Memory Areas Of The S7-1200 - Siemens SIMATIC S7-1200 Getting Started

Quick review
1.1 Basic information about how the CPU works
1.1.5

Memory areas of the S7-1200

Note
Using symbolic addresses instead of absolute addresses
STEP 7 Basic facilitates symbolic programming. You create symbolic names or "tags" for the
addresses of the data, whether as PLC tags relating to memory addresses and I/O points or
as local variables used within a code block. To use these tags in your user program, simply
enter the tag name for the instruction parameter. For a better understanding of how the CPU
structures and addresses the memory areas, the following paragraphs explain the "absolute"
addressing that is referenced by the PLC tags.
The CPU provides a variety of specialized memory areas, including inputs (I), outputs (Q), bit
memory (M), data block (DB), and local or temporary memory (L). Your user program
accesses (reads from and writes to) the data stored in these memory areas. Each different
memory location has a unique address. Your user program uses these addresses to access
the information in the memory location.
Memory area
I
Process image
input
Q
Process image
output
M
Bit memory
L
"Temp" memory
DB
Data block
Whether you use a tag (such as "Start" or "Stop") or an absolute address (such as "I0.3" or
"Q1.7"), a reference to the input (I) or output (Q) memory areas accesses the process image
and not the physical output. To immediately access or force the physical (peripheral) input or
output in your user program, append the reference with ":P" (such as "Stop:P" or "Q0.3:P").
12
Description
The CPU copies the state of the physical inputs to I memory at the beginning of
the scan cycle. To immediately access or to force the physical inputs, append a
":P" to the address or tag (such as "Start:P" or I0.3:P).
The CPU copies the state of Q memory to the physical outputs at the beginning
of the scan cycle. To immediately access or to force the physical outputs,
append a ":P" to the address or tag (such as "Stop:P" or Q0.3:P).
The user program reads and writes the data stored in M memory. Any code
block can access the M memory. You can configure addresses within M memory
to retain the values of the data after a power cycle.
Whenever a code block is called, the CPU allocates the temporary, or local,
memory (L) to be used during the execution of the block. When the execution of
the code block finishes, the CPU reallocates the local memory for the execution
of other code blocks.
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
specify a data block to be either read/write or read only. 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.
Getting started with S7-1200
Getting Started, 11/2009, A5E02486791-01