3.8.10.3 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 rst eld 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 ushes the incomplete message and
assumes that the next byte is the address eld 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 nal CRC eld is not valid for the
combined messages.
3.8.10.4
Address Field
The address eld 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 eld of the message.
When the slave sends its response, it places its own
address in this address eld to let the master know which
slave is responding.
3.8.10.5
Function Field
The function eld of a message frame contains 8 bits. Valid
codes are in the range of 1–FF. Function elds are used to
send messages between master and slave. When a
message is sent from a master to a slave device, the
function code eld tells the slave what kind of action to
perform. When the slave responds to the master, it uses
the function code eld 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 signicant bit set to
logic 1. In addition, the slave places a unique code into the
data eld of the response message. This tells the master
what kind of error occurred, or the reason for the
exception. Also refer to chapter 3.8.10.10 Function Codes
Supported by Modbus RTU and chapter 3.8.10.11 Modbus
Exception Codes.
3.8.10.6
Data Field
The data eld is constructed using sets of two hexadecimal
digits, in the range of 00–FF hexadecimal. These are made
up of one RTU character. The data eld of messages sent
from a master to slave device contains additional
information that the slave must use to take the action
dened 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 eld.
3.8.10.7 CRC Check Field
Messages include an error-checking eld, operating based
on a cyclical redundancy check (CRC) method. The CRC
eld checks the contents of the entire message. It is
applied regardless of any parity check method used for the
individual characters of the message. The CRC value is
calculated by the transmitting device, which appends the
CRC as the last eld in the message. The receiving device
recalculates a CRC during receipt of the message and
compares the calculated value to the actual value received
in the CRC
eld. If the two values are unequal, a bus
timeout results. The error-checking eld contains a 16-bit
binary value implemented as two 8-bit bytes. When this is
done, the low-order byte of the eld is appended rst,
followed by the high-order byte. The CRC high-order byte
is the last byte sent in the message.
3.8.10.8
Coil Register Addressing
In Modbus, all data is organized in coils and holding
registers. Coils hold a single bit, whereas holding registers
hold a 2-byte word (16 bits). All data addresses in Modbus
messages are referenced to zero. The rst occurrence of a
data item is addressed as item number 0. For example: The
coil known as coil 1 in a programmable controller is
addressed as the data address
eld of a Modbus message.
Coil 127 decimal is addressed as coil 007EHEX (126 decimal).
Holding register 40001 is addressed as register 0000 in the
data address
eld of the message. The function code eld
already species a holding-register operation. Therefore,
the 4XXXX reference is implicit. Holding register 40108 is
addressed as register 006BHEX (107 decimal).
System Integration
Design Guide
MG20N622 Danfoss A/S © 09/2014 All rights reserved. 89
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