Online Serial Monitor and Plotter

How to use the serial monitor

1. Plug in your board or USB-to-serial adapter. Close any other program that has its port open (an IDE's serial monitor, a terminal, a slicer): a port can be open in only one program at a time.
2. Pick the baud rate the device uses, the number in its code's Serial.begin(…) or in its manual. Most devices use 8 data bits, no parity and 1 stop bit (8N1), the defaults here.
3. Press Connect and choose the port in the browser's list. What the device prints appears in the monitor at once.
4. Type in the box at the bottom and press Enter to send. Pick the line ending your device expects: most command parsers wait for a newline (LF), some for CR LF.

Everything happens in your browser: what your device sends and what you type are never uploaded. Saving the log writes a file on your own computer.

Monitor tools

• Timestamps show when each line's first byte arrived, to the millisecond, so you can time a boot sequence or a sensor loop.
• Hex shows the raw bytes, line endings included (0D 0A is CR LF). Use it for binary protocols or to find a stray character. Control characters also show as symbols such as ␀ and ␛ in the text view.
• Filter shows only the lines containing what you type, for example ERROR or a sensor name.
• Auto-scroll follows new lines. Scroll up to read and it pauses by itself; scroll back to the bottom to follow again.
• Save .txt keeps the log as you see it, Save .csv as a table with the time, direction, text and hex bytes of every line, ready for a spreadsheet.
• Up and Down arrows in the send box bring back what you sent before. Tick Hex next to it to send bytes such as 01 FF 0A.

Plotter

Switch to Plotter to draw the numbers your device prints as a live chart, the same format as the Arduino IDE's serial plotter: one reading per line, values separated by spaces, tabs or commas, each optionally named (temp:21.5,hum:40). Up to 8 values per line get their own colour and can be hidden from the legend. Lines with words in them are skipped, so status messages don't break the chart. Pause freezes the view while data keeps arriving, and Save .csv in plotter view writes the plotted values.

DTR, RTS and resets

DTR and RTS are control lines the computer sets on the port. Many development boards wire them to their reset circuit. On most systems the computer raises DTR as soon as a program opens the port, before this page can change it, so boards like the Arduino Uno restart when you connect and you see their boot messages. After connecting, the DTR and RTS boxes set the lines directly: switch DTR off and on again to reset such a board on purpose, or leave it off to avoid further resets while you stay connected. Unticking it can't prevent the reset on connect. CTS, DSR, DCD and RI light up when the device sets them.

If the board restarts into a different USB mode (while you upload new firmware, for example), the port disappears for a moment. With Reconnect automatically on, the monitor reopens it with the same settings when it comes back.

Troubleshooting

• Garbled characters: the baud rate doesn't match the device. Try the rate from its code, then 115200 and 9600.
• The port isn't in the list: try another USB cable (some only charge) and port, and check that the adapter's driver is installed. On Linux your user needs access to the serial device (often the dialout group).
• "Couldn't open the port": another program has it open. Close it and press Connect again.
• Nothing arrives: check the baud rate and that TX and RX are crossed (the adapter's TX to the device's RX), and that the device and adapter share ground.

Which browsers can use it?

The serial monitor uses the Web Serial API. As of October 2026 it works in Chrome, Edge and Opera on Windows, macOS, Linux and ChromeOS, in Chrome for Android from version 148, and in Firefox on computers from version 151, which first asks you to install a small site permission add-on. Safari doesn't support it, and on iPhone and iPad no browser does. There, use a desktop terminal program such as PuTTY (Windows) or screen (macOS, Linux), or the serial monitor in your board's IDE.

Related tools

Reading the parts on your board? The resistor color code calculator decodes colour bands and SMD codes. Building a controller or a MIDI device? Check it with the MIDI tester or the joystick tester, and make a test signal with the tone generator.