Siemens Simatic S7-400 - Manual (Page 143)

Digital Modules
4-21
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Terminal Assignment and Block Diagram of the SM 421; DI 32 24 VDC
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Process
Data register and bus control
Module
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Figure 4-3 Terminal Assignment and Block Diagram of the SM 421; DI 32 24 VDC