Siemens Simatic S7 Series System Manual

PLC concepts
4.1 Execution of the control logic
S7-200 SMART
56 System Manual, 09/2015, A5E03822230-AC
Tasks in a scan cycle
The CPU executes a series of tasks repetitively. This cyclical execution of tasks is called the
scan cycle. The execution of the user program is dependent upon whether the CPU is in
STOP mode or in RUN mode. In RUN mode, your program is executed; in STOP mode, your
program is not executed.
Table 4- 1 Tasks performed by the CPU in a scan cycle
Scan cycle
Description
Reading the inputs: The CPU copies the state of the physi-
cal inputs to the process image input register.
Executing the control logic in the program: The CPU exe-
cutes the instructions of the program and stores the values
in the various memory areas.
Processing any communications requests: The CPU per-
forms any tasks required for communications.
Executing the CPU self-test diagnostics: The CPU ensures
that the firmware, the program memory, and any expansion
modules are working properly.
Writing to the outputs: The values stored in the process
image output register are written to the physical outputs.

Siemens Simatic S7 Series Specifications

General

CPU	Varies by model (e.g., S7-300, S7-400, S7-1200, S7-1500)
Communication Interfaces	PROFIBUS, PROFINET, Ethernet, MPI
Programming	TIA Portal, STEP 7
Programming Languages	LAD, FBD, STL, SCL, GRAPH
I/O Modules	Digital, Analog, Specialty modules available
Operating Temperature	0°C to 60°C (standard models)
Mounting	DIN rail mounting
Operating Voltage	24V DC or 120/230V AC (varies by model)
Protection Class	IP20 (standard models)
Memory	Varies depending on the S7 model. Includes work memory (RAM) for program execution, load memory (EEPROM or Flash) for program storage, and system memory for operating system functions. Memory sizes range from kilobytes to megabytes.