core/homeassistant/components/modbus/services.yaml

write_coil:
  description: Write to a modbus coil.
  fields:
    address: {description: Address of the register to write to., example: 0}
    state: {description: State to write., example: false}
    unit: {description: Address of the modbus unit., example: 21}
write_register:
  description: Write to a modbus holding register.
  fields:
    address: {description: Address of the holding register to write to., example: 0}
    unit: {description: Address of the modbus unit., example: 21}
    value: {description: Value (single value or array) to write., example: "0 or [4,0]"}