Modbus TCP/IP Driver Manual
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Appendix B. Troubleshooting
Appendix B.1. Server Configuration of System Station Address
When using the FieldServer as a Modbus Server, the FieldServer System Station address must be
configured to be different from any of the configured Modbus Server Node_ID’s. Configuring these to be
the same invokes proprietary system information to be transmitted and should therefore be avoided.
Appendix B.2. Modbus TCP/IP Connection Error Descriptions
No Start – An Ethernet connection cannot be established to the specified IP Address.
TCP Connection Lost – An Ethernet connection was established to the specified IP Address but the
connection was terminated or lost.
Timeout – An Ethernet connection was established to the specified IP Address and a Modbus request
was sent but the device did not respond before the timeout interval expired.
Appendix B.3. Understanding Max Concurrent Messages
The FieldServer Max_Concurrent_Messages parameter enables polling multiple “threads” for the Modbus
TCP/IP Driver; increasing network communications speed if the FieldServer is a Modbus Master. This
feature can significantly improve communication speeds but may also create minor communication errors
with other vendors’ devices. Most often, this does not prevent communication but creates a small
percentage of communication errors. These errors must be accepted to utilize this feature.
Consider an application where one communication thread is used to poll four devices. This sequence
includes the following steps:
1. Poll device 1 and wait for a response to be received.
2. Poll device 2 and wait for a response to be received.
3. Poll device 3 and wait for a response to be received.
4. Poll device 4 and wait for a response to be received.
5. Poll device 5 and wait for a response, then go back to step 1.
Now consider five threads for the communication. Then the sequence follows:
1. Poll devices 1,2,3,4 and 5 at the same time and wait for the individual responses to come back.
2. As each thread is freed up, poll the next device in line and continue.
This example shows how using multiple threads is substantially faster but additionally, if one of the
devices is faulty then the network is influenced far less when five threads are used.
• In the single thread scenario, communication must wait for the faulty device and when it doesn’t
respond (after 2 seconds) it moves on. This is a significant delay.
• In the five thread scenario, only one thread waits while the other threads communicate freely.