Siemens digital input output module sb 1221 series siemens 6es7221 3bd30 0xb0 the siemens 6es7221 3bd30 0xb0 is a sb 1221 digital input output signal module that converts the level of the external digital signals from the process to the internal signal level of the s7 1200 the 24v signal board is plugged direct into the simatic s7 1200 cpu and linked electrically and mechanically with the cpu in this way the installation dimensions of the cpu remain unchanged siemens 6es7221 3bd30 0xb0