Danfoss VLT Series Design Manual page 149

VLT ® HVAC Drive Design Guide
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 exception. Please
also refer to the sections
7.8.6. Data Field
The data field is constructed using sets of two hexadecimal digits, in the range of 00 to FF hexa-
decimal. These are made up of one RTU character. The data field of messages sent from a master
to slave device contains additional information that 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.
7.8.7. 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 calcu-
lated value to the actual value received in the CRC field. If the two values are unequal, a bus
timeout 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.
7.8.8. 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).
Function Codes Supported by Modbus RTU
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
7. RS-485 Installation and Set-up
and Exception Codes.
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