Process Image
Process Image
Process Image
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One of the internal tasks of the operating system (OS) is to read the status of inputs into the process image
input table (PII). Once this step is complete, the user program is executed with all blocks that are called in it.
The cycle ends with writing the process image output table (PIQ) to the outputs for the modules. Reading in
the process image input table and writing the process image output table to the outputs for the modules is all
independently controlled by the operating system.
One of the internal tasks of the operating system (OS) is to write the process image output table (PIQ) to the
outputs for the modules and to read in the status of inputs into the process image input table (PII). Once this
step is complete, the user program is executed with all blocks that are called in it. Writing the process image
output table to the outputs for the modules and reading in the process image input table is all independently
controlled by the operating system.
Advantages of the Process Image
Compared with direct access to the input/output modules, the main advantage of accessing the process
image is that the CPU has a consistent image of the process signals for the duration of one program cycle.
If a signal state on an input module changes while the program is being executed, the signal state in the
process image is retained until the process image is updated again in the next cycle. The process of
repeatedly scanning an input signal within a user program ensures that consistent input information is always
available.
Access to the process image also requires far less time than direct access to the signal modules since the
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The process image areas used as bit memory must be located outside of the
parameter assignment for "Size of the Process Image" or.
You assign the parameters for the CPU, along with the priority of the OBs, to indicate which process-image
partition is assigned to which OB.
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S7-300: No entry is made in the diagnostic buffer, no OB is called, the corresponding input bytes are
reset to "0" and will remain at "0" until the fault is gone again.
S7-400: An entry is made in the diagnostic buffer and OB85 is started for each I/O access for each
update of the corresponding process image. The faulty input bytes are reset to "0" each time the process
image is accessed.
For new CPUs (as of 4/99), you can reassign parameters for the reaction to I/O access errors so that the
CPU functions in one of the following manners:
Generates an entry in the diagnostic buffer and starts OB85 only for incoming and outgoing PZF (before
OB 85 is called, the faulty input bytes are reset to "0" and are no longer overwritten by the operating
system until the outgoing PZF)
Produces the default reaction of an S7-300 (does not call OB85; the corresponding input bytes are reset
to "0" and are no longer overwritten by the operating system until the fault is cleared.)
Produces the default reaction of an S7-400 (calls OB85 for each individual access; the faulty input bytes
are reset to "0" each time the process image is accessed.)
See also:
Operating Modes and Mode Transitions
Error OBs as a Reaction to Detected Errors
Configuring Short and Equal-Length Process Reaction Times on PROFIBUS-DP
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