7.8.6 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.
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 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.
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).
Coil Number Description Signal Direction
1-16 Frequency converter control word (see table below) Master to slave
17-32 Frequency converter speed or set-point reference Range 0x0 – 0xFFFF (-200% ... ~200%) Master to slave
33-48 Frequency converter status word (see table below) Slave to master
49-64 Open loop mode: Frequency converter output frequency Closed loop mode: Frequency
converter feedback signal
Slave to master
65 Parameter write control (master to slave) Master to slave
0 = Parameter changes are written to the RAM of the frequency con-
verter
1 = Parameter changes are written to the RAM and EEPROM of the
frequency converter.
66-65536 Reserved
7 RS-485 Installation and Set-up VLT
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AQUA Drive Design Guide
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