S7-1200 SM1221 Digital Inputs

S7-1200 SM1221 Digital Inputs is the Siemens S7-1200 signal-module branch for adding 24 VDC digital input capacity when the PLC platform is already fixed and the remaining choice is how many field input points the station needs. Compared with SM1222 digital outputs, SM1221 is the input-side family for reading switches, sensors, and status signals rather than driving loads. Compared with SM1231 analog inputs, it is the simpler branch when the field devices are discrete on/off points and the project does not need analog measurement channels or scaling behavior. Keep this family in scope when the controller is already S7-1200 and the next question is how much discrete input expansion belongs in the rack. Read the product list by input count and electrical fit first, then narrow by wiring density, terminal layout, and the exact field-device mix.