SIMATIC S7-300 SIEMENS 6ES7321-1BL00-0AA0 SM 321 Digital Input Module A digital input module for collecting discrete field signals in SIMATIC S7-300 automation systems. Module Overview Discrete Signal Interface Field-level switching information is transferred to the S7-300 control program. Input Signal Collection The SM 321 receives switching information from field devices and makes the resulting input states available to the controller. Siemens specifies its use with