What this SM 321 module does on an S7-300 rack
The Siemens 6ES7321-1BH81-0AA0 is a 16-channel digital input module from the SM 321 family. It reads 16 discrete 24 VDC field signals — limit switches, proximity sensors, pushbuttons — and optically isolates each channel from the backplane, so a fault on one field wire stays on that wire and doesn't take out the CPU.
Lifecycle reality: EOL hot, not dead yet
For a line-down situation or a BOM freeze, the practical move is to source this exact order code while it's still available through independent channels.
What the 16 x DC 24 V optically isolated spec means on site
Sixteen channels at 24 VDC — that's enough for a small station's worth of field sensors without needing an expansion rack. Each channel is optically isolated, which means the 24 V field wiring and the 5 V backplane logic are separated by an optocoupler. In practice, this prevents ground loops between the field devices and the PLC, and it means a short on one input won't cascade to the CPU or other modules. The module snaps onto the DIN rail and connects to the backplane bus via the standard front connector. The 24 V supply for the field side comes through the module's own terminals, not the backplane — so if you lose field power, only this module's inputs drop out, not the whole rack.