fail-safe timer has expired. Although the fail-safe feature is not intended for this scenario, the feature provides a predictable time of tD(OFS) until the bus is ready to be read after switching modes.

THVD2410 can be tested with these conditions, and THVD2450 can also have similar results.