Siemens SIMATIC S7-300 Hardware And Installation Manual page 232

Glossary
Programming device
Programming devices are essentially personal computers which are compact,
portable and suitable for industrial applications. They are equipped with special
hardware and software for SIMATIC PLCs.
RAM
RAM (Random Access Memory) is a semiconductor read/write memory.
Reduction factor
The reduction rate determines the send/receive frequency for --> GD packets on
the basis of the CPU cycle.
Reference ground
--> Ground
Reference potential
Potential with reference to which the voltages of participating circuits are observed
and/or measured.
Restart
When a central processing unit is started up (e.g. by switching the mode selector
from STOP to RUN or by switching the power on), organization block OB 100
(complete restart) is executed before cyclic program execution commences (OB 1).
On restart, the input process image is read in and the STEP 7 user program is
executed, starting at the first instruction in OB 1.
Retentivity
A memory area is retentive if its contents are retained even after a power failure
and a change from STOP to RUN. The volatile area of memory bits, timers and
counters is reset after a power failure and transition from STOP to RUN mode.
The following can be made retentive:
• Bit memories
• S7 timers
• S7 counters
• Data areas (only with Memory Card or integrated EPROM)
Runtime error
These are PLC errors occurring during user program execution (that is, not in the
process itself).
12-12
S7-300 Programmable Controller Hardware and Installation
A5E00105492-01