Features Of The S7-200 - Siemens Simatic S7-200 System Manual

Features of the S7-200

The S7-200 provides several special features that allow you to customize how the S7-200 functions to
better fit your application.
The S7-200 Allows Your Program to Immediately Read or Write the I/O
The S7-200 instruction set provides instructions that immediately read from or write to the physical I/O.
These immediate I/O instructions allow direct access to the actual input or output point, even though the
image registers are normally used as either the source or the destination for I/O accesses.
The corresponding process-image input register location is not modified when you use an immediate
instruction to access an input point. The corresponding process-image output register location is updated
simultaneously when you use an immediate instruction to access an output point.
Tip
The S7-200 handles reads of analog inputs as immediate data, unless you enable analog input filtering.
When you write a value to an analog output, the output is updated immediately.
It is usually advantageous to use the process-image register rather than to directly access inputs or
outputs during the execution of your program. There are three reasons for using the image registers:
-
The sampling of all inputs at the start of the scan synchronizes and freezes the values of the inputs
for the program execution phase of the scan cycle. The outputs are updated from the image register
after the execution of the program is complete. This provides a stabilizing effect on the system.
-
Your program can access the image register much more quickly than it can access I/O points,
allowing faster execution of the program.
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I/O points are bit entities and must be accessed as bits or bytes, but you can access the image
register as bits, bytes, words, or double words. Thus, the image registers provide additional
flexibility.
The S7-200 Allows Your Program to Interrupt the Scan Cycle
If you use interrupts, the routines associated with each interrupt event are stored as part of the program.
The interrupt routines are not executed as part of the normal scan cycle, but are executed when the
interrupt event occurs (which could be at any point in the scan cycle).
Interrupts are serviced by the S7-200 on a first-come-first-served basis within their respective priority
assignments. See the Interrupt instructions in Chapter 6 for more information.
PLC Concepts Chapter 4
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