Hi I am trying to use STRUCT to parse commands from Modbus/TCP, Modbus client will fill the bytes related to a specific command (type, parameters), then PLC (Codesys) will access the command parameters. A STRUCT will simplify a lot the process of parsing the data, so I don't need to parse byte per byte, address will be mapped to a struct and I can use it for parsing the data.

From what I understood, the compiler does not allow me to specify the location of the struct for a %MW register.

VAR_COMMAND AT %MW1002: structGenericModbusCommand;

This will throw:

Bad declaration, use '%ML' for 'structGenericModbusCommand' variable

But If I change it to %ML, when you go online you see that it is actually allocating the struct in another memory address.

When we go online, on GVL we can see:

VAR_COMMAND.     structGenericModbusCommand     %ML1002
     commandType        COMMANDTYPE             %MB8016
     ...                                        %MB8020

I am not specifying a pointer or a reference it is a struct type, why is it mapping to a %MW location (random one). How can I control where it is mapping to?

I need a consistent way to map STRUCT to specific modbus address. Is it possible?

Thanks,
Paulo

Note I am using a Codesys OEM: Schneider Machine Expert Logic Builder (M241)

This is normal and correct since in CoDeSys static addressing of variables uses the IEC method. A LONG tag has the size of 8 bytes, so %ML0 coincides with %MB0 ... %MB7; %ML1000 with %MB8000 ... %MB8007. (Similar reason for word and other formats).
Also the reason the compiler won't let you map the structure to a %MW depends on the fact that the individual base elements are manipulated to 64 bits regardless of the smaller size.

That said, it makes me strange that you cannot write a parser without making use of static memory allocation.