Priority Classes - Siemens SIMATIC S7-400 Installation Manual

Parameters, Static
In contrast to dynamic parameters, static parameters of modules cannot be
changed by means of the user program, but only via STEP 7 (not in the RUN
state); for example, input delay of a digital signal input module.
PG
→ Programming Device
PROFIBUS DP
Digital, analog and intelligent modules, as well as a wide range of field devices to
EN 50170, Part 3, such as drives or valve modules are moved to the local process
by the PLC across distances of up to 23 km.
The modules and field devices are interconnected with the PLC via PROFIBUS DP
field bus and are addressed in the same way as local I/O.

Priority classes

The S7 CPU operating system provides up to 26 priority classes (or "program
execution levels") which are assigned different OBs. These classes determine
which OB can interrupt other OBs. Several OBs belonging to the same priority
class do not interrupt each other, and are executed in sequential order.
Process Image
The process image is a component part of the system memory of the S7-400 CPU.
At the beginning of the cyclic program, the signal states of the input modules are
transferred to the process-image input table (PII). At the end of the cyclic program,
the process-image output table (PIQ) is transferred to the output modules as the
signal state.
Programming Device (PG)
A personal computer with a special compact design, suitable for industrial
conditions. A Siemens programming device is completely equipped for
programming the SIMATIC programmable logic controllers.
Protective Ground
Connection via protective conductor to a common ground conductor for the
exposed, conductive parts of electrical apparatus which are not normally live, but at
which a voltage may be present in the event of a fault, and which are jointly
protected via a protective device.
Automation System S7-400 Hardware and Installation
A5E00850741-01
Glossary
Glossary-11