I am exploring the use of STRUCT in parsing Modbus/TCP commands. In this setup, the Modbus client populates bytes with data related to a particular command (such as type and parameters), which the PLC (Codesys) accesses. By implementing a STRUCT, the process of parsing the data is streamlined, eliminating the need to parse byte by byte. Instead, the data can be easily accessed by mapping addresses to a struct. However, I have encountered an issue where the compiler does not allow me to specify the location of the struct for a %MW register. Specifically, using the code block VAR_COMMAND AT %MW1002: structGenericModbusCommand results in an error message stating "Bad declaration, use '%ML' for 'structGenericModbusCommand' variable." However, changing it to %ML allocates the struct to a different memory address upon going online. This inconsistency raises the question of why a struct type is being mapped to a %MW location instead of a specified one. Is there a way to control where the struct is mapped? In order to establish a reliable method for mapping STRUCT to specific Modbus addresses, I am seeking a solution to this issue. Any insights or suggestions would be greatly appreciated. Thank you. - Paulo (Using Codesys OEM: Schneider Machine Expert Logic Builder M241)
The size of the structGenericModbusCommand varies depending on the type of memory being used: MB (1 byte), MW (2 bytes), MD (4 bytes), and possibly ML (8 bytes). It appears that the memory addressing may be overlapping, with memory addresses progressing in the sequence MW1002 -> MB2004 -> MD501. For further information on memory mapping, visit: https://product-help.schneider-electric.com/Machine%20Expert/V1.1/en/m241prg/m241prg/M2xx_-_Memory_Mapping/M2xx_-_Memory_Mapping-3.htm.
Hi Paulo! It sounds like you're working on quite an interesting project. The issue you're experiencing might be due to %MW being a word type area, intended for unsigned integers, while %ML is for double words, and is likely used for allocating structures. Altering that would require deeper manipulation of the system, so instead, I'd suggest creating an array of words (%MW) of the size of your structure. Then, use a UNION to superimpose your structure over the array. Essentially, it’s a workaround to manually allocate memory for your structured data using the array language construct. This should allow you to control the mapping location precisely. Hope this helps!
Hi Paulo, it seems like you're trying to directly map the STRUCT to a %MW memory address, which could be causing the issue. Generally, Codesys isn't really fond of mixing memory areas (%ML, %MW, etc.) when it concerns complex data types like STRUCT. A common workaround is to use a MOVE block to copy the data from your %MW area to the STRUCT, and vice versa for output. Thus, effectively maintaining the desired address association while still utilizing the ease of structured variables. This might be a limitation based on Codesys's memory management constraints, or due to the architecture of the PLC you're using. I suggest giving this method a try and see if it resolves your issue. Good luck!
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Answer: 1. Why am I encountering an error when trying to specify the location of a struct for a %MW register in CODESYS? - The error message "Bad declaration, use '%ML' for 'structGenericModbusCommand' variable" indicates that the compiler does not allow mapping a struct to a %MW register. Instead, it suggests using '%ML' for the struct variable.
Answer: - In order to specify a specific memory address for a struct in CODESYS, you should use '%ML' instead of '%MW'. Mapping a struct to a %MW location may lead to inconsistencies in memory allocation.
Answer: - To ensure consistency and reliability in mapping a struct to Modbus addresses in CODESYS, it is advisable to use '%ML' for specifying the memory location of the struct variable. This helps in avoiding allocation inconsistencies when going online.
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