Web Serial Terminal

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arduino serial monitor online

Arduino serial monitor online

Use the same number that is in your sketch's Serial.begin, open the port and read the lines Serial.println sends. To send commands back, choose the line ending the sketch expects and type in the send field.

Serial.begin, println and the line ending

The Arduino reference defines Serial.begin(speed) as the data rate in bits per second (baud), and the optional second argument sets data bits, parity and stop bits, with a default of 8 bits, no parity and 1 stop, SERIAL_8N1. So the monitor number must equal the one in Serial.begin: a sketch with Serial.begin(9600) calls for 9600 here. If meaningless symbols show up, the baud rate differs.
Serial.println writes the value as text and appends a carriage return (CR, ASCII 13) and a newline (LF, ASCII 10), per the same reference. So Serial.println("OK") sends bytes 4F 4B 0D 0A, and the monitor shows one line, OK. Serial.print adds nothing, so several calls appear glued on the same line until a terminator arrives. Serial.write(65) sends the byte 0x41, which text shows as A.
To send, the line ending has to match the sketch. If it reads with Serial.readStringUntil('\n'), choose LF. If it compares text without handling \r, avoid CRLF. With no line ending, only the typed bytes go out. Opening the port can restart many Arduino boards, which is normal, and the DTR pulse button triggers a restart on purpose. The API accepts 7 or 8 data bits, and the Arduino also accepts 5 and 6, which are out of the browser's reach.

What each call sends

Serial.println("OK")

Input: OK + CR LF
Expected output: 4F 4B 0D 0A

The monitor shows the line OK.

Serial.print("OK")

Input: OK
Expected output: 4F 4B

With no terminator, the line stays open until one arrives.

Serial.write(65)

Input: 65
Expected output: 41 (A)

A raw byte, shown as the character A.

Command for a sketch reading until \n

Input: LED_ON + LF
Expected output: 4C 45 44 5F 4F 4E 0A

Seven bytes, the last is the \n.

Classic baud rate of the basic example

Input: 9600 baud, 8N1
Expected output: 960 bytes/s

Data ceiling on the wire, with no gaps between bytes.

Full tool FAQ

What is Web Serial and which browsers have it?

It is an API that lets a page access serial ports with your permission. Per caniuse on 2026-11-02, it works in Chrome and Edge 89 or newer, Opera 76 or newer, Firefox on desktop from version 151 and Chrome for Android from 156. It does not work in Safari, on iPhone, in Firefox for Android or in Samsung Internet.

How do I connect an Arduino or an ESP board?

Which baud rate should I use?

What does 8N1 mean?

Why does it not work on iPhone, Safari or older Firefox?

Which line ending should I choose when sending?

How do I send bytes in hexadecimal?

Does the log grow without limit?

What if I pull the cable mid-session?

Is it safe? Does my data go to a server?

Frequently asked questions

Why does the board restart when I connect?

Many Arduino boards wire the DTR signal to the reset pin, and opening the port moves that signal. It is board behavior, not a tool error, and it is even how firmware gets uploaded. The DTR pulse button does the same on purpose.

Can I keep the IDE and the monitor open together?

Only one program at a time can open the port. Close the IDE serial monitor before connecting here, or disconnect here before uploading a new sketch.

Does the monitor work with ESP32 and ESP8266?

It works with any board that appears as a serial port, which includes ESP ones. Use the baud rate your code sets and strip the ANSI colors, common in those logs, with the toolbar option.

Next steps

Serial.begin, println and the line ending

The Arduino reference defines Serial.begin(speed) as the data rate in bits per second (baud), and the optional second argument sets data bits, parity and stop bits, with a default of 8 bits, no parity and 1 stop, SERIAL_8N1. So the monitor number must equal the one in Serial.begin: a sketch with Serial.begin(9600) calls for 9600 here. If meaningless symbols show up, the baud rate differs.

Serial.println writes the value as text and appends a carriage return (CR, ASCII 13) and a newline (LF, ASCII 10), per the same reference. So Serial.println("OK") sends bytes 4F 4B 0D 0A, and the monitor shows one line, OK. Serial.print adds nothing, so several calls appear glued on the same line until a terminator arrives. Serial.write(65) sends the byte 0x41, which text shows as A.

To send, the line ending has to match the sketch. If it reads with Serial.readStringUntil('\n'), choose LF. If it compares text without handling \r, avoid CRLF. With no line ending, only the typed bytes go out. Opening the port can restart many Arduino boards, which is normal, and the DTR pulse button triggers a restart on purpose. The API accepts 7 or 8 data bits, and the Arduino also accepts 5 and 6, which are out of the browser's reach.

What each call sends

Serial.println("OK")

Input: OK + CR LF
Expected output: 4F 4B 0D 0A

The monitor shows the line OK.

Serial.print("OK")

Input: OK
Expected output: 4F 4B

With no terminator, the line stays open until one arrives.

Serial.write(65)

Input: 65
Expected output: 41 (A)

A raw byte, shown as the character A.

Command for a sketch reading until \n

Input: LED_ON + LF
Expected output: 4C 45 44 5F 4F 4E 0A

Seven bytes, the last is the \n.

Classic baud rate of the basic example

Input: 9600 baud, 8N1
Expected output: 960 bytes/s

Data ceiling on the wire, with no gaps between bytes.

Full tool FAQ

What is Web Serial and which browsers have it?

How do I connect an Arduino or an ESP board?

Which baud rate should I use?

What does 8N1 mean?

Why does it not work on iPhone, Safari or older Firefox?

Which line ending should I choose when sending?

How do I send bytes in hexadecimal?

Does the log grow without limit?

What if I pull the cable mid-session?

Is it safe? Does my data go to a server?

Frequently asked questions

Why does the board restart when I connect?

Can I keep the IDE and the monitor open together?

Only one program at a time can open the port. Close the IDE serial monitor before connecting here, or disconnect here before uploading a new sketch.

Does the monitor work with ESP32 and ESP8266?

It works with any board that appears as a serial port, which includes ESP ones. Use the baud rate your code sets and strip the ANSI colors, common in those logs, with the toolbar option.