Danfoss VLT AQUA Drive Design Manual page 138

7. RS-485 Installation and Set-up
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
7
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
ber
1-16
17-32
33-48
49-64
65
66-65536
138
Num- Description
Adjustable frequency drive control word (see table be-
low)
Adjustable frequency drive speed or setpoint reference
Range 0x0 – 0xFFFF (-200% ... ~200%)
Adjustable frequency drive status word (see table below) Slave to master
Open-loop mode: Adjustable frequency drive output fre-
quency closed-loop mode: Adjustable frequency drive
feedback signal
Parameter write control (master to slave)
0 = Parameter changes are written to the
RAM of the adjustable frequency drive.
1 = Parameter changes are written to the
RAM and EEPROM of the adjustable fre-
quency drive.
Reserved
MG.20.N2.22 - VLT
Signal Direction
Master to slave
Master to slave
Slave to master
Master to slave
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