Write register

The Write register action writes a value to one or more Modbus holding registers on a connected device. Use it to send setpoints, commands, or other numeric values directly to your hardware.

You can write a single value or a list of values. A single value uses Modbus function code 0x06 (write single register), while a list uses function code 0x10 (write multiple registers).

Using this action from the user interface

If you prefer building automations and scripts visually, Home Assistant walks you through this action step by step. You pick what to target, tweak a few options, and save. No YAML knowledge required.

To use this action in an automation or script:

1. Go to Settings > Automations & scenes.
2. Open an existing automation or script, or select Create to start a new one.
3. If you’re setting up a new automation, add a trigger in the When section. Scripts don’t need a trigger.
4. In the Then do section, select Add action.
5. From the search box, search for and select Modbus: Write register.
6. Fill in the address, the value to write, and optionally the device address and hub.
7. Select Save.

This action does not support targets. In the UI, you are not prompted to choose an area, device, entity, or label. Instead, you select the Modbus hub by name.

Options in the UI

Using this action in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, refer to this action as modbus.write_register. A basic example looks like this:

action: modbus.write_register
data:
  address: 138
  value: 42

This writes the value 42 to register 138 on the default hub.

Options in YAML

Good to know

• Whether you need to reverse the byte or word order depends on the device and the byte order of your CPU.

More examples

Real scenarios where this action shows up in automations and scripts. Copy any example and adapt it to your setup.

Write a float32 value

To write a float32 value, use the network format. For example, 10.0 is 0x41200000 in network-order hexadecimal, which you write as two 16-bit registers.

action: modbus.write_register
data:
  address: 138
  slave: 1
  hub: modbus_hub
  value:
    - 0x4120
    - 0x0000

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with the action you’re calling and what you expected to happen, or share on our subreddit /r/homeassistant.

Related actions

These actions work well alongside this one:

• Write coil: Writes a state to one or more Modbus coils.

• Stop hub: Stops a Modbus hub and closes its connection.

• Reload Modbus configuration: Reloads the Modbus configuration from your YAML configuration.