Modbus Online: a free web Modbus tester
Looking for a Modbus online tool? WebModbus is a web Modbus master that reads and writes registers and coils from the browser, with no desktop program to install. It handles Modbus RTU, ASCII, TCP, and UDP, and its request windows follow the familiar Modbus Poll workflow.
What you can do
- Read coils (01), discrete inputs (02), holding registers (03), and input registers (04), once or by polling.
- Write single and multiple coils and registers (05, 06, 15, 16), with a confirmation before anything is sent.
- Send diagnostics and other function codes (08, 11, 17, 22, 23, 43/14) or type a raw frame in the Test Center; the CRC or LRC is added for you.
- Scan addresses or Slave IDs to find out what a device answers.
- View values as signed, unsigned, hex, binary, 32/64-bit integers, or floats, with any byte or word order.
- Watch values on a real-time chart, check the communication log, and export the table to CSV.
How to connect
Modbus RTU / ASCII (serial)
Plug in a USB-RS485 or RS232 adapter, open WebModbus in desktop Chrome or Edge, choose a serial port, set the baud rate, parity, and stop bits, and connect. Web Serial needs an HTTPS page, which this site is.
Modbus TCP / UDP
Browsers cannot open raw TCP or UDP sockets, so start the small bridge on your own computer and point WebModbus at it. The bridge listens on loopback only and requires an access token. It is not a public proxy, so a static web page can never reach devices on your private network on its own.
Quick start
- Open webmodbus.com and connect to your port.
- Click "New request" and press F8: set the Slave ID, function code, start address, and quantity.
- Read once or start polling, then change the format from the Display menu.
- Need help? Read the guide.
FAQ
Is it really free, and do I need an account?
It is free and works without an account. Signing in with Google only syncs settings across your devices.
Do my values leave my computer?
No. Communication happens in your browser or the local bridge. If you turn on live sharing, values go only to guests you approve. See the privacy policy.
Which browsers work?
Serial connections need Chrome or Edge. Other browsers can use TCP and UDP through the local bridge.
Is it affiliated with Modbus Poll or the Modbus Organization?
No. WebModbus is an independent project. "Modbus" is a registered trademark of Schneider Electric USA, Inc.