Software 4.1.1.3 Influence of timer interrupts on the cycle time The programmable timers depend on highest precision. This is ensured by a quartz crystal and the relevant fre- quency dividers that generat impulses which, in turn, gene- rate interrupts at the intervals set by the programmable ti- mers (10 ms, 100 ms, 1 s, 10 s). If the timers are enabled, these timer interrupts lead to the current time values being incremented or decremented which means that the timer outputs may have to be adju- sted. This is added to by the updating of the clock pulse markers (C00.00-03).
Processing of the current program is therefore to be inter- rupted, the contents of the CPU registers is to be saved and stored for continuation later. 4.1.1.3.1 Extension of the cyle time The amount of time by which the cycle is extended due to the handling of timer interrupts depends on the number of currently active programmable timers. Worst case Every 10 ms, the PLC cycle is extended by c. 2 ms if all of the 32 possible timers have been programmed as clock pulses with the same time on the basis of 10 ms and if they are all enabled. Best case In the best case, the cycle time is extended by only 0.4 ms. 39