Modbus Rtu Message Framing Structure - Danfoss VLT Refrigeration Drive FC 103 Design Manual

System Integration
3.9.9.4 Network Configuration
To enable Modbus RTU on the frequency converter, set the
following parameters:
Parameter
Parameter 8-30 Protocol
Parameter 8-31 Address
Parameter 8-32 Baud Rate
Parameter 8-33 Parity / Stop Bits
Table 3.31 Modbus RTU Parameters
3.9.10 Modbus RTU Message Framing
Structure
3.9.10.1 Frequency Converter with Modbus
RTU
The controllers are set up to communicate on the Modbus
network using RTU mode, with each byte in a message
containing 2 4-bit hexadecimal characters. The format for
each byte is shown in Table 3.32.
Start Data byte
bit
Table 3.32 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 Cyclic redundancy check (CRC).
3.9.10.2 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 recognise when the message is
completed. Partial messages are detected and errors set as
a result. Characters for transmission must be in
hexadecimal 00 to FF format in each field. The frequency
converter continuously monitors the network bus, also
during silent intervals. When the first field (the address
field) is received, each frequency converter 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 3.33.
MG16G202
Design Guide
Setting
Modbus RTU
1–247
2400–115200
Even parity, 1 stop bit (default)
Stop/ Stop
parity
Danfoss A/S © 08/2015 All rights reserved.
Start Address Function
T1-T2-T3- 8 bits 8 bits
T4
Table 3.33 Typical Modbus RTU Message Structure
3.9.10.3 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 slave), since
the value in the final CRC field is not valid for the
combined messages.
3.9.10.4 Address Field
The address field of a message frame contains 8 bits. Valid
slave device addresses are in the range of 0–247 decimal.
The individual slave devices are assigned addresses in the
range of 1–247. (0 is reserved for broadcast mode, which
all slaves recognise.) A master addresses a slave by placing
the slave address in the address field of the message.
When the slave sends its response, it places its own
address in this address field to let the master know which
slave is responding.
3.9.10.5 Function Field
The function field of a message frame contains 8 bits. Valid
codes are in the range of 1–FF. Function fields are used to
send messages between master and slave. When a
message is sent from a master to a slave device, the
function code field tells the slave what action to perform.
When the slave responds to the master, it uses the
function code field to indicate either a normal (error-free)
response, or that an error occurred (called an exception
response). For a normal response, the slave simply echoes
the original function code. For an exception response, the
slave returns a code that is equivalent to the original
function code with its most significant bit set to logic 1. In
addition, the slave places a unique code into the data field
of the response message. This tells the master what error
occurred, or the reason for the exception. Also refer to
Data CRC End
check
N x 8 bits 16 bits T1-T2-T3-
T4
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