Siemens Simatic S7-400 - Manual (Page 176)

Digital Modules
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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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Figure 4-12 Terminal Assignment and Block Diagram of the SM 421; DI 16 120/230 VUC