Controllers communicate using a master/follower technique in which only the master can initiate transactions (called queries). Followers
respond by supplying the requested data to the master, or by acting as requested in the query. The master can address individual
followers, or initiate a broadcast telegram to all followers. Followers return a response to queries that are addressed to them individually.
No responses are returned to broadcast queries from the master.
The Modbus RTU protocol establishes the format for the master query by providing the following information:
l The device (or broadcast) address.
l A function code defining the requested action.
l Any data to be sent.
l An error-checking field.
The response telegram of the follower device is also constructed using Modbus protocol. It contains fields confirming the action taken,
any data to be returned, and an error-checking field. If an error occurs in receipt of the telegram, or if the follower is unable to perform
the requested action, the follower constructs and sends an error message. Alternatively, a timeout occurs.
9.6.3 Drive with Modbus RTU
The drive communicates in Modbus RTU format over the built-in RS485 interface. Modbus RTU provides access to the control word and
bus reference of the drive.
The control word allows the Modbus master to control several important functions of the drive:
l Start.
l Various stops:
¢ Coast stop.
¢ Quick stop.
¢ DC brake stop.
¢ Normal (ramp) stop.
l Reset after a fault trip.
l Run at various preset speeds.
l Run in reverse.
l Change the active setup.
l Control built-in relay of the drive.
The bus reference is commonly used for speed control. It is also possible to access the parameters, read their values, and, where possible,
write values to them. Accessing the parameters offers a range of control options, including controlling the setpoint of the drive when its
internal PI controller is used.
9.7 Modbus RTU Network Configuration
To enable Modbus RTU on the drive, set the following parameters:
Table 39: Parameters to Enable the Modbus RTU
Parameter Setting
Parameter 8-30 Protocol Modbus RTU
Parameter 8-31 Address 1–247
Parameter 8-32 Baud Rate 2400–115200
Parameter 8-33 Parity/Stop Bits Even parity, 1 stop bit (default)
100 | Danfoss A/S © 2024.01 AJ435824192086en-000101 / 130R1295
Design Guide | VLT® AutomationDrive FC 360
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