Serial Monitor Online

This serial monitor online lets you read and send data on any serial port your computer can see: an Arduino, ESP32, ESP8266 or Raspberry Pi Pico, a USB to serial adapter with a CH340, CP2102, FTDI or PL2303 chip, a 3D printer, a GPS module or an old RS-232 device. Pick the port, set the baud rate, data bits, parity, stop bits and flow control, and the output appears with optional timestamps and a hex view. You can switch the DTR and RTS lines, watch CTS, DSR, DCD and RI, reset a board and run a loopback test that checks a port or adapter end to end. It works in Chrome, Edge and Opera on computers, in Firefox 151 and later, and with some limits in Chrome on Android.

How to use the serial monitor

1. Connect the board or adapter with a USB cable that carries data, not a charge-only cable.
2. Set the baud rate to match the device. For an Arduino sketch, use the number in Serial.begin().
3. Press Choose port, select the device in the list and press Connect. Output starts appearing.
4. Type a command in the box and press Enter. Choose the line ending the device expects: most Arduino sketches that read lines want New line.
5. Turn on Hex view to see every byte, including ones that do not print. Save log downloads everything as a text file.

Serial settings explained

The usual combination is written 8N1: 8 data bits, no parity, 1 stop bit. Change these only when a device's manual says so, as some industrial and older equipment uses 7E1.

The serial monitor is not showing anything

Work through these from the most common:

• The baud rate is wrong. A mismatch usually shows garbage, but very far off rates show nothing at all.
• The sketch never prints. Check that Serial.begin() runs before your first print.
• The board printed before you connected. Opening the port resets an Uno, Nano or Mega through the DTR line, which restarts the sketch so you catch setup(). Boards with native USB do not reset this way. Press Reset Arduino or Reset ESP32, or the board's reset button.
• Native USB boards need a setting. On an ESP32-S3 or C3 in the Arduino IDE, set USB CDC On Boot to Enabled. On a Leonardo or Micro, add while (!Serial); after Serial.begin().
• TX and RX are not crossed. With a USB to serial adapter, the adapter's TX goes to the device's RX and RX to TX, and the grounds must be joined.
• The device expects a different line ending. If it answers commands only sometimes, try Both CR and LF.

Garbled characters or line errors

Random symbols almost always mean a baud rate mismatch. If the Line errors count climbs, the port is reporting framing or parity errors, which also point to mismatched settings or electrical noise on long wires. Garbage only during startup is normal on ESP8266 boards. A 3.3 V device on a 5 V adapter, or a long unshielded cable, can also corrupt data. Check the voltage jumper on the adapter.

The port does not appear

If the board is missing from the list, the computer has not created a serial port for it:

• Drivers. Boards with a CH340 or CH9102 chip may need WCH's driver on older Windows or Mac versions, CP210x boards need Silicon Labs' driver if Windows Update does not supply it, and FTDI chips work out of the box on Windows, macOS and Linux. The tester names the chip once the port opens.
• Prolific PL2303 adapters. Windows rejects many old or copied PL2303 chips with a "phased out" message in Device Manager. Replace the adapter with a CP2102, CH340 or FTDI one.
• Linux permissions. Add yourself to the dialout group with sudo usermod -aG dialout $USER, then log out and back in. On Ubuntu, a CH340 port that vanishes a second after plugging in is grabbed by the brltty package. Remove it with sudo apt remove brltty.
• The cable or port. Try another cable and a port on the computer itself. If Windows shows an unknown USB device, follow the usb device not recognized guide.
• Another program holds the port. Close the Arduino IDE's Serial Monitor, PuTTY, a slicer such as Cura or another browser tab. Only one program can use a port at a time.

Arduino and ESP32 reset and boot mode

Classic Arduino boards wire DTR to the reset pin through a capacitor, so DTR going on resets the board. Reset Arduino turns DTR off and on again. Most ESP32 boards use DTR and RTS together through two transistors: RTS pulls the EN (reset) pin low and DTR pulls GPIO0 low. Reset ESP32 pulses EN with GPIO0 high, so the board restarts and runs its program. ESP32 boot mode holds GPIO0 low while EN is released, and the board waits for a firmware upload, which shows up at 115200 baud as "waiting for download". If an upload tool reports a wrong boot mode, hold the board's BOOT button while it connects.

Test a serial port with a loopback

A loopback test proves a port or adapter works without any other device. Join TX to RX with a jumper wire, open the port and press Loopback test. The tester sends 1,024 bytes and checks every one comes back, measures the round trip and the throughput, and checks the control lines. On a 9 pin RS-232 connector, the loopback links pins 2 and 3 for data, 7 and 8 for RTS and CTS, and 4 to 6 and 1 for DTR, DSR and DCD.

Other ports and devices

For music gear on USB or 5 pin DIN, the midi tester reads notes and controls. Wireless modules that use Bluetooth Low Energy instead of a serial profile show up in the bluetooth tester. 3D printers often accept G-code over serial at 115200 or 250000 baud. On Marlin firmware, send M503 to list the stored settings before you work through the 3d printer calibration steps, and the 3d printer test prints page has models to check the result. GPS modules print NMEA sentences, usually at 9600 baud. To check the GPS in a phone instead, use the gps test.

What this serial monitor cannot do

The browser only opens ports you pick, one tab at a time, so it cannot watch the traffic between another program and a device the way a port sniffer does. It cannot open a port another program is using, and virtual ports made by some emulators and remote desktop tools may not appear. For terminal work on a computer without a supported browser, use the Arduino IDE's Serial Monitor, PuTTY on Windows, or screen /dev/ttyUSB0 115200 on Mac and Linux.

Serial monitor questions