S7 Scan/Cycle Time Problem

Good day

I have a simple program running on an S7-315 CPU. Scan cycle time is never more than 10ms EXCEPT when I attempt to monitor a block from my PC (via Ethernet Interface) in which case the cycle time shoots up to almost 500ms and the CPU obviously doesn't like this. I've never had this before and am frankly thoroughly puzzled...

Any tips anyone please?

Are you using a -PN CPU or are you communicating over the backplane via a CP?

In the latter case, is your IO also on the same backplane as the CP/CPU or is it separate over Profibus?

The other area where I've seen problems is having a mix of 10MBit/s and 100MBit/s on the same Ethernet link.

I am using a CP 343-1.
IO is over Profibus - using the built-in PB port of the CPU.
Will check my link speeds - my PC is 1000Mbit/s connected to switch, CP is 100MB/s connected to separate switch, packets are routed via separate router.

This is very odd indeed. It should not do like that.

Is it the same if you remove the LAN from the CP343-1 and connect directly from your PC, i.e. there are no routers or switches in between ?

And, in the CPU Properties .. Cycle/Clock memory tab, what is Scan cycle load from communication set to ? Normal is 20%

You can also try to go to CPU Properties .. Protection tab, and select Mode = Process mode. And then set Permissible cycle increase via test functions = 5 ms.

And finally, are there any entries in the diagnostics buffer ?