Dear Experts,

I’m trying to study and learn more deeply about PROFINET, and now there is an issue that really makes me confuse, which took a lot of time and effort to think about, but still cannot figure it out, please kindly give me some hints or help.

Here is the description:

S7-1500 as IOC and XC208 as IOD, try to setup the RT communication.

1. Set the send clock = 250us, then the update time can only set as 128ms, but cannot set to 256ms or 512ms:
   
   please refer to 1.png, 2.png, 3.png, 4.png in attachment.
   
   
   
2. Set the send clock = 500us, then the update time can only set as 128ms and 256ms, but cannot set to 512ms:
   
   please refer to 5.png, 6.png, 7.png, 8.png in attachment.
   
   
   

And TIA only tell me that “The set update time cannot be used” as above, or “a minimum update of xxx ms is required” when compile, and there is no other information that I can find:

please refer to 9.png in attachment.

And the properties of device XC208 as following (from GSDML file):

please refer to 10.png, 11.png, 12.png in attachment.

And to my understanding, SendCycle(UpdateTime)=ReductionRatio*SendClockFactor*TimeBase

Where

TimeBase =31.25us;

So, if UpdateTime=256ms, then there is available combination of ReductionRatio and SendClockFactor of the device (e.g. ReductionRatio=256, SendClockFactor=32).

So, what’s the reason that cannot set UpdateTime to 256ms or 512ms when Send Clock=250us?

And why cannot set UpdateTime to 512ms when Send Clock=500us?

Thank you so much for any help in advance!