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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?

  • Nope. Did you read the question and the answer on that post? This question was about connecting multiple TX to one RX with the UART (something that it was not designed to do). The UART actively drives the RX and TX pins both low and high and does not normally need pull ups (or pull downs for that matter). If you're planning to cross larger distances, you should look at RS232, RS422 or 485. – StarCat Feb 21 at 7:20
  • @StarCat But they said" Master will probably need 2.2K to 4.7K pullup resistor to bring the line high reliably/quickly." – ShoutOutAndCalculate Feb 21 at 7:22
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    Please read the question and the answer for that post. This was not about a simple TX->RX connection but a connection of multiple TX to one RX via a diode on every TX and a pull up on the RX. This is not the way that the UART is usually used and requires bus arbitration between multiple devices. – StarCat Feb 21 at 7:24
  • In my experience the BIGGEST gain in reliability came from using the hardware port instead of SoftwareSerial. MASSIVE gain... – Wendall Feb 21 at 20:03
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    Please show your circuit and elaborate on your problem. – the busybee Feb 22 at 15:00
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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.

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