It might be an obvious answer to this, but I can't find it via a quick google search. To give my question a bit of more context, I'm writing an application which simulates a communication with Arduino via serial port, therefore I need to know how it is implemented internally.

From my previous experience with micro-controllers, the serial port is normally presented in micro-controller as two separate queues, one for TX and another for RX. For transmit out of MC, my software writes a byte to the end of transmit queue, and MC hardware reads byte from the beginning of the queue, converts it to bits, and toggles the pin to transmit data/parity/stop bits. And it keep doing that until queue is exhausted. The opposite happens for receive queue.

Am I right in assuming that Arduino has similar implementation with two separate queues? In particular, I want to read and write to serial port simultaneously.

  • 1
    It depends on which Arduino and which serial port you are talking about. It can be a raw UART (TX and RX pins), an UART tunneled through USB by another chip (e.g. in the Uno communicating with the host computer), or a completely virtual UART seen through USB (e.g. in the Leonardo). Apr 27, 2017 at 13:22
  • but to answer the question, there are two separate buffers, the arduino can only read or write at a time, but the hardware side can be treated as being full duplex with separate buffers.
    – James Kent
    Apr 27, 2017 at 13:25
  • 1
    The AVR UART has a simple one step hardware queue for both the RX and the TX. The hardware has a register for the parallel to serial conversion and a queue register to allow write/read while a transmit/receive is in progress. The Arduino HardwareSerial class adds a software queue (32 or 64 bytes). These buffers are used to feed the hardware queues on interrupts from the UART hardware module. Apr 27, 2017 at 15:28
  • 1
    Please do not cross-post the same question on different Stack Exchange sites. See Is cross-posting a question on multiple Stack Exchange sites permitted if the question is on-topic for each site?.
    – Nick Gammon
    Apr 27, 2017 at 23:08

2 Answers 2


Yes. Arduino has two buffers. See the HardwareSerial.h and HardwareSerial.cpp


I have a lengthy post about Arduino serial communications: How does serial communications work on the Arduino?

In short, the hardware (on an Atmega328 like on the Uno) has a two-byte receive queue (so you can be putting one byte into memory while the next byte is being received). It has a one-byte send queue.

Lengthier queues are implemented in software using interrupts to move data from the RAM queues in/out of the hardware queues.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.