Siemens Simatic series Manual page 29

Parameters/address space
4.2 Address space
Feedback interface
The figure below shows the address assignment of the module in the process image input.
The feedback interface for the counters (channels 0 and 1) begins at input byte x+4.
Via the feedback interface, the user program receives current counted values and status
information from the module.
Figure 4-7
28
Assignment of the address space of the feedback interface of the DI 16x24VDC Count
(bytes 0 to 11)
DI 16x24VDC HF Digital Input Module (6ES7521-1BH00-0AB0)
Manual, 08/2018, A5E03485952-AG