Global Memory - Siemens S7-1200 Programming Manualline

3 General Programming
3.4 Storage concept
Advantages
Program can be made up modularly from ready blocks with partial tasks.
Program is easy to expand and maintain.
Program code is easier to read since there are no hidden cross accesses.
Recommendation
If possible, only use the local tags. This is how the blocks can be used
universally and modularly.
Use the data exchange via the block interfaces (In, Out, InOut), to ensure the
reusability of blocks.
Only use the instance data blocks as local memory for the respective function
block. Other blocks shall not be written into instance data blocks.
Figure 3-17: Avoiding accesses to instance data blocks
OB
If only the block interfaces are used for the data exchange it can be ensured that
all blocks can be used independent from each other.
Figure 3-18: Block interfaces for data exchange
OB
FC
FB
Local
3.4.2

Global memory

Memories are called global when they can be accessed from any location of the
user program. There are hardware-dependent memories (e.g. bit memory, timers,
counters, etc.) and global DBs. For hardware-dependent memory areas there is the
danger that the program may not be portable to any controller because the areas
there may already be used. This is why you should use global DBs instead of
hardware-dependent memory areas.
Programming Guideline for S7-1200/1500
V1.2, Entry ID: 81318674
FB
Local
FC FB
Local
FB
Local
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