Siemens SIMATIC S7-300 Series Reference Manual page 172

Glossary
Backup memory
Backup memory ensures buffering of CPU memory areas
³ CPU, using no battery. A configurable number of timers, counters, memories
and data bytes (retentive timers, counters, memories and data bytes) is backed
up.
Bit memory
Memory bits are objects of ³ CPU system memory, used for storing
intermediate results. They can be accessed in units of a bit, byte, word or
DWORD.
Bus
A bus is a communication medium connecting several nodes. Data transmission
can be serial or parallel across electrical conductors or fiber-optic cables.
Bus segment
A bus segment is a self-contained section of a serial bus system. Bus segments
are interconnected using repeaters.
Chassis ground
Chassis ground is the totality of all the interconnected inactive parts of a piece of
equipment on which a hazardous touch voltage cannot build up even in the event
of a fault.
Clock memories
Memories that can be used for clocking purposes in the user program (1 memory
byte).
Note
Note in the case of S7-300 CPUs that the clock memory byte is not exceeded in
the user program.
Glossary-2
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01