RuggedCom RuggedRouter RX1100 - Manual (Page 219)

24. Configuring Serial Protocols
Revision 1.14.3 219 RX1000/RX1100™
24.1.4.2. 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.
24.1.5. TcpModbus Concepts And Issues
24.1.5.1. 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.
24.1.5.2. 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.
Note
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.
24.1.5.3. 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.
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.
24.1.5.4. 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.
24.1.5.5. TcpModbus Performance Determinants
The following description provides some insight into the possible sources of delay and error in an
end-to-end TcpModbus exchange.