Siemens Memory Specifications download pdf (Page 42)

3 General Programming

3.4 Storage concept

Programming Guideline for S7-1200/1500

V1.2, Entry ID: 81318674

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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

Local

FC FB

Local

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

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.

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Siemens Memory Specifications Page 42