16-channel 48-125 VDC input with railway-grade temperature range
The Siemens 6AG1321-1CH20-2AA0 is a SIPLUS SM 321 digital input module with 16 channels accepting 48 VDC to 125 VDC nominal input voltage. The wide signal-1 threshold of 30 VDC to 146 VDC means it registers a logic high across a broad range, tolerating voltage sag on long cable runs without false off-states. The EN 50155 certification (sections 4, 5, 12; T1, Category 1, Class A/B) confirms it for rolling-stock vibration, temperature cycling, and supply interruptions — not just a wider temp range but a full railway qualification.
Derating and channel count at elevated temperature
At 50 °C and below, all 16 channels are usable at any input voltage. At 60 °C, the module still delivers 8 channels at 48 VDC but drops to 6 channels when the input voltage reaches 146 VDC. At the 70 °C ceiling, it supports 6 channels at 48 VDC and only 4 at 146 VDC. This derating is driven by internal dissipation at higher input current and ambient — plan the channel assignment accordingly if the module sits near a heat source. Typical power loss is 4.3 W, and the module draws 40 mA max from the 5 VDC backplane bus. The 20-pin front connector (required, not included) carries the field wiring; unshielded cable runs up to 600 m, shielded up to 1000 m.