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Chapter 22 - Configuring Serial Protocols
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.
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.
The decision to use retransmissions, and the number to use depends upon factors such
as:
• The probability of a line failure
• The number of RTUs and amount of traffic on the port
• The cost of retransmitting the request from the server vs. timing-out and
retransmitting at the master. This cost is affected by the speed of the ports
and of the network.
ModBus Exception Handling
If the Server Gateway receives a request for an unconfigured RTU, it will respond to
the originator with a special message called an exception (type 10). A type 11
exception is returned by the server if the RTU fails to respond to requests.
Native TcpModbus polling packages will want to receive these messages. Immediate
indication of a failure can accelerate recovery sequences and reduce the need for long
timeouts.
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