5.8.6 Data Field
The data field is constructed using sets of 2 hexadecimal
digits, in the range of 00–FF hexadecimal. These digits are
made up of 1 RTU character. The data field of telegrams
sent from a master to a slave device contains additional
information which the slave must use to perform
accordingly.
The information can include items such as:
•
Coil or register addresses.
•
The quantity of items to be handled.
•
The count of actual data bytes in the field.
5.8.7 CRC Check Field
Telegrams include an error-checking field, operating based
on a cyclic redundancy check (CRC) method. The CRC field
checks the contents of the entire telegram. It is applied
regardless of any parity check method used for the
individual characters of the telegram. The transmitting
device calculates the CRC value and appends the CRC as
the last field in the telegram. The receiving device
recalculates a CRC during receipt of the telegram and
compares the calculated value to the actual value received
in the CRC field. 2 unequal values result in bus timeout.
The error-checking field contains a 16-bit binary value
implemented as 2 8-bit bytes. After the implementation,
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 telegram.
5.8.8 Coil Register Addressing
In Modbus, all data is 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 telegrams are referenced to 0. The first occurrence
of a data item is addressed as item number 0. 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 telegram. 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 telegram. 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
).
Master to slave
17–32
Frequency converter speed or
setpoint reference range 0x0–
0xFFFF (-200% ... ~200%).
Master to slave
33–48
Frequency converter status word
(see
).
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
converter.
1 = Parameter changes are written
to the RAM and EEPROM of the
frequency converter.
66–65536 Reserved.
–
Table 5.15 Coil Register
Coil
0
1
01
Preset reference lsb
02
Preset reference msb
03
DC brake
No DC brake
04
Coast stop
No coast stop
05
Quick stop
No quick stop
06
Freeze frequency
No freeze frequency
07
Ramp stop
Start
08
No reset
Reset
09
No jog
Jog
10
Ramp 1
Ramp 2
11
Data not valid
Data valid
12
Relay 1 off
Relay 1 on
13
Relay 2 off
Relay 2 on
14
Set up lsb
15
–
16
No reversing
Reversing
Table 5.16 Frequency Converter Control Word (FC Profile)
RS485 Installation and Set-...
VLT
®
Midi Drive FC 280
62
Danfoss A/S © 03/2016 All rights reserved.
MG07B102
5
5