Danfoss VLT AutomationDrive FC 301 Series Design Manual page 199

RS-485 Installation and Set...
12.10 Network Configuration
To enable Modbus RTU on the adjustable frequency drive,
set the following parameters
Parameter Setting
8-30 Protocol Modbus RTU
8-31 Address 1–247
8-32 Baud Rate 2400–115200
8-33 Parity / Stop Bits Even parity, 1 stop bit (default)
Table 12.10 Modbus RTU Parameters
12.11 Modbus RTU Message Framing
Structure
12.11.1 Adjustable Frequency Drive with
Modbus RTU
The controllers are set up to communicate on the Modbus
network using RTU (Remote Terminal Unit) mode, with
each byte in a message containing two 4-bit hexadecimal
characters. The format for each byte is shown in
Table 12.11.
Start Data byte
bit
Table 12.11 Format for Each Byte
Coding System 8-bit binary, hexadecimal 0-9, A-F. 2
hexadecimal characters contained in each
8-bit field of the message
Bits Per Byte 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
Error Check Field Cyclical Redundancy Check (CRC)
MG33BF22
Design Guide
12.11.2 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 in
Table 12.12.
Start
T1-T2-T3-
T4
Table 12.12 Typical Modbus RTU Message Structure
Stop/ Stop
parity
12.11.3 Start/Stop Field
Messages start with a silent period of at least 3.5 character
intervals. This is implemented as a multiple of character
intervals at the selected network baud rate (shown as Start
T1-T2-T3-T4). The first field to be transmitted is the device
address. Following the last transmitted character, a similar
period of at least 3.5 character intervals marks the end of
the message. A new message can begin after this period.
The entire message frame must be transmitted as a
continuous stream. If a silent period of more than 1.5
character intervals occurs before completion of the frame,
the receiving device flushes the incomplete message and
assumes that the next byte is the address field of a new
message. Similarly, if a new message begins before 3.5
character intervals after a previous message, the receiving
device considers it a continuation of the previous message.
This causes a timeout (no response from the follower),
since the value in the final CRC field is not valid for the
combined messages.
Rev. 2014-04-04 All rights reserved.
Address Function Data
8 bits 8 bits N x 8
bits
CRC End
check
16 bits T1-T2-T3-
T4
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