dchartier

One clue: compare the size of the working memory for a Siemens PLC to that of a Controllogix or Quantum CPU. A CPU 315-2DP uses 128 kB RAM; our latest Controllogix L63 has 4 MB. And the S7-315 can do almost as much as the comparable Rockwell PLC (a CPU 317-2DP would be a better equivalent). But to do so, you have to organize your program differently.

So directly accessible memory (known set addresses in the CPU memory) is limited to the input/output image tables and a zone of flag bits (M-memory). All other memory is organised in DBs; these are simply STRUCT variables (C/C++ users use this concept constantly) created by the user, but available in RAM only when needed. The CPU loads them from load memory into work memory only when required (called), as all other program blocks anyway; the working address of the variables is known only when the DB is loaded, much as a C/C++ variable's address is known only when it is used. It's a memalloc() function: create the memory space for the required variable when it is used, and when your need for it is finished, free the used memory for other variables.