Table of Contents
Advertisement
Chapter 5
Setup and Configuration
Figure 80: Sources of Delay and Error in an End-to-End Exchange
1. Master 2. Client Gateway 3. Server Gateway 4. Remote Terminal Unit (RTU)
In step 1a, 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
Modbus TCP 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 Modbus TCP exception code 10 ("No
Response"). If the forwarding of Modbus TCP 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 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 responses.
134
1
2
1a
1b
3b
8
9d
3
2
3a
6
7
9c
Modbus TCP Performance Determinants
RUGGEDCOM ROS
User Guide
4
4
5
9a
9b
Advertisement
Table of Contents
