Siemens Simatic S7-300 CPU 31xC Series Installation And Operating Instructions Manual page 264

Glossary
System diagnostics
System diagnostics refers to the detection, evaluation and signaling of errors which occur
within the PLC, Examples of such error/faults include: Program errors or failures on modules.
System errors can be indicated by LEDs or in STEP 7.
System function
A system function (SFC) is a --> function integrated in the operating system of the CPU that
can be called when necessary in the STEP 7 user program.
System function block
A system function block (SFB) is a --> function block integrated in the operating system of
the CPU that can be called when necessary in the STEP 7 user program.
System memory
System memory is an integrated RAM memory in the CPU. System memory contains the
address areas (e.g. timers, counters, flag bits) and data areas that are required internally by
the operating system (for example, communication buffers).
System status list
The system status list contains data that describes the current status of an S7-300. You can
always use this list to obtain an overview of:
• The configuration of the S7-300
• the current CPU configuration and configurable signal modules
• the current status and processes in the CPU and in configurable signal modules.
Terminating resistor
The terminating resistor is used to avoid reflections on data links.
Timer
See Timers
Timers
Timers are part of CPU system memory. The content of timer cells is automatically updated
by the operating system, asynchronously to the user program. STEP 7 instructions are used
to define the precise function of the timer cell (for example, on-delay) and to initiate their
execution (for example, start).
TOD interrupt
See Interrupt, time-of-day
Glossary-22
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05