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 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 (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).
Coil Number Description Signal Direction
1-16
Adjustable frequency drive control word (see Table 7.15)
Master to follower
17-32 Adjustable frequency drive speed or setpoint reference Range 0x0–0xFFFF
(-200% ... -200%)
Master to follower
33-48
Adjustable frequency drive status word (see Table 7.15 and Table 7.16)
Follower to master
49-64 Open-loop mode: Adjustable frequency drive output frequency
Closed-loop mode: Adjustable frequency drive feedback signal
Follower to master
65 Parameter write control (master to follower) Master to follower
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 frequency drive.
66-65536 Reserved
Table 7.14 Coil Register
RS-485 Installation and Set... Design Guide
96 Danfoss A/S © Rev. 2014-01-14 All rights reserved. MG18C522
77
To Next Page
Loading...
Need help?
General