10.10 Network Configuration
10.10.1 Adjustable Frequency Drive with
Modbus RTU
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)
10.11 Modbus RTU Message Framing
Structure
10.11.1 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 2 4-bit hexadecimal
characters. The format for each byte is shown in
Start
bit
Data byte
Stop
Parity
Stop
Table 10.9 Example Format
Coding System
8-bit binary, hexadecimal 0–9, A-F. two
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)
Table 10.10 Bit Detail
10.11.2 Modbus RTU Message Structure
The transmitting device places a Modbus RTU message
into a frame with a known beginning and ending point.
Receiving devices are able 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 recognise 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
.
Start
Address Function
Data
CRC
check
End
T1-T2-T3-
T4
8 bits
8 bits
N x 8
bits
16 bits
T1-T2-T3-
T4
Table 10.11 Typical Modbus RTU Message Structure
10.11.3 Start/Stop Field
Messages start with a silent period of at least 3.5 character
intervals, implemented as a multiple of character intervals
at the selected network baud rate (shown as Start T1-T2-
T3-T4). The first transmitted field 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,
causing a timeout (no response from the follower), since
the value in the final CRC field is not valid for the
combined messages.
RS-485 Installation and Set...
Design Guide
MG34S222
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