Pinging AC800M Controllers, Responsetime gets up
when we ping our AC800M Controllers (800xA 6.1.0 RU1) the response time is typically 1 ms for some Minutes but then increases immediately to somewhat like 120ms and returns slowly to 1 ms. Then the game starts from the beginning.
When pinging several controllers at the same time, the increase of responsetime is not synchron
Any idea what can cause such a behaviour?
Voted best answer
The 61131 tasks are scheduled on a strict periodicity whereas your "ping" have intermittent firing timing... :-)
AC 800M execute 61131 application code with higher priority than it exchanges Ethernet telegrams. The Task Delay you can (=should!) insert acts as "minor breathing holes" where the controller can expedite Ethernet telegrams with the surrounding world. With continuous 61131 execution (Task Delay = 0) the code will execute up to 700ms when the Cyclic Task Load maximum will suspend 61131 execution for up to 300ms to meet the Cyclic Load Limit of 70%.
At start of ping you may unknowingly hit a "hole" in the 61131 workload schedule, but as your ping eventually gets off-phase (by its own nature) you will sooner or later "hit" a 61131 code block and notice a delay in ping response. Some time later the delay is magically "gone" (because you are once more in a Task Delay "sweet spot").
A well tuned PA-controller will not have any 61131 block executing more than 200ms (lesser is better) and have a number of manually inserted Task Delays to disperse the "free time" when Ethernet communication can take place. For a HI-controller the maximum recommended continuous task execution time is 100ms.