Tcpmodbus Performance Determinants; Figure 175: Sources Of Delay And Error In An End To End Exchange - RuggedCom RuggedRouter RX1000 User Manual

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RuggedRouter User Guide

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 175: 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.
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