Hello everyone, I am a new user of Modbus and Redlion CR1000-07000. I have set up a project to communicate with remote RTUs using a simple radio link. Initially, the system worked flawlessly with a Master to Slaves setup on a simplex link. However, we now need to extend the coverage by adding a repeater due to obstacles obstructing the line of sight. While the repeater successfully transmits the packets, a problem arises when the Redlion receives the repeated packets and misinterprets them as RTU replies. Despite our efforts to find a solution by searching online, we have not been able to resolve this issue of repeated Modbus packets. Can anyone offer any guidance on how to address this problem?
Could you please explain that to me again? The image displayed here depicts a Phoenix Contact Repeater, which acts as a conduit for messages, forwarding them and their responses. It does not generate messages on its own. Is it essentially a hybrid of radio and repeater technology?
The radios are connected directly through RS485. The radio repeater, on the other hand, uses RS232 with the RXD and TXD lines linked to resend any received packets automatically. As a result, the Remote RTU is unable to detect the Poll from HMI, but it can detect the retransmitted packet. However, an issue arises as the HMI receives the repeated packet and mistakenly interprets it as the response from the RTU.
If you're not familiar with radio repeaters, it may be beneficial to seek input from an expert on the topic. It's important to note that simply connecting the RXD and TXD lines of a Friendcom device may not turn it into a modbus repeater; this function is typically performed by a specialized device designed for the task. One alternative could be using two radio link devices at the midpoint with different frequencies. For example, Radio 1 communicates with repeater side 1 on frequency 1, which then relays the data to repeater side 2 on frequency 2, allowing communication with Radio 2. It's worth noting that using basic radios for Modbus RTU can also present some challenges, as found in online research.
One way to improve communication efficiency is by utilizing two frequencies and a second repeater. By splitting your 12.5 kHz channel into two 6.25 kHz channels and connecting the second repeater back-to-back with the first repeater through their serial ports, you can create a more effective communication system. Utilize one 6.25 kHz channel between your master radio and the first repeater, and another 6.25 kHz channel between the second repeater and the end radio. It is important to consider potential issues with channel separation, especially in relation to how the Red Lion system handles interference. While this setup is worth exploring, it may be more beneficial to acquire an additional 12.5 kHz frequency, assuming proper licensing.
Thank you for the valuable feedback. The TXD to RXD connection on the single radio performed effectively, ensuring that packets transmitted and received remained uncorrupted. However, there was an issue with sending the master poll back to the HMI due to compatibility issues. Despite this setback, the single radio solution proved to be both elegant and cost-effective, with low current consumption. To address connectivity challenges with RTUs not within line of sight of the master HMI, we plan to implement a two-frequency split using a second radio. Additionally, the repeater site is powered by solar energy, necessitating further work on power management. Fortunately, solar power solutions are now more affordable. In obtaining a second point to multipoint license in NZ, we encountered no significant hurdles. As someone new to Modbus, I found it surprising that the master did not recognize its own packet and disregarded it.
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Answer: - To troubleshoot this issue, you can start by checking the configuration settings of the repeater and Redlion RTUs to ensure they are set up correctly for communication. Additionally, consider adjusting signal strengths, data rates, or error-checking mechanisms to minimize packet misinterpretation.
Answer: - Common reasons for misinterpretation of repeated packets could include signal interference, incorrect data framing, synchronization issues, or packet collisions. It is essential to investigate these factors when troubleshooting the problem.
Answer: - When setting up Modbus repeaters in a radio link network with Redlion RTUs, ensure that the repeater is compatible with Modbus communication protocols and configured to forward packets accurately without introducing errors. Proper positioning and optimization of signal strength can also help improve communication reliability.
Answer: - To differentiate between repeated packets and genuine RTU replies, you can analyze the packet timestamps, data content, and sequence numbers to identify any patterns or discrepancies. Monitoring the network traffic and using diagnostic tools can also aid in distinguishing between the two types of data.
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