RuggedCom RuggedRouter RX1100 - Manual (Page 220)

24. Configuring Serial Protocols
Revision 1.14.3 220 RX1000/RX1100™
Figure 24.1. 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 represents the possibility that the Server Gateway does not have 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 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 offline, 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 offline, the RTU receives the request in error or that
the Server Gateway receives the RTU response in error. If the Server Gateway does not retry the
request, it will issue an exception to the originator.