22. Serial Protocols
ROX™ v2.2 User Guide
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RuggedBackbone™ RX1500
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.
22.1.4. 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.
22.1.4.1. 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.
22.1.4.2. 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.
22.1.5. TcpModbus Concepts And Issues
22.1.5.1. Host And Remote Roles
Client gateways (such as that implemented by the RX1500) always make the TCP connection to the
Server Gateway. The Server Gateway can only accept a connection.
22.1.5.2. Port Numbers
The TCP port number dedicated to Modbus use is port 502. The Server Gateway can also be configured
to accept a connection on a configurable port number. This auxiliary port can be used by masters that
do not support port 502.
The Server Gateway is capable of creating only one connection on the specified auxiliary
port, whereas when Modbus is configured to use the default port, 502, it may connect to
multiple RTUs.
22.1.5.3. Retransmissions
The Server Gateway offers the ability to resend a request to an RTU should the RTU receive the request
in error or the Server Gateway receives the RTU response in error.
