Modbus cyclic writes

[Smeagol]

Hi,

I have a modbus slave on which i want to write a register periodically. Can this be done via the Modbus UI? I have the following profile prepared:

Code:

{
    "manufacturer": "IDM",
    "description": "Heat Pump Injection",
    "mapping": [        
        {
            "name": "Current Injection",
            "bus_datatype": "float32",
            "type": "register",
            "address": 74,            
            "writable": 1,
            "write_only": 1,
            "datatype": "float32",
            "units": "Kw",
            "read_swap": true,
    "value_multiplier": "0.001"
        }
    ]
}

But i have no clue when the value will be written? Is it only on mapped address change? Of can it be set to update no matter what every 30 seconds? If so, how? Or should this be done differently? 

Thanks for the help!