In STEP 7 (TIA Portal) V15.1, why is it not possible to configure as isochronous centrally slotted modules in S7-1500 clock-synchronized?
With firmware V2.6 and higher of the S7-1500 CPUs isochronous mode is supported on the central backplane bus. The reason for the behavior described above is a non-updated module description of the centrally slotted modules. It is not enough just to update the firmware of the S7-1500 CPUs.
Update the module description of each module. For this you open the "Properties" of the module and in the "General" tab under "Catalog information" you click the "Update module description" button.
Then save and compile the device configuration via the menu command "Compile > Hardware (rebuild all)". You can then configure the centrally slotted modules as isochronous.
You can have multiple modules of the same type upgrade synchronously.
More information is available in the manual "SIMATIC STEP 7 Basic/Professional V15 and SIMATIC WinCC V15" under "Update Module Version".