Modbus Rtu Message Structure; Start/Stop Field; Address Field; Function Field - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

RS-485 Installation and Set...
Start
bit
Table 7.12 Format for Each Byte
Coding System
Bits Per Byte
Error Check Field
Table 7.13
7 7

7.8.2 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 7.14.
Start Address
T1-T2-T3- 8 bits
T4
Table 7.14 Typical Modbus RTU Message Structure

7.8.3 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
84
®
VLT
Data byte Stop/
parity
8-bit binary, hexadecimal 0-9, A-F. 2
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)
Function Data CRC
check
8 bits N x 8 16 bits
bits
MG18C402 - VLT
HVAC Basic Drive Design Guide
assumes that the next byte is the address field of a new
Stop
message. Similarly, if a new message begins prior to 3.5
character intervals after a previous message, the receiving
device will consider it a continuation of the previous
message. This causes a time-out (no response from the
slave), since the value in the final CRC field is not valid for
the combined messages.

7.8.4 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 recognize.) 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.

7.8.5 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 kind of 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 some kind of 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 kind of error occurred, or the reason for the
End
exception. Please also refer to 7.8.10 Function Codes
Supported by Modbus RTU and 7.8.11 Modbus Exception
T1-T2-T3-
Codes
T4

7.8.6 Data Field

The data field is constructed using sets of two hexadecimal
digits, in the range of 00 to FF hexadecimal. These are
made up of one RTU character. The data field of messages
sent from a master to slave device contains additional
information which the slave must use to take the action
defined by the function code. This can include items such
as coil or register addresses, the quantity of items to be
handled, and the count of actual data bytes in the field.
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