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Configuring the fieldbus
6.2 Communication via RS485
Frequency inverters with Control Units CU230P-2 HVAC, CU230P-2 DP, CU230P-2 CAN
Operating Instructions, 01/2011, FW 4.4, A5E02430659B AD
135
6.2.3.3
Modbus RTU telegram
Description
For Modbus, there is precisely one master and up to 247 slaves. Communication is always
triggered by the master. The slaves can only transfer data at the request of the master.
Slave-to-slave communication is not possible. The Control Unit always operates as slave.
The following figure shows the structure of a Modbus RTU telegram.
Char
ac
ter
dela
y
t
im
e
Char
ac
ter
dela
y
t
im
e
Char
ac
ter
dela
y
t
im
e
Char
ac
ter
dela
y
t
im
e
Char
ac
ter
dela
y
t
im
e
Char
ac
ter
dela
y
t
im
e
≥ 3.5 bytes
≥ 3.5 bytes
Initial pause
Interframe
delay
Function
code
Data
Initial
pause
Interframe
delay
Interframe
delay
End pause
Slave
1 Byte
1 Byte
1 Byte
1 Byte
1 Byte
1 Byte
1 Byte
CRC low
CRC high
1 Byte
0 ... 252 Bytes
2 Byte
CRC
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Figure 6-12 Modbus with delay times
The data area of the telegram is structured according to the mapping tables.
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