Hey everyone, I've been struggling to establish communication between Modbus TCP and Rockwell's Modbus TCP AOI for reading purposes. I've encountered some issues and was hoping to get some assistance. It appears that when I attempt to request data from holding registers (a total of 34), only about ten registers are successfully read. At times, I receive an STS_Overload alarm, while other times there is no alarm, but the transaction status always shows as "exception". I have gone through the documentation thoroughly and have experimented with the execution period of the Periodic Task and the Poll Interval of the AOI, but the issue persists. Any advice would be greatly appreciated. Thank you!
If your ACD upload is showing a faulted state and you need assistance with wireshark analysis, referring to the Modbus Slave device manual can provide valuable insight. Overlap issues may indicate that your peer device failed to respond in time for the next scheduled read attempt. By checking the exception code, you can pinpoint the specific issue at hand. It is possible that you are trying to read data that is not supported by the device.
If you are unable to share your complete automated call distribution (ACD) settings, you can still export and upload just one specific ladder logic rung from your alarm output indicator (AOI). By doing so, you will be able to preserve all of the important configuration and exception values within that particular rung.
Recently, I utilized the Modbus TCP Add-On Instruction (AOI) from Rockwell and encountered a problem similar to what has been described here. Can someone provide assistance with this issue?
Hey there, your issue might be related to the quantity of registers you're trying to read in a single request. Modbus TCP typically allows a maximum of approx 125 registers per read due to the byte limit of TCP packets. If the data of each register you are reading is too large, the request may overload. I'd suggest trying to split your request into smaller chunks to see if that resolves the STS_Overload and "exception" statuses you're encountering. Don't forget to check the data rate limitation of your AOI - it can also impact the successful reading of data.
It sounds like you’re running into a frustrating issue! Based on what you've described, it might help to double-check the configuration of your Modbus TCP settings—specifically, ensure that the slave device is set up to handle multiple requests at once. The STS_Overload alarm could indicate that your requests are coming in too quickly for the device to handle. Have you tried increasing the Poll Interval further or experimenting with different execution periods? Sometimes, adding a slight delay in your requests can also make a difference. Hope this helps, and good luck!
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Answer: Answer: The issue of only about ten registers being successfully read while attempting to request data from 34 holding registers could be related to communication overlap issues or configuration settings. It's important to review the setup and configuration of the communication parameters to ensure proper data exchange.
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