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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
exception. Please also refer to the sections

8.8.10 Function

Codes Supported by Modbus RTU

and

8.8.11 Modbus

Exception Codes

8.8.6 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.

8.8.7 CRC Check Field

Messages include an error-checking field, operating on the
basis of 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 unequal, a bus time-
out 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.

8.8.8 Coil Register Addressing

In Modbus, all data are organized in coils and holding
registers. Coils hold a single bit, whereas holding registers
hold a 2-byte word (i.e. 16 bits). 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 007EHEX (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 006BHEX (107 decimal).

Coil
Number

Description

Signal Direction

1-16

Frequency converter control word
(see

Table 8.2

)

Master to slave

17-32

Frequency converter speed or set-
point reference Range 0x0 – 0xFFFF
(-200% ... ~200%)

Master to slave

33-48