04/10 MN04020001Z-EN
Modbus parameters
137
For Modbus to function at least the following parameters must be
set:
Another requirement is that the arrow
C in the lower status line
of the LCD display is pointing to BUS (adjustable via the LOC/REM
button). The PLC (master) must also be provided with an RS485
serial interface and the necessary Modbus RTU driver software.
Operating mode Modbus RTU
Operating mode Modbus RTU (Remote Terminal Unit) transfers
the data in binary format (faster data rate) and determines the
transfer format for the data request and the data response. Each
message byte that is sent contains two hexadecimal characters
(0 … 9, A … F).
The data transfer between a master (PLC) and the frequency
inverter (M-Max
TM
) is carried out according to the following
sequence:
• Master request: the master sends a Modbus frame to the
frequency inverter.
• Slave response: the frequency inverter sends a Modbus frame as
a response to the master.
PNU Value Notes
S2.2 1 To activate Modbus.
S2.3 1 - 255 Set differently at each slave (MMX); 0 is used by the master for broadcasts.
S2.4 0-8 Same setting at the master and slave.
S2.6 0/1 Same setting at the master and slave.
6.1 3 Fieldbus selected as a control level.
6.2
2 Set setpoint value via the fieldbus; other setpoint sources also possible, fixed frequencies overlay all setpoint values,
also a fieldbus setpoint value.
Figure 107: Data exchange between master and slave
a Master request
b Slave response, not with broadcast
Start
MMX... (Slave)Master
Address (1 Byte)
Function code (1 Byte)
Data (N x 1 Byte)
CRC (2 Bytes)
End
Start
Address (1 Byte)
Function code (1 Byte)
Data (N x 1 Byte)
CRC (2 Bytes)
End
h
The frequency inverter (slave) only sends a response if it
has received a request from the master beforehand.
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