22. Serial Protocols
ROX™ v2.2 User Guide
222
RuggedBackbone™ RX1500
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.
22.1.5.4. 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.
22.1.5.5. TcpModbus Performance Determinants
The following description provides some insight into the possible sources of delay and error in an end-
to-end TcpModbus exchange.
Figure 22.2. Sources of Delay and Error in an End to End Exchange
