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Entry type: FAQ Entry ID: 291495, Entry date: 07/28/2021

How are the variables stored in the temporary local data?

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In the following we explain the local data addressing with S7-300/S7-400.

The L-stack addressing always starts at address "0". For each block, bytes are reserved on the L-stack, which corresponds to the amount of static data and/or local data. When a block is terminated, its area is released again.

The pointer always points to the first byte of the currently opened block. The following table shows an example:

Runtime level  L stack
in bytes 
Call OB1 (with 20 static [fixed] and 10 additional local data)
Call FC1 (with 30 local data) 30 bytes (OB1) +30 bytes (FC1)
 Call FC10 (with 20 local data)
60 bytes (OB1 + FC1) +20 bytes FC10
 Call FC11 (with 20 local data)
60 bytes (OB1 + FC1) +20 bytes FC11
 Call FC12 (with 30 local data)
60 bytes (OB1 + FC1) +30 bytes FC12

Call FC2 (with 50 local data)
30 bytes (OB1) +50 bytes (FC2)

Call FC20 (with 10 local data)
80 bytes (OB1 + FC2) +10 bytes FC20
Call FC21 (with 10 local data)
80 bytes (OB1 + FC2) +10 bytes FC21
Call FC22 (with 20 local data)
80 bytes (OB1 + FC2) +20 bytes FC22
 Call FC221 (with 10 local data)
100 bytes (OB1 + FC2 + FC22) + 10 bytes FC221

The L stack is used by the CPU for internal processing. The STL editor itself also uses local data, for example for the parameter transfer during a block call. This is why you should not change the contents of the L stack.

The local data size was changed in the firmware V3.x and higher (new hardware) for all S7-300 CPUs and increased to 32 KB per process level, for example. More detailed information is available in the technical data of each CPU.

For the CPU 319-3 PN/DP, 2MB, 6ES7318-3EL01-0AB0 in the technical data under "Data areas and their retentivity" you will find the information about the local data:

  • Per priority class, max. 32768 bytes; max. 2048 bytes per block

With the S7-400, 256 bytes per priority class are provided for each program processing level (runtime level), which you may set in the CPU properties of the hardware configuration.

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