Eaton MMX34 Series - Serial Interface (Modbus RTU); General Information about Modbus; Communications in a Modbus Network

04/10 MN04020001Z-EN
135
7 Serial interface (Modbus RTU)
General information about Modbus
Modbus is a centrally polled bus system in which a so-called
master (PLC) controls the entire data transfer on the bus.
Cross-traffic between the individual slaves is not possible.
Each data exchange is initiated only on request of the master. Only
one request can be issued on the cable. A slave cannot initiate a
transfer but only react to a request with a response.
Two types of dialog are possible between master and slave:
• The master sends a request to a slave and waits for a response.
• The master sends a request to all slaves and does not wait for a
response (broadcast).
Communications in a Modbus network
The figures shows a typical arrangement with a host computer
(master) and any number (maximum 31 slaves) of frequency
inverter M-Max
TM
(slaves). Each frequency inverter has a unique
address in the network. This addressing is executed individually for
each M-Max
TM
via system parameter S2.3 and is independent of
the physical connection (position) in the network.
Serial interface A-B
The electrical connection between master and the slaves
connected in parallel is implemented via the serial interface A-B
(A = negative, B = positive) with a shielded RS485 twisted pair
cable.
The position of the connection terminals in the M-Max
TM
for the
serial interface A-B (see
figure 52, page 49).
The M-Max
TM
’s built-in RS -485 port supports the Modbus RTU
protocol and therefore allows a direct network connection without
an additional interface module.
The network cable must be provided at each physical end (last
station) with a bus termination resistor (120 O) in order to prevent
signal reflections and the resulting transfer errors. This necessary
resistor is already integrated in the M-Max
TM
frequency inverter
and is switched on via microswitch S4 (see
figure 52, page 49).
h
More information on Modbus can be found under
www.modbus.org.
Figure 106: Modbus network with M-Max
TM
Host computer
A
B
A
B
A
B
S4
120 O
12 n
S4
120 O
S4
120 O
RS485
- term
RS485
- term
RS485
- term