Filter
Top Products
Serial Protocols
ROS™ v3.5 60 RS400
Master
Client
Gateway
Server
Gateway
RTU
Transmission time from
Master to Client Gateway
Network transmission time
Transmission time from
Server Gateway to RTU
RTU "think" and transmission
times to Server Gateway
Network transmission time
Transmission time from
Client Gateway to Master
Queuing time
1
5
8
6
3b
3a
4
2
9b
9a
9d
9c
7
Time-out / Retransmissions
complete, Exception sent
1a
Figure 35: 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 1b.
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. The Server Gateway will issue