8 AC inputs at 120/230 V — signal thresholds and isolation
The 6ES7321-1FF01-0AA0 is an 8-channel digital input module for the SIMATIC S7-300 rack, accepting 120/230 V AC (47–63 Hz) on each channel. The signal-1 threshold is 79–264 V AC, drawing 6.5 mA typical at 120 V and 11 mA at 230 V; signal-0 is 0–40 V. That 79 V turn-on floor means it cleanly discriminates line-level AC signals from leakage or coupled noise on long runs — important when the unshielded cable length reaches 600 m. Channel-to-channel isolation is grouped in pairs (groups of 2), with optocoupler isolation between the input channels and the backplane bus. The isolation is tested at 4000 V DC, so the module handles mixed AC supplies on adjacent groups without coupling issues.
Lifecycle status — end-of-life hot, sourcing reality
The module draws 29 mA max from the 5 V DC backplane bus and dissipates 4.9 W typical power loss. It requires a 20-pin front connector (not included). The form factor is 40 mm wide, 125 mm high, 120 mm deep — standard S7-300 single-width slot.
Integration notes — wiring, diagnostics, and 2-wire sensors
The module accepts 2-wire proximity sensors, with a permissible quiescent current up to 2 mA — so standard AC proximity switches with a dark current below that threshold will not false-trigger the input. There is no parameterisable input filter; the switching delay is fixed at 25 ms for both 0→1 and 1→0 transitions, which is adequate for 50/60 Hz AC signals but means fast pulse trains (above ~20 Hz) will be missed. No diagnostic alarm, no hardware interrupt, no group error LED — the only status indication is a green LED per channel. The module is not isolated between channels (only between channel groups and the backplane).