Impact of Profibus I\O on AC800M Cyclic Load
Consider Configuration with Red. AC800M (PM866) and Red. CI854. All I/O modules used in configuraiton are S800 I/O's and are connected to Controller using FCI CI840.
1. Does I\O on Profibus cause an impact of CPU Cylic Load ?. In most of manuals I have read that profibus does not cause any impact on CPU Load as it is managed by CI854. But what about load for reading data structure from CI854 to Controller ?
2. How much of Cyclic Load is acounted for Variable copying (i.e Reading\ Writing a Variable value from\to I\O channel. This is up & above the cylic load contributed by Runtime POU instances for an application.
Everything is handled by the CI854, including copying the IO values ( which of course are "data structures" ) in and out of the CPU memory. There is no CPU load for this. None.
One thing to note is that updating IO is not synchronous with the execution of the application code. Updating IO channels happens at a rate dependednt on the speed of the Profibus. With program execution in the range of 0.5 to 1 seconds this is typically not noticable (assuming you have not reduced the bus speed) but it does mean that profibus is not suitable for some high speed applications such as positioning, rotation detection etc where the input may be changing many times per second. You need to use modulebus IO for this.