Siemens Simatic S7-400 - Manual (Page 619)

Glossary
Glossary-13
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Priority class
The operating system of an S7 CPU has a maximum of 28 priority classes
(= program execution levels) – for cyclic program scanning or program scanning
controlled by hardware interrupt, for example.
Each priority class is assigned
³
organization blocks in which the user can
program a response. By default, the OBs have different priorities determining the
order in which they are executed or interrupted in the event that they are
activated simultaneously. The user can change the default priorities.
Process image
The signal states of the digital input and output modules are stored in the CPU in
a process image.
A distinction is made between the process input image and the process output
image. The process input image (PII) is read by the input modules before the
operating system scans the user program. The process output image (PIQ) is
transferred to the output modules at the end of program scanning.
Product status
Products having an identical order number are distinguished by their product
status. The product status is incremented for upwards compatible extensions,
modifications due to production reasons (use of new component parts and
components) and troubleshooting.
PROFIBUS-DP
Digital, analog and intelligent I/O modules and a wide range of field devices
complying with EN 50170, Part 3, such as drives or valve terminals, are moved
by the automation system to the process on site, over a distance of up to 23 km.
The modules and field devices are connected to the programmable controller by
means of the PROFIBUS-DP fieldbus and addressed in the same way as central
I/O.
Programmable controller
Programmable controllers (PLCs) are electronic controllers whose function is sto-
red as a program on the control device. The design and wiring of the device do
not therefore depend on the function of the controller.
The architecture of a programmable controller is similar to that of a computer; it
consists of a
³ CPU (central processing unit) with memory, input/output modules
and an internal bus system. The I/O and the programming language are
designed to meet the requirements of open-loop control.
Programmable controller
A programmable controller
³
consists of a
³
central device, a CPU and diverse
input/output modules.