Crc Check Field - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

RS-485 Installation and Set...

7.8.7 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 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 (that is, 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).
®
VLT HVAC Basic Drive Design Guide
Coil
Number
1-16
17-32
33-48
49-64
65
66-65536
Table 7.15
Coil
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
Table 7.16 Frequency Converter Control Word (FC Profile)
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MG18C402 - VLT is a registered Danfoss trademark
Description
Frequency converter control word
(see Table 7.16)
Frequency converter speed or set-
point reference Range 0x0-0xFFFF
(-200% ... -200%)
Frequency converter status word
(see Table 7.16 and Table 7.17)
Open loop mode: Frequency
converter output frequency
Closed loop mode: Frequency
converter feedback signal
Parameter write control (master to
slave)
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.
Reserved
0 1
Preset reference LSB
Preset reference MSB
DC brake No DC brake
Coast stop No coast stop
Quick stop No quick stop
Freeze freq. No freeze freq.
Ramp stop Start
No reset Reset
No jog Jog
Ramp 1 Ramp 2
Data not valid Data valid
Relay 1 off Relay 1 on
Relay 2 off Relay 2 on
Set up LSB
No reversing Reversing
Signal
Direction
Master to slave
Master to slave
Slave to master
Slave to master
Master to slave
7 7
85