Specifications

Basic Concepts for Programming an S7-200 CPU
4-23
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
The CPU does not update analog inputs as part of the normal scan cycle unless
digital filtering of analog inputs is enabled. Digital filtering is provided as a user
selectable option and can be individually enabled for each analog input point.
Digital filtering is intended for use with low-cost analog modules that do not provide
filtering internal to the module. Digital filtering should be used in applications where
the input signal varies slowly with time. If the signal is a high-speed signal, then
digital filtering should not be enabled.
When analog input filtering is enabled for an analog input, the CPU updates that
analog input once per scan cycle, performs the filtering function, and stores the
filtered value internally. The filtered value is then supplied each time your program
accesses the analog input.
When analog filtering is not enabled for an analog input, the CPU reads the value
of the analog input from the physical module each time your program accesses the
analog input.
Executing the Program
During the execution phase of the scan cycle, the CPU executes your program,
starting with the first instruction and proceeding to the end instruction. The
immediate I/O instructions give you immediate access to inputs and outputs during
the execution of either the program or an interrupt routine.
If you use interrupts in your program, the interrupt routines that are associated with
the interrupt events are stored as part of the program. (See Section 4.5.) The
interrupt routines are not executed as part of the normal scan cycle, but are
executed when the interrupt event occurs (which may be at any point in the scan
cycle).
Processing the Communication Requests
During the message-processing phase of the scan cycle, the CPU processes any
messages that were received from the communications port.
Executing the CPU Self-Diagnostic Test
During this phase of the scan cycle, the CPU checks its firmware and your
program memory (RUN mode only). It also checks the status of any I/O modules.
Writing to the Digital Outputs
At the end of every scan cycle, the CPU writes the values stored in the
process-image output register to the digital outputs.