97BPLC concepts
4.2 Data storage, memory areas, I/O and addressing
S7-1200 Programmable controller
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System Manual, 11/2011, A5E02486680-05
4.1.6.2
Configuring the outputs on a RUN-to-STOP transition
You can configure the behavior of the digital and analog outputs when the CPU is in STOP
mode. For any output of a CPU, SB or SM, you can set the outputs to either freeze the value
or use a substitute value:
●
Substituting a specified output value (default): You enter a substitute value for each
output (channel) of that CPU, SB, or SM device.
The default substitute value for digital output channels is OFF, and the default substitute
value for analog output channels is 0.
●
Freezing the outputs to remain in last state: The outputs retain their current value at the
time of the transition from RUN to STOP. After power up, the outputs are set to the
default substitute value.
You configure the behavior of the outputs in Device Configuration. Select the individual
devices and use the "Properties" tab to configure the outputs for each device.
When the CPU changes from RUN to STOP, the CPU retains the process image and writes
the appropriate values for both the digital and analog outputs, based upon the configuration.
4.2
Data storage, memory areas, I/O and addressing
4.2.1
Accessing the data of the S7-1200
STEP 7 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 several options for storing data during the execution of the user program:
●
Global memory: The CPU provides a variety of specialized memory areas, including
inputs (I), outputs (Q) and bit memory (M). This memory is accessible by all code blocks
without restriction
●
PLC tag table: You can enter symbolic names in the STEP 7 PLC tag table for specific
memory locations. These tags are global to the STEP 7 program and allow programming
with names that are meaningful for your application.
●
Data block (DB): You can include DBs in your user program to store data for the code
blocks. The data stored persists when the execution of the associated code block comes
to an end. A "global" DB stores data that can be used by all code blocks, while an
instance DB stores data for a specific FB and is structured by the parameters for the FB.
●
Temp memory: Whenever a code block is called, the operating system of 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.