Data Field; Crc Check Field; Coil Register Addressing - Danfoss VLT AutomationDrive FC 300 Design Manual

RS-485 Installation and Set...

10.11.6 Data Field

The data field is constructed using sets of 2 hexadecimal digits, in the range of 00 to FF hexadecimal. These sequences are
made up of one RTU character. The data field of messages sent from a master to follower device contains more information,
which the follower must use to do what is defined by the function code. This information can include items such as coil or
register addresses, the quantity of items, and the count of actual data bytes in the field.

10.11.7 CRC Check Field

Messages include an error-checking field, operating 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 transmitting device calculates the CRC value then 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 2 values are unequal, a bus time-out results. The error-checking field contains a
16-bit binary value implemented as 2 8-bit bytes. After error-checking, 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.

10.11.8 Coil Register Addressing

In Modbus, all data are organised in coils and holding registers. Coils hold a single bit, whereas holding registers hold a 2
byte word (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
1–16 Frequency converter control word (see Table 10.14)
17–32 Frequency converter speed or set-point reference Range 0x0–0xFFFF (-200% ...
~200%)
33–48 Frequency converter status word (see Table 10.14)
49–64 Open loop mode: Frequency converter output frequency Closed loop mode:
Frequency converter feedback signal
65 Parameter write control (master to follower)
0 = Parameter changes are written to the RAM of the frequency converter
1 =Parameter changes are written to the RAM and EEPROM of the frequency
converter.
66-65536 Reserved
Table 10.12 Coils and Holding Registers
®
VLT AutomationDrive FC 300 Design Guide 90-1200 kW
MG34S202 - Rev. 2013-08-19
Signal direction
Master to follower
Master to follower
Follower to master
Follower to master
Master to follower
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