Write coil

The Write coil action writes an on or off state to one or more Modbus coils on a connected device. Use it to switch outputs, relays, or other binary controls directly on your hardware.

You can write a single state or a list of states. A single state uses Modbus function code 0x05 (write single coil), while a list uses function code 0x0F (write multiple coils).

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 coil.
6. Fill in the address, the state 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_coil. A basic example looks like this:

action: modbus.write_coil
data:
  address: 17
  state: true

This switches the coil at address 17 on, using the default hub.

Options in YAML

Try it yourself

Ready to test this? Open Developer tools > Actions, search for this action, fill in the fields, and select Perform action. You see what happens on your actual entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more] without writing a line of YAML.

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 register: Writes a value to one or more Modbus holding registers.

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

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