Operating instructions

VLT
®
2800, VLT
®
6000 HVAC and VLT
®
8000 AQUA Modbus RTU
Network
Configuration
■ 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,itplacesitsownaddressinthisaddressfield
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-FF. (See section
Modbus RTU Function Codes.) 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
oftheresponsemessage. Thistellsthemaster
what kind of error occurred, or the reason for the
exception. See the chapter Exception Codes in
these operating instructions 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 coil or register
addresses, the quantity of items to be handled, and
the count of actual data bytes in the field.
■ 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, 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
byteisthelastbytesentinthemessage.
■ 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).
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