THVD1500: D pin need a pullup resistor?

Beida Song said:

Why there isn't a pull up resistor for the D pin? And there is a 10k pull up resistor for R pin?

The D pin is an input pin. In the block diagram, the MCU/UART TxD is usually a push pull output. So normally the D pin is always biased. A pull up resistor on the D pin isn't necessary in that situation. But if the MCU/UART is unpowered and the THVD1500 is powered and the DE is enabled then having a pull up resistor would be helpful to set a known state on the RS485 lines. So it depends if the power sequence of the system has it so that the MCU is unpowered while the THVD is powered. 

The pull up resistor on the R pin isn't a requirement. But if the THVD1500 RE# is disabled then the R output would become high impedance. Without the resistor, the net would float to an unknown value during that situation. So if you ever disable the THVD1500 receiver then we recommend an external pull up resistor on the R pin to set a known output state for RxD (to prevent any UART glitches to the MCU).

Beida Song said:

If customer set a 10k pull up resistor in D pin, what will be effect, can you help evaluate it?

There is nothing to evaluate. You would just have a little more current consumption when the TxD pin of the MCU drives low. If you used a 10k resistor to Vcc then this would be approximately 500uA (assuming Vcc is 5V). 5V/10kohms = 0.5mA

-Bobby