@kaoshihchuan the following two links will help you understanding the new object model of RepRapFirmware version 3 (RRF3):
https://duet3d.dozuki.com/Wiki/Object_Model_of_RepRapFirmware
https://duet3d.github.io/DuetSoftwareFramework/api/DuetAPI.Machine.MachineModel.html#properties

In the first link is explained, how the Duet can be accessed and explaining of the json structure.
You should use the current RRF version (today: 3.1.1).