Siemens SINAMICS G120 CU240B-2 - Modbus RTU Telegram

Connection to a fieldbus
6.4 Communication via RS485
Inverter with CU240B-2 and CU240E-2 Control Units
146 Operating Instructions, 07/2010, FW 4.3.2, A5E02299792B AA
Possible causes of a timeout
Alarm
No.
Parameter
name
Note
A1910 Setpoint timeout
The alarm is generated when p2040 ≠ 0 ms and one of the following causes is present:
 The bus connection is interrupted
 The MODBUS master is switched off
 Communication error (CRC, parity bit, logical error)
 An excessively low value for the fieldbus monitoring time (p2040)
6.4.3.2 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.
Character delay time
Character delay time
Character delay time
Character delay time
Character delay time
Character delay time
≥ 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 6-19 Modbus with delay times
The data area of the telegram is structured according to the mapping tables.