I am working with a Micro850 PLC system to retrieve data from a DEPOLOX 400M water analyzer. I have successfully extracted the first two parameters (registers 100 & 115), but I am unable to access the last parameter I need, which is located at register 160. The Rockwell help page states that there is a 125-word limit per MSG_MODBUS when reading registers. Is there a way to create a new MSG_MODBUS configuration that allows for an offset start or extends the word limit beyond 125?
To access data from specific Modbus addresses, simply utilize (2) Msg_Modbus function blocks. Each block can read up to 125 words starting from your designated address. Please note that a maximum of 16 Msg_Modbus blocks can be used simultaneously. You have the flexibility to specify the exact number of words to read, up to the maximum of 125. For example, if you require data from addresses 100 and 115, you can instruct the system to read in 6 words. In this scenario, the first address read would be 100 and the sixth address read would be 115.
The limit of 125 registers is not specific to the Micro850 PLC, but rather a requirement of the Modbus protocol. This limitation stems from the historical need to encode a count of bytes into a single byte, with 125 registers being equivalent to 250 bytes, close to the maximum value of 255. This limitation is also influenced by practical considerations related to data packet sizes in serial systems. While one might expect packet sizes to increase when transitioning from serial to Ethernet protocols like Modbus and AB DF1, this was not the case, except for potentially General Electric's SRTP. Ethernet/IP, specifically designed for Ethernet, allows for larger packet sizes, but much of this extra space is required for protocol overhead such as address segments, data typing, and tag names. In contrast, Modbus and DF1 have minimal overhead. Despite this limitation, modern protocols are able to handle complex data efficiently, making them a valuable choice for industrial applications.
I made a mistake with the address section on Facebook by setting it to 1 instead of 100. By adjusting it to 100, I was able to skip over parameters 1-99 and begin higher on the list. This adjustment proved to be beneficial in prioritizing certain tasks.
Yes, you can indeed do this. You can create another MSG_MODBUS where you set the start address to 126. This will allow you to read additional registers above the 125 words limitation of the first message. Remember, the word limit is imposed on each individual message, not as a total sum. You just need to readjust your offset. Make sure you test it on a small scale first to ensure it's working as you expect. It might take a few tweaks to get the configuration just right.
I faced a similar issue a while back where I also had to access data beyond the 125-word limit. Rather than extending the word limit, which can get complicated, I solved the problem by making two Modbus read commands. I would recommend you to split your reads; carry out the first read for registers 100 & 115, then a separate read for register 160. This way, the second read command will carry out an offset start. It worked seamlessly for me and I hope it solves your problem too.
You should be able to overcome this issue with the 125-word limit by creating multiple MODBUS messages. Break down the register access into two or more operations, ensuring each does not exceed the 125-word limit. The offset start feature can also be utilized to point the second operation to the register 160 from where data need to be retrieved. This should help resolve your problem!
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Answer: Answer: According to the Rockwell help page, the word limit per MSG_MODBUS when reading registers in a Micro850 PLC system is 125. Currently, there is no built-in way to extend this limit.
Answer: Answer: Unfortunately, as of now, there is no direct support for offset start in MSG_MODBUS configurations for the Micro850 PLC system. You may need to explore alternative solutions or workarounds to access register 160.
Answer: Answer: Since the word limit per MSG_MODBUS is 125 in a Micro850 PLC system, accessing register 160 directly may not be possible. You might consider splitting the data retrieval process into multiple transactions or optimizing the data retrieval strategy to work within the 125-word limit constraint.
Answer: Answer: To maximize the use of the 125-word limit per MSG_MODBUS in a Micro
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