# Changing the function code of a data item of a Modbus RTU server

The selected function code determines:

- whether the data item is read or written to,
- the size of the data item,
- whether the value of the data item can be set by an application program or is provided externally (e. g. by an I/O system).

1. You must add a data item to the Modbus RTU server.

1. Open the device view.
2. In the device tree, double-click the server whose data item you want to edit.
3. Select the data item you want to change.
4. Double-click the list box Function code.
5. In the drop-down list, select the new function code.
6. Press Enter.

Supported Modbus RTU function codes