Siemens 6ES7421-1FH00-0AA0 Digital Input Module — Legacy Replacement. Siemens 6ES7421-1FH00-0AA0 is the exact manufacturer order number for a digital input module in the SIMATIC S7-400 portfolio. Siemens describes this exact reference as: SIMATIC S7-400, SM 421 DIGITAL INPUT MODULE, OPTIC. ISOLATED, 16 DI, 120/230V AC. The verified manufacturer lifecycle record is PM500:Discontinued Product or end of PLM & Support.