Online Serial Monitor
Connect an Arduino, ESP32, Raspberry Pi Pico or any USB-to-serial adapter straight from your browser. Read timestamped output, send text or hex commands, graph numbers in the plotter and save the log, with nothing to install.
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Port settings (data bits, parity, stop bits, flow control, DTR/RTS)
Settings apply on the next connect. 8 data bits, no parity, 1 stop bit (8N1) is right for Arduino, ESP32, Raspberry Pi Pico and almost every USB-serial adapter.
Every line that contains numbers becomes one sample. Use 1.2 3.4, 1.2,3.4 or labelled values like temp:21.5 hum:40, the same format as the Arduino IDE Serial Plotter.
How to use the Online Serial Monitor
- Plug in your board with a data-capable USB cable and close any other program that is using the port, such as the Arduino IDE Serial Monitor.
- Choose the baud rate that matches
Serial.begin()in your sketch (9600 and 115200 are the most common). - Click Connect port, select your device in the browser's port picker and click Connect. Output appears in the log with timestamps.
- Send commands in the box under the log. Pick the line ending your code expects, or switch to Hex bytes to send raw binary.
- Open the Plotter tab to graph numeric lines live, then download the data as CSV or the chart as PNG.
- Save the log as a text file when you are done, and click Disconnect to release the port.
How a browser serial monitor works
A serial monitor shows the bytes a microcontroller sends over its UART, the same stream you print with Serial.println() in an Arduino sketch or print() in MicroPython. Your board's USB chip (an ATmega16U2, CH340, CP2102, FTDI or the microcontroller's own USB) appears to the computer as a virtual COM port. The Web Serial API lets a web page open that port after you pick it in a browser dialog, so the page can read and write bytes just like the Arduino IDE, PuTTY or screen would.
Serial links have no clock wire, so both sides must agree on the timing in advance. The baud rate is the number of signal changes per second. With the common 8N1 framing each byte is sent as 10 bits (1 start bit, 8 data bits, no parity, 1 stop bit), so the maximum throughput is roughly baud rate ÷ 10 bytes per second: 960 bytes/s at 9600 baud and 11,520 bytes/s at 115200 baud.
Common baud rates and where they are used
| Baud rate | Max bytes/s (8N1) | Typical use |
|---|---|---|
| 9600 | 960 | Arduino examples, GPS modules (NMEA), HC-05/HC-06 Bluetooth in data mode, many sensors |
| 38400 | 3,840 | HC-05 AT command mode, some older modems and industrial devices |
| 57600 | 5,760 | Arduino bootloaders on older boards, telemetry radios |
| 74880 | 7,488 | ESP8266 ROM boot messages (the garbage you see at reset at other rates) |
| 115200 | 11,520 | ESP32, ESP8266 sketches, Raspberry Pi console, Pico, most modern boards |
| 250000 | 25,000 | Marlin 3D-printer firmware |
| 921600 | 92,160 | Fast data logging and firmware upload on ESP32 and USB-serial adapters |
Boards with native USB, such as the Arduino Leonardo, Raspberry Pi Pico or ESP32-S3 in USB CDC mode, ignore the baud rate entirely and always run at full USB speed, so almost any value works for them. The exception is 1200 baud: opening and closing the port at 1200 is the "touch" signal that makes many of these boards restart into their bootloader.
Troubleshooting tips
- "Failed to open" or "port is busy": only one program can hold a serial port. Close the Arduino IDE Serial Monitor or Plotter, Cura, OctoPrint, PuTTY or another tab, then connect again. Uploading a sketch also needs the port, so disconnect here before you upload.
- No output at all: check that the sketch calls
Serial.begin(), that TX and RX are crossed (TX to RX) when you use a separate USB-serial adapter, and that both sides share a ground wire. - Missing first lines: on boards with native USB, add
while (!Serial) {}afterSerial.begin()so the sketch waits for the monitor to connect. - Debugging binary protocols: switch the display to Hex or Text + hex to see every byte, including invisible carriage returns (
0D) and line feeds (0A). Unprintable bytes are shown as a middle dot in text view. - Parity, framing or break errors in the log mean the electrical timing does not match: check the baud rate, data bits, parity and stop bits in Port settings.
Frequently asked questions
Which browsers support this online serial monitor?
It uses the Web Serial API, which is available in desktop Chrome, Microsoft Edge and Opera version 89 and newer on Windows, macOS, Linux and ChromeOS. Firefox and Safari do not support Web Serial, and mobile browsers cannot open serial ports. The demo device works in any browser.
Why can't I see my Arduino in the port list?
Use a USB cable that carries data, not a charge-only cable, and install the USB-serial driver if your board uses a CH340 or CP210x chip. On Linux your user must be allowed to access the port (usually by joining the dialout group). Ports that are open in another program can appear but will fail to open.
Why is the output garbled or full of strange characters?
Garbled text almost always means the baud rate does not match the one set in Serial.begin() on the device. ESP8266 and ESP32 boards also print a boot message at 74880 or 115200 baud, which looks like noise at other rates. Disconnect, pick the correct rate and connect again.
Which line ending should I choose?
Pick what your sketch reads up to. Code that uses Serial.readStringUntil('\n') needs Newline (LF). Many AT-command modules such as HC-05, ESP-01 and SIM800 expect Both CR + LF. Choose No line ending when you send single-character commands.
What data format does the plotter understand?
Each line that contains numbers becomes one sample. Separate values with spaces, commas, semicolons or tabs, for example 21.5,40.2, or label them as name:value pairs like temp:21.5 hum:40.2, which is the same format as the Arduino IDE Serial Plotter. A line of words alone sets the names of the following unlabelled columns.
Does my board reset when I connect?
Many Arduino boards, including the Uno and Nano, reset when the DTR line goes active, so the sketch restarts and setup() runs again when you connect. Untick DTR in Port settings before connecting to avoid that, or use the Reset board button to restart it on purpose.
Is any of my serial data sent to a server?
No. The browser talks to the port directly and everything is shown and stored only on this page. The log is held in memory until you clear it or close the tab, and only the baud rate, line ending and command history are remembered in this browser.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.