10 Voltage Drop Workarounds

If a voltage drop on the RXD line is causing issues for a system, there are a variety of methods to solve this voltage drop.

• The main workaround is to end the last transmission with a voltage high on the TXD line. This eliminates the voltage drop on the RXD line. Across the various tests discussed previously, a voltage low was simulated on the TXD line with 0V. When the TXD line is held at a voltage high, there is no voltage drop present as seen for both THVD1400 and THVD2410 in Figure 10-1 and Figure 10-2.

Figure 10-1 THVD1400 1000pF TXD High

Figure 10-2 THVD2410 1000pF TXD High

Holding the last TXD bit high works for both single-ended and differential capacitance setups for THVD1400 and THVD2410. This workaround can also apply to most RS-485 devices experiencing a voltage drop on the RXD line due to high capacitance.

Figure 10-3 DE and RE Shorted with D=High

• Another workaround is to separately control the nRE and DE pins using the MCU. Rather than shorting these together, nRE and DE can be toggled off and on with a slight delay between modes such that the bus has enough time to discharge. This allows the capacitors to fully discharge before the transceiver starts to read the bus.
• Lastly, if the other methods are not possible for a particular system, capacitance on the bus need to be minimized to shorten the RXD voltage drop time. Unnecessary filtering capacitors or other capacitive components need to be removed.