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MG.10.P2.22
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.
Function Field
The function field of a message frame contains
8 bits. Valid codes are in the range of 1 – 255
decimal. (See Appendix A for a description of
supported Modbus functions.) 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 a 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 exception. See
the
Exception Codes
section in this manual
for definitions.
Data Field
The data field is constructed using sets of two
hexadecimal digits, in the range of 00 to FF
hexadecimal. These are made from 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 like discrete and
register addresses, the quantity of items to
be handled, and the count of actual data bytes
in the field. The data field can have a length of
zero.
CRC Check Field
Messages include an error-checking field that
is based on a cyclical redundancy check
(CRC) method. The CRC field 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 field 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
field. If the two values are not equal, an error
results.
The error checking field contains a 16-bit
binary value implemented as two 8-bit bytes.
When this is done, the low-order byte of the
field is appended first, followed by the high-
order byte. The CRC high-order byte is the
last byte sent in the message.
Coil/Register Addressing
All data addresses in Modbus messages are
referenced to zero. The first occurrence of a
data item is addressed as item number zero.
For example:
The coil known as ‘coil 1’ in a programmable
controller is addressed as coil 0000 in the data
address field of a Modbus message. Coil 127
decimal is addressed as coil 007E
HEX
(126
decimal).
Holding register 40001 is addressed as
register 0000 in the data address field of the
message. The function code field already
specifies a ‘holding register’ operation.
Therefore, the ‘4XXXX’ reference is implicit.
Holding register 40108 is addressed as
register 006B
HEX
(107 decimal).
Modbus
Message
Structure
(continued)
