Modbus RTU Explorer

Chipkin Automation Systems. A quick diagnostic bench for Modbus RTU over a serial port, running entirely in your browser. Connect a USB-to-RS485 or RS232 adapter, send any of the eight standard requests, and read the answer back as every data type at once — so you can see at a glance whether a register really is a Float32, and in which byte order.

version · Modbus RTU only · nothing leaves this browser

1 Serial port

Choose your adapter, set the character format to match the device, then connect. An empty picker means the adapter is unplugged or its driver is not installed.

Modbus RTU devices most often use 9600 8N1 or 19200 8E1. If the device answers with a CRC error every time, the character format is the first thing to change.

2 Request
3 Response

Nothing sent yet.

4 Communication log
Nothing logged yet.
Notes and troubleshooting

This is a diagnostic bench, not the full product

It speaks Modbus RTU only — the serial form. For Modbus TCP, large point lists, saved configurations, logging and reporting, use the Chipkin Modbus Explorer desktop application. This page is for the ten minutes when you have a laptop, a serial adapter and a device that is not answering.

Addressing

The number in a device manual is often not the number that goes on the wire. A holding register listed as 40001 is almost always protocol address 0, and 40100 is 99. Set Address base to 1-based and type the documented number if the manual counts from one; the line under the form always shows the value actually being transmitted.

Common failures

  • Nothing at all comes back. Wrong unit ID, wrong baud rate, A and B swapped on RS-485, or the adapter is not switched to the right mode. Try the unit ID first — it is the most common.
  • CRC errors on every reply. Almost always the character format: try 8E1 if you are on 8N1, and check the baud rate.
  • Exception 2, Illegal Data Address. The register exists in the manual but not at that offset. Switch the address base, or read one register at a time to find where the block really starts.
  • The number looks like nonsense. It is probably a 32-bit value in a byte order you are not expecting. The 32-bit and 64-bit table shows all four orders side by side — the one that reads as a sensible engineering value is the right one.

Browser support

Web Serial works in Chrome and Edge on Windows, macOS, Linux and ChromeOS. Firefox and Safari do not implement it. The page must be served over https:// or from localhost.

Testing without a device

The simulator page pretends to be a Modbus RTU device on a second serial port. With a linked virtual pair such as com0com's COM3 and COM4, you can run both pages side by side with no hardware at all.