Siemens SIMATIC S7-400 Installation Manual page 220

Glossary
R
RAM
The RAM (Random Access Memory) is a semiconductor memory with random
access (read/write memory). May be used as for storing interim data for later use.
This memory is not retentive, that is, its data are lost after power failure.
Reference Ground
→ Ground
Reference Potential
The potential on which the voltages of the various circuits are based and according
to which they are measured.
Retentive Data
Retentive data are not lost after a power failure, if a backup battery is provided.
Release version
Used to distinguish products with the same order number. Incremented with each
up-compatible functional enhancement, manufacturing-specific change (use of new
parts / components) and for bugfixes.
Run-Time Error
Errors that occur in the programmable controller (that is, not in the process) during
execution of the user program.
S
Scan Cycle Time
The scan cycle time is the time the CPU takes to run the user program once
through.
Shared Data
Shared data can be accessed by any code block (FC, FB, OB). These include bit
memory M, inputs I, outputs Q, timers T, counters C, and data blocks DB. Global
data can be accessed either absolutely or symbolically.
Glossary-12
Automation System S7-400 Hardware and Installation
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