Vlt Aqua With Modbus Rtu; Modbus Rtu Message Framing Structure - Danfoss VLT AQUA Drive Design Manual

7. RS-485 Installation and Set-up
7.7. Network Configuration

7.7.1. VLT AQUA with Modbus RTU

To enable modbus RTU on the VLT AQUA, set the following parameters:
Parameter
ber
8-30
8-31
8-32
8-33

7.8. Modbus RTU Message Framing Structure

7.8.1. VLT AQUA with Modbus RTU
7
The controllers are set up to communicate on the Modbus network in RTU (Remote Terminal Unit)
mode, with each 8-bit byte in a message containing two 4-bit hexadecimal characters. The format
for each byte is shown below.
Start bit
Coding System
Bits Per Byte
Error Check Field
7.8.2. Modbus RTU Message Structure
The transmitting device places a Modbus RTU message into a frame with a known beginning and
ending point. This allows receiving devices to begin at the start of the message, read the address
portion, determine which device is addressed (or all devices, if the message is broadcast), and to
recognize when the message is completed. Partial messages are detected, and errors are set as
a result. Characters for transmission must be in hexadecimal 00 to FF format in each field. The
adjustable frequency drive continuously monitors the network bus, also during 'silent' intervals.
When the first field (the address field) is received, each adjustable frequency drive or device
decodes it to determine which device is being addressed. Modbus RTU messages addressed to
zero are broadcast messages. No response is permitted for broadcast messages. A typical message
frame is shown below.
136
Num- Parameter name
Protocol
Address
Baud Rate
Parity/Stop bits
Data bit
8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters
contained in each 8-bit field of the message
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Cyclical Redundancy Check (CRC)
MG.20.N2.22 - VLT ®
VLT
Setting
Modbus RTU
1 - 247
2400 - 115200
Even parity, 1 stop bit (default)
is a registered Danfoss trademark.
® AQUA Drive Design Guide
Stop Stop
parity