Siemens 6ES7521-1BL00-0AA0 Manual page 16

Address space
4.1 Address space
Value status (Quality Information, QI)
The meaning of the value status depends on the submodule involved.
For the 1st submodule (=basic submodule), the value status is not relevant.
For the 2nd to 4th submodule (=MSI submodule), the value status 0 indicates that the value
is incorrect or the basic submodule has not yet been configured (not ready).
The figure below shows the assignment of the address space with submodules 1 and 2.
Figure 4-3 Address space for configuration as 1 x 32-channel DI 32x24VDC BA MSI with value status
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Digital input module DI 32x24VDC BA (6ES7521-1BL10-0AA0)
Manual, 12/2016, A5E32363711-AD