Siemens SIMATIC S7-1200 - Page 121

PLC concepts
5.2 Data storage, memory areas, I/O and addressing
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
121
M (bit memory area):
Use the bit memory area (M memory) for both control relays and data
to store the intermediate status of an operation or other control information. You can access
the bit memory area in bits, bytes, words, or double words. Both read and write access is
permitted for M memory.
Table 5- 26 Absolute addressing for M memory
Bit
M[byte address].[bit address]
M26.7
Byte, Word, or Double Word
M[size][starting byte address]
MB20, MW30, MD50
Temp (temporary memory):
The CPU allocates the temp memory on an as-needed basis.
The CPU allocates the temp memory for the code block and initializes the memory locations
to 0 at the time when it starts the code block (for an OB) or calls the code block (for an FC or
FB).
Temp memory is similar to M memory with one major exception: M memory has a "global"
scope, and temp memory has a "local" scope:
● M memory: Any OB, FC, or FB can access the data in M memory, meaning that the data
is available globally for all of the elements of the user program.
● Temp memory: The CPU restricts access to the data in temp memory to the OB, FC, or
FB that created or declared the temp memory location. Temp memory locations remain
local and different code blocks do not share temp memory, even when the code block
calls another code block. For example: When an OB calls an FC, the FC cannot access
the temp memory of the OB that called it.
The CPU provides temp (local) memory for each OB priority level:
● 16 Kbytes for startup and program cycle, including associated FBs and FCs
● 6 Kbytes for each additional interrupt event thread, including associated FBs and FCs
You access temp memory by symbolic addressing only.
You can find out the amount of temp (local) memory that the blocks in your program use
through the call structure in STEP 7. From the project tree select Program info and then
select the Call structure tab. You will see all of the OBs in your program and you can drill
down to see the blocks that they call. For each block, you can see the local data allocation.
You can also access the Call structure display from the STEP 7
Tools > Call structure
menu
command.
DB (data block):
Use the DB memory for storing various types of data, including intermediate
status of an operation or other control information parameters for FBs, and data structures
required for many instructions such as timers and counters. You can access data block
memory in bits, bytes, words, or double words. Both read and write access is permitted for
read/write data blocks. Only read access is permitted for read-only data blocks.
Table 5- 27 Absolute addressing for DB memory
Bit DB[data block number].DBX[byte ad-
dress].[bit address]
DB1.DBX2.3
Byte, Word, or Double
Word
DB[data block number].DB [size][starting
byte address]
DB1.DBB4, DB10.DBW2,
DB20.DBD8