Remote Terminal Unit; Modbus Rtu Message Framing Structure - Danfoss VLT 5000 Instruction Manual

Remote The controllers are setup to communicate on
the Modbus network using RTU (Remote
Terminal
Terminal Unit) mode, with each 8-bit byte in
Unit
A Modbus RTU message is placed by the
Modbus RTU
transmitting device into a frame with a known
Message
beginning and ending point. This allows
Framing
receiving devices to begin at the start of the
Structure
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'
Start/Stop Field
Modbus RTU
Messages start with a silent interval of at least
Message
3.5 character times. This is implemented as
Framing
a multiple of character times at the 9600
Structure
network baud rate (shown as Start T1-T2-
(continued)
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
14
Coding System:
Bits Per Byte:
Error Check Field:
Start Address
T1-T2-T3-T4 8 Bits
Typical Modbus RTU Message Structure
a message contains two 4-bit hexadecimal
characters. The format for each byte is shown
below.
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)
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 converted to broadcast messages using
the FC protocol. 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.
Function Data
8 Bits n x 8 Bits
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 set an error, since the value in the
final CRC field is not valid for the combined
messages.
MG.10.P2.22 - VLT is a registered Danfoss trademark
CRC Check End
16 Bits T1-T2-T3-T4