2. Serial Protocols
ROS® v3.11User Guide 50 RMC30
r Gateway to RTU
RTU "think" and transmission
time
Gateway to Master
Queuing time
Figure 2.6. Sources of Delay and Error in an End-to-End Exchange
In step 1, the master issues a request to the Client Gateway. If the Client Gateway validates
the message, it will forward it to the network as step 2.
The Client Gateway can respond immediately in certain circumstances, as shown in step 1a.
When the Client Gateway does not have a configuration for the specified RTU, it will respond
to the master with an exception using TCPModbus exception code 11 (“No Path”). When the
Client Gateway has a configured RTU but the connection is not yet active, it will respond to
the master with an exception using TCPModbus exception code 10 (“No Response”). If the
forwarding of TCPModbus exceptions is disabled, the client will not issue any responses.
Steps 3a and 3b represent the possibility that the Server Gateway does not have a
configuration for the specified RTU. The Server Gateway will always respond with a type 10
(“No Path”) in step 3a, which the client will forward in step 3b.
Step 4 represents the possibility of a queuing delay. The Server Gateway may have to queue
the request while it awaits the response to a previous request. The worst case occurs when a
number of requests are queued for an RTU that has gone off-line, especially when the server
is programmed to retry the request upon failure.
Steps 5-8 represent the case where the request is responded to by the RTU and is forwarded
successfully to the master. It includes the “think time” for the RTU to process the request and
build the response.
Step 9a represents the possibility that the RTU is off-line, the RTU receives the request in
error or that the Server Gateway receives the RTU response in error. The Server Gateway will
issue an exception to the originator. If sending exceptions has not been enabled, the Server
Gateway will not send any response.
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