7
Modbus RTU
Message
Framing
Structure
The controllers are setup to communicate on
the Modbus network using RTU (Remote
Terminal Unit) mode, with each 8-bit byte in a
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)
A Modbus RTU message is placed by the
transmitting device 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 know when
the message is completed. Partial messages
are detected and errors set as a result.
The allowable characters transmitted for all
fields are hexadecimal 0-9, A-F. The adjustable
frequency drives monitor the network bus
continuously, including ‘silent’ intervals. When
the first field (the address field) is received, each
drive or device decodes it to determine
whether it is the addressed device.
Modbus RTU messages addressed to zero
are broadcast messages. No response is
needed on broadcast messages.
To ensure the attribute data returned is the
most current, each attribute access must
include one attribute only.
A typical message frame is shown below.
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
Remote
Terminal
Unit
Start/Stop Field
Messages start with a silent interval of at least
3.5 character times. This is implemented as a
multiple of character times at the selected
network baud rate (shown as Start T1-T2-
T3-T4). The first field then transmitted is the
device address. Following the last transmitted
character, a similar interval of at least 3.5
character times marks the end of the
message. A new message can begin after
this interval.
The entire message frame must be transmitted
as a continuous stream. If a silent interval of
more than 1.5 character times occurs before
completion of the frame, the receiving device
flushes the incomplete message and assumes
that the next byte will be the address field of a
new message.
Similarly, if a new message begins earlier that
3.5 character times following a previous
message, the receiving device will consider it
a continuation of the previous message. This
will cause a timeout (no response from the
slave), since the value in the final CRC field is
not valid for the combined messages.
Typical Modbus RTU Message Structure
Modbus RTU
Message
Framing
Structure
(continued)
message contains two 4-bit hexadecimal
characters. The format for each byte is shown
below.
Summary of Contents for Mobrus RTU TR1 Series
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