Siemens SIMATIC IM 151/CPU Manual page 204

Glossary
Operating system of the CPU
The operating system of the CPU organizes all the functions and processes of
the CPU which are not associated with a special control task.
Organization block
Organization blocks (OBs) represent the interface between the operating system
of the CPU and the user program. The processing sequence of the user program
is defined in the organization blocks.
Parameter
1st variable of a STEP 7 code block
2nd variable to set the behavior of a module (one or more per module). Each
module is delivered with a suitable default setting, which can be changed by
configuring the parameters in STEP 7 .
PG Programming Device
PLC Programmable Controller
Priority Class
The operating system of an S7-CPU provides up to 26 priority classes (or
"runtime levels") to which various organization blocks are assigned. 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 the
states of the inputs are written into the process input image at the start of the
cyclic program. At the end of the cyclic program, the signal states in the process
output image are transferred to the outputs.
Glossary-8
system memory of the CPU. The signal
ET 200S Interface Module IM 151/CPU
A5E00058783-01