Moeller DF51 - The network protocol

10/05 AWB8230-1541GB
147
6 Serial interface (Modbus)
This section describes the mounting and function of the serial
interface.
General information about Modbus
Modbus is a centrally polled bus system in which the master (PLC)
controls the entire data flow on the bus. Internode communication
between the individual stations (slaves) is not possible.
Every data transfer is initiated by a request from the master. Only
one signal at a time can be transferred along the bus line. Slaves
cannot initiate a transmission; they can only respond to a request.
Two types of dialog are possible between master and slave:
• The master sends a message to a slave and waits for a response.
• The master sends a message to all slaves and does not wait for
a response (broadcast).
The RS 485 port
The DF51’s built-in RS 485 port supports the Modbus RTU protocol
and therefore allows a direct network connection without an
additional interface module.
Communications in a Modbus network
Figure 153 shows a typical arrangement with a host computer
(master) and any number of DF51 frequency inverters (up to 31
stations). Each frequency inverter has a unique address in the
network. The addresses are defined through PNU C072 and
independent of the DF51's physical position within the network.
Caution!
The master cyclically polls slaves’ fault messages. It is
therefore advisable to send device-specific and safety-
relevant fault messages directly through the control signal
terminals (for example fault indication relays DF51).
Example:
A short-circuit in the motor conductor at the output of the
DF51 switches on the mechanical brake directly.
Figure 152:RS 485 interface (RJ 45 socket)
A
RUN
PRG
Hz
PRG
ENTER
I
O
POWER
ALARM
Figure 153:Modbus network with DF51
Host computer
A
RUN
PRG
Hz
PRG
ENTER
IO
POWER
ALARM
A
RUN
PRG
Hz
PRG
ENTER
IO
POWER
ALARM
A
RUN
PRG
Hz
PRG
ENTER
IO
POWER
ALARM
DF51
DF51
DF51
1
2
32