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
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 Num-
ber
Description
Signal Direction
1-16
Frequency converter control word (see table below)
Master to slave
17-32
Frequency converter speed or set-point reference Range
0x0 – 0xFFFF (-200% ... ~200%)
Master to slave
33-48
Frequency converter status word (see table below)
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 con-
verter.
66-65536
Reserved
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 freq.
No freeze freq.
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
Set up MSB
16
No reversing
Reversing
Frequency converter control word
(FC profile)
Coil 0
1
33
Control not
ready
Control ready
34
Frequency con-
verter not ready
Frequency con-
verter ready
35
Coasting stop
Safety closed
36
No alarm
Alarm
37
Not used
Not used
38
Not used
Not used
39
Not used
Not used
40
No warning
Warning
41
Not at reference At reference
42
Hand mode
Auto mode
43
Out of freq.
range
In frequency
range
44
Stopped
Running
45
Not used
Not used
46
No voltage warn-
ing
Voltage warning
47
Not in current
limit
Current limit
48
No thermal
warning
Thermal warning
Frequency converter status word
(FC profile)
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®
HVAC Drive Design Guide
7. RS-485 Installation and Set-up
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