2.2 Modbus RTU - Features
BU 0050 GB-3111 Subject to technical amendments 61
2 Modbus RTU
(SK 540E and above)
2.1 The bus system
Modbus is an open communication protocol, which is based on a Master/Slave architecture. The bus system
must be set up in a linear configuration, whose ends are terminated with terminating resistors. In principle, up
to 256 participants are possible within a bus system. These communicate with each other via RS485.
2.2 Features
As standard, frequency inverters of version SK 540E and above provide the Modbus in version Modbus RTU
<8, E, 1>. Installation is by means of a two-core cable with an additional GND connection.
Electrically isolated bus interface
Modbus RTU <8, E, 1>
up to 32 participants can be connected to a segment (communication via RS485)
Point to point communication (between 2 participants) is possible via RS232
Connection via terminal bar X7:73/74 (SK 520E and above) or
connection via RJ12 socket (X11)
Caution: with more than 2 participants, communication via RS232 is not possible, and for their protection,
the RS232 contacts TXD and RXD must not be connected.
Termination resistors can be connected via DIP switch on the FI (DIP 1).
Address range 0,1,3 … 30
Address 2 must not be used!
Address 0 is reserved for the master (Broadcast Mode)
The baud rate is adjustable (4800Baud … 38400Baud)
With communication between only 2 participants, an RS232 connection can be set up. In this case, care must
be taken that the communication speed may be lower, especially with long cable lengths.
For a network with more than 2 participants, the RS485 interface must always be used.
Switchover between the Modbus and the USS protocol is automatic. The condition for this is that address 2 is
not set in parameter (P512).
The frequency inverter can process 2 versions of the Modbus protocol.
1. Communication via Bus IOs: If the frequency inverter is to be accessed via Bus IO Bits, the functions
must be assigned in parameters (P480) and (P481). The source for the control word and the setpoints
(P509/P510) must be set to "Control terminals". (For details: see "Coil list")
2. Process data communication: If process data is to be exchanged or parameters changed, the source for
the control word and the setpoints (P509/P510) must be set to "USS". The definition of the parameters is
made in the parameters (P543) to (P548). (For details: see "Process data")
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