Siemens Simatic S7-400 - Manual (Page 173)

Digital Modules
4-51
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Terminal Assignment and Block Diagram of the SM 421; DI 16 120/230 VUC
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Data register and bus control
Process Module
Figure 4-11 Terminal Assignment and Block Diagram of the SM 421; DI 16 120/230 VUC