Siemens SIMATIC S7-300 Hardware And Installation Manual page 250

Glossary
Timers
Timers are part of CPU --> system memory. The contents of the "timer cells" are
updated automatically by the operating system asynchronously to the user
program. STEP 7 instructions are used to define the exact function of the timer
cells (for example on-delay) and initiate their execution (e.g. start).
Cycle Time
The term cycle time describes the time required by a --> CPU to run through a -->
user program once.
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-02