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Data Types DDT: Mapping Rules At a Glance The DDTs are stored in the PLC’s memory in the order in which its elements are declared. However, the following rules apply. Principle for Premium and Quantum The storage principle for Premium and Quantum is as follows: the elements are stored in the order in which they are declared in the structure, z the basic element is the byte (alignment of data on the memory bytes), z each element has an alignment rule: z the BOOL and BYTE types are indiscriminately aligned on the odd or even bytes, z all the other elementary types are aligned on the even bytes, z the structures and tables are aligned according to the alignment rule for the BOOL and BYTE types if they only contain BOOL and BYTE elements, otherwise they are aligned on the memory’s even bytes. z WARNING RISK OF INCOMPATIBILITY AFTER CONCEPT CONVERSION With the Concept programming application, the data structures do not handle any shift in offsets (each element is set one after the other in the memory, regardless of its type). Consequently, we recommend that you check everything, in particular the consistency of the data when using DDTs located on the "State RAM" (risk of shifts) or functions for communication with other devices (transfers with a different size from those programmed in Concept). Failure to follow these instructions can result in death, serious injury, or equipment damage. 272 35006144 07/2011
