Modbus Tcp Applications - Siemens RUGGEDCOM ROX II User Manual

Chapter 3
Device Management
Host And Remote Roles
The raw socket protocol can either initiate or accept a TCP connection for serial encapsulation. It can establish a
connection initiated from a remote host, vice versa, or bidirectionally.
Configure the device at the host-end to establish a connection with the remote host when:
• The host-end uses a port redirector that must make the connection
• The host-end is only occasionally activated and will make the connection when it becomes active
• A host-end firewall requires the connection to be made outbound
If the host-end wants to open multiple connections with the remote-ends in order to implement broadcast polling,
configure the device to accept connections with the remote-ends.
Configure the device to connect from each side (host or remote) to the other if both sides support this
functionality.
Message Packetization
The serial server buffers receive characters into packets in order to improve network efficiency and demarcate
messages.
The serial server uses three methods to decide when to packetize and forward the buffered characters to the
network:
• packetize on a specific character
• packetize on timeout
• packetize on a full packet
If configured to packetize on a specific character, the serial server will examine each received character,
packetize and forward it upon receiving the specific character. The character is usually a <CR> or an <LF>
character but may be any ASCII character.
If configured to packetize on a timeout, the serial server will wait for a configurable time after receiving a
character before packetizing and forwarding it. If another character arrives during the waiting interval, the timer is
restarted. This method allows characters transmitted as part of an entire message to be forwarded to the network
in a single packet, when the timer expires after receiving the very last character of the message. This is usually
the only packetizer selected when supporting Modbus TCP communications.
Finally, the serial server will always packetize and forward on a full packet, specifically when the number of
characters fills its communications buffer (1024 bytes).
Section 3.20.1.2

Modbus TCP Applications

The Modbus TCP Server application is used to transport Modbus requests and responses across IP networks.
The source of the polls is a Modbus master, a host computer that issues the polls to a remote host (RTU)
connected to the serial port of the device running the Modbus TCP Server application. The Modbus polls
encapsulated in TCP packets received by the device will be forwarded to the remote host via the serial port
based on the host's address defined in the RTU list. The responses from remote host are TCP encapsulated and
returned to the master that originated the polls.
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RUGGEDCOM ROX II
CLI User Guide
Modbus TCP Applications