Siemens S7-400 Installation Manual page 305

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.
Programmable Logic Controller (PLC)
Programmable controllers are electronic control devices whose functions are
stored in the controller in the form of a program. The structure and wiring of a PLC
therefore do not depend on the actual functions of the controller. Programmable
controllers and computers have similar structure: they consist of a CPU (central
processing unit) with memory, input/output modules, and an internal bus system.
The I/O and programming language are tailored to the requirements of open-loop
control technnology.
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.
R
RAM
The RAM (Random Access Memory) is a semiconductor memory with random
access (read/write memory).
Reference Ground
³ Ground
S7-400 and M7-400 Programmable Controllers Hardware and Installation
A5E00069481-04
Glossary
Glossary-13