[Bug Report] Abnormal Polling Speed

[CWZ7605]

Steps to Reproduce:

1. Create multiple Address Tables and set them to different MODBUS Point Types.
2. Without connecting a Modbus Slave device, click "Connect", select the appropriate COM port, and proceed with the connection.
3. At this point, OpenModScan will attempt to communicate with the Modbus Slave device. However, since the Modbus Slave device is not connected, the communication will time out.
4. The Scan Rate set for the Address Table is 1 scan per second. Observe the "Number of Polls"—you will find that the value increases faster than once per second (starts at 40 second in the video).
5. In this state, wait for a few seconds and then connect the Modbus Slave device. After OpenModScan successfully establishes communication with the Modbus Slave device, the value of "Number of Polls" grows rapidly, and you can also observe that the serial port converter is sending data very frequently. After a period of time, OpenModScan will resume sending data at the set Scan Rate (starts at 1 minute and 28 seconds in the video).

Video:
https://github.com/user-attachments/assets/18174c5c-8dcc-491d-8221-f3f80d8bb4cb

I hope this issue can be fixed.

[sanny32]

Hello @CWZ7605. Thank you for using OpenModScan program.
The behavior you described is not bug.
When OpenModScan send a request and did not receive response with "Slave Response Timeout", it send a new retry request. The number of retries setup in the 'Modbus Protocol Selection' dialog

[CWZ7605]

Thank you for your reply. @sanny32

I think I may not have expressed my meaning clearly.

After the Modbus Slave Device is connected normally, the Request sending frequency should be consistent with the Scan Rate.
However, it now appears that the Requests sent before the normal connection are cached instead of being discarded. As a result, after the normal connection is established, these cached Requests are re-sent.

[sanny32]

I understand. I will test this one.

[sanny32]

So, after tests, I can't confirm that the Requests sent before the normal connection are cached. If you have any other experience just let me know about it.

[CWZ7605]

I have discovered a new phenomenon that may be related to the issue we are currently discussing.

I used a virtual serial port to set up a mapping between COM1 and COM2.

OpenModSim is connected to COM2 to simulate the Modbus Slave.When I use OpenModScan to establish a connection with OpenModSim via COM1, OpenModScan immediately sends two data requests right after the connection is established.

I have recorded a video to illustrate this.

Test.mp4

[sanny32]

OpenModSim is connected to COM2 to simulate the Modbus Slave.When I use OpenModScan to establish a connection with OpenModSim via COM1, OpenModScan immediately sends two data requests right after the connection is established.

1. Sending request on device connection establishing is correct behavior.
2. Sending second request on device connection establishing is incorrect behavior and will be fixed.
3. OpenModsim with device id 255 should not respond to requests with device id 1. It is a OpenModSim bug. Please create issue at https://github.com/sanny32/OpenModSim/issues

[CWZ7605]

Thank you for your reply—you have a keen eye for detail.
I will submit an issue in OpenModSim.