Siemens Simatic S7-300 CPU 31xC Series Installation And Operating Instructions Manual page 261

Retentive memory
A memory area is considered retentive if its contents are retained even after a power loss
and transitions from STOP to RUN. The non-retentive area of memory flag bits, timers and
counters is reset following a power failure and a transition from the STOP mode to the RUN
mode.
Retentive can be the:
• Flag bits
• S7 timers
• S7 counters
• Data areas
Router
See Default Router
See Switch
A router works in a way similar to a switch. With a router, however, it is also possible to
specify which communications nodes can communicate via the router and which cannot.
Communication nodes on different sides of a router can only communicate with each other if
you have explicitly enabled communication via the router between the two nodes.
RT
See Real Time
Runtime error
Errors occurred in the PLC (that is, not in the process itself) during user program execution.
Segment
See Bus segment
SFB
See System function block
SFC
See System function
Signal module
Signal modules (SM) form the interface between the process and the PLC. There are digital
input and output modules (input/output module, digital) and analog input and output
modules. (input/output module, analog)
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05
Glossary
Glossary-19