Siemens SIMATIC S7-300 Hardware And Installation Manual page 231

PLC
--> Programmable controller
Priority class
The S7 CPU operating system provides up to 26 priority classes (or "Program
execution levels"). Specific OBs are assigned to these classes. The priority classes
determine which OBs interrupt other OBs. If a priority class includes several OBs,
they do not interrupt each other, but are executed sequentially.
Process Image
The process image is part of CPU --> system memory. At the start of cyclic
program execution, the status of the signal module inputs is written to the input
process image. At the end of cyclic program execution, the signal status of the
output process image is transferred to the output modules.
Process interrupt
A process interrupt is triggered by interrupt-triggering modules as a result of a
specific event in the process. The process interrupt is reported to the CPU. The
assigned --> organization block is then processed, according to interrupt priority.
Product version
The product version differentiates between products which have the same order
number. The product version is increased with each upwardly compatible function
extension, production-related modification (use of new components) or bug-fix.
PROFIBUS-DP
The PLC distributes controls for digital, analog and intelligent modules as well as a
wide range of field devices to EN 50170, part 3, for example, drives or valve
blocks, to processes at external locations - even across distances exceeding
23 km.
The modules and field devices are connected to the programmable controller via
the PROFIBUS-DP fieldbus and addressed in the same way as centralized I/Os.
Programmable controller
Programmable controllers (PLCs) are electronic controllers whose function is
saved as a program in the control unit. The configuration and wiring of the unit are
therefore independent of the function of the control system. The PLC has a
computer structure; it consists of the --> CPU (Central Processing Unit) with
memories, I/O modules and internal bus system. The I/Os and the programming
language are oriented to control engineering needs.
S7-300 Programmable Controller Hardware and Installation
A5E00105492-01
Glossary
12-11