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RuggedRouter® User Guide
Use of Turnaround Delays
Some RTU protocols (such as ModBus) use the concept of a turnaround delay. When
the host sends a message (such as a broadcast) that does not invoke an RTU response,
it waits a turnaround delay time. This delay ensures that the RTU has time to process
the broadcast message before it has to receive the next poll.
When polling is performed, network delays may cause the broadcast and next poll to
arrive at the remote server at the same time. Configuring a turnaround delay will
enforce a minimum separation time between each message sent out the port. Note
that turnaround delays do not need to be configured at the host computer side and may
be disabled there.
TcpModBus Server Application
The TcpModbus 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
over a serial line.
A TcpModbus Client application, such as that implemented by the RuggedServer
accepts Modbus polls on a serial line from a master and determines the IP address of
the corresponding RTU. The client then encapsulates the message in TCP and
forwards the frame to a Server Gateway or native TcpModbus RTU. Returning
responses are stripped of their TCP headers and issued to the master.
The TcpModbus Server application accepts TCP encapsulated modbus messages from
Client Gateways and native masters. After removing the TCP headers the messages
are issued to the RTU. Responses are TCP encapsulated and returned to the
originator.
A “native” TcpModbus master is one that can encapsulate the Modbus polls in TCP
and directly issue them to the network.
Local Routing At The Server Gateway
The Server Gateway supports up to 32 RTUs on any of its four ports. When a request
for a specific RTU arrives the server will route it to the correct port.
MultiMaster Capability
It is possible for multiple masters to simultaneously issue requests for the same RTU.
The Server Gateway will queue the requests and deliver them to the RTU in turn.
This “multimaster” capability allows widely distributed masters to configure and
extract information from the RTU.
TcpModbus Concepts And Issues
Host And Remote Roles
Client gateways (such as that implemented by the RuggedRouter) always make the
TCP connection to the Server Gateway. The Server Gateway can only accept a
connection.
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