Modbus Rtu Telegram - Siemens SINAMICS G120 CU240B-2 Operating Instructions Manual

Connection to a fieldbus
6.4 Communication via RS485
Possible causes of a timeout
Alarm
No.
A1910
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.
Initial
pause
Initial pause
Slave
≥ 3.5 bytes 1 Byte
≥ 3.5 bytes
Figure 6-19 Modbus with delay times
The data area of the telegram is structured according to the mapping tables.
146
Parameter Note
name
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)

Interframe
delay
Function
code
1 Byte
1 Byte 1 Byte
Interframe
delay
Data
0 ... 252 Bytes
1 Byte 1 Byte
Operating Instructions, 07/2010, FW 4.3.2, A5E02299792B AA
CRC
2 Byte
CRC low CRC high
1 Byte 1 Byte
Inverter with CU240B-2 and CU240E-2 Control Units
End pause
Interframe
delay