On A post from Arduino forum , someone mentioned that a pull up resistor over RX/TX pin could help the reliably of UART communication.
Is that really the case for nano and uno board?
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No. That is not the case. Adding a pullup resistor on a normal UART will do nothing except maybe reduce your reliability.
You only want pullup resistors on something which is "open drain" (AKA "open collector") which has no way of raising the voltage of a signal line, only lowering it - such as I2C signals, or a shared open-drain, interrupt signal.