Siemens SIMATIC S7-400, SM 421 digital input module, optically isolated, 16 DI, 120/230V AC, order code 6ES7421-1FH00-0AA0.
The Siemens 6ES7421-1FH00-0AA0 is an SM 421 digital input module for the S7-400 rack. It gives you 16 optically isolated inputs rated for 120/230V AC — meaning you can wire line-voltage sensors (limit switches, pushbuttons, proximity detectors) direct into the PLC without an interposing relay. The optical isolation separates field wiring from the backplane, which keeps noise on the AC side from coupling into the CPU.
No direct pin-compatible alternative is documented in the available records. The SM 421 family includes other input modules, but differences in voltage rating, channel count, or isolation type mean a swap requires verifying the slot configuration and wiring scheme. Stick with the exact order code unless you are prepared to re-engineer the cabinet.
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