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9
Connections and
communications
Interface RS-485
Data transfer method RS-485 MS/TP, half-duplex
Transfer cable STP (Shielded Twisted Pair), type Belden 9841 or similar
Connector 2.5 mm
2
Electrical isolation Functional
Modbus RTU As described in “Modicon Modbus Protocol Reference Guide”
Bitrate
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, 115200
and 230400 bits/s
Addresses 1 to 247
Connections and
communications
Interface 100BaseTX, IEEE 802.3 compatible
Data transfer method Ethernet half/full -duplex
Data transfer speed 10/100 MBit/s, autosensing
Protocol Modbus TCP
Connector Shielded RJ45 connector
Cable type CAT5e STP
Modbus TCP As described in Modbus Messaging Implementation Guide
Default IP Selectable: Fixed or DHCP (AutoIP) Another difference between TCP and UDP is that UDP is connectionless. TCP connections are always opened with TCP
SYN messages and closed with TCP FIN or TCP RST. With UDP, the first packet is already a Modbus query. IP address
and port combination is treated as a connection. If port number changes, it is considered as a new connection or as a
second connection if both stay active.
When using UDP, it is not guaranteed that the sent frame reaches its destination. The PLC must keep track of the
Modbus requests by using the Modbus transaction id-field. It actually must do this also when using TCP. If the PLC does
not receive response in time from the AC drive in UDP connection, it needs to send the query again. When using TCP,
the TCP/IP stack will keep resending the request until it has been acknowledged by the receiver (see Figure 4). If the PLC
sends new queries during this time, some of those may not be sent to the network (by TCP/IP stack) until previous sent
package(s) has been acknowledged. This can cause small packet storms when the connection is resumed between the
PLC and the AC drive (see Figure 5).
11716_uk
Modbus TCP Communication
PLC
Open
Connection
Communicate
Communicate
Close
Connection
Drive
TCP, SYN
TCP, SYN, ACK
TCP, ACK
Modbus Query
Modbus Query
Modbus Response, TCP, ACK
TCP, ACK
TCP, ACK
TCP, ACK
TCP, ACK
TCP, FIN, ACK
Modbus UDP Communication
PLC Drive
Modbus Query
Modbus Response
Modbus Query
Figure 3. Modbus TCP and UDP communication comparison