Modbus for home automation: test the device before the config

Why test first

A Home Assistant or openHAB config has several chances to go wrong: the Unit ID, the register address, the data type, the word order and the scale. If the sensor shows nonsense, you can’t tell which one it is. FieldKit lets you check each one without restarting anything.

How to check a device

1. Enable Modbus TCP on the device. Many devices ship with it off. Check its web page or manual.
2. Find it. Tap Find devices on the same Wi-Fi network.
3. Read one register you can verify, such as a temperature shown on the device. If the value looks shifted, compare 0-based and 1-based. Why this happens.
4. Work out the data type. Two registers that read as huge numbers point to a word order issue. The float converter shows all four orders.
5. Watch it for a while. A chart over an hour shows whether the value and scale make sense.

Then copy the working values into your config. Save them in a profile too, in case you need them next year.

Writes need extra care

A write to a heat pump or a wallbox changes real equipment. FieldKit stops each write on a confirmation sheet, but the sheet can’t tell you whether a value is safe for your device. Read the vendor’s documentation first, and start with reads.

Limits to know

• FieldKit doesn’t support solar data loggers that speak Solarman V5, even on port 8899.
• RS-485 devices need a Wi-Fi or Ethernet converter in transparent mode.
• Many devices allow few simultaneous Modbus clients. Stop your home automation’s polling while you test, or the device may not answer.
• FieldKit is a test tool and doesn’t run in the background. Keep your integration for the long-term polling.