What this module does in an S7-300 rack
The Siemens 6ES7321-7BH00-0AB0 is a SIMATIC S7-300 SM 321 digital input module that provides 16 optically isolated inputs at 24 VDC, with a 20-pin front connector for field wiring. It supports both diagnostic alarms and hardware interrupts, so the CPU can immediately flag an external fault or a change-of-state on any input without polling the module.
Input characteristics and wiring
The module accepts 2-wire proximity sensors with a permissible quiescent current up to 1.5 mA, and the input filter is parameterizable to 0.1 / 0.5 / 3 / 15 / 20 ms — choose the slower setting to debounce mechanical contacts or the faster one for high-speed sensor edges. Signal "1" is detected from 13 to 30 V (typical current draw 7 mA), while signal "0" is -30 to +5 V, giving a clean threshold at 24 VDC nominal with a permissible range of 20.4 to 28.8 V DC. All 16 inputs share a common group isolated from the backplane bus via optocouplers, tested at 500 V DC — no channel-to-channel isolation, so the field supply must be common across all inputs.
The module snaps onto a DIN rail in the S7-300 rack, occupying one slot with a width of 40 mm, depth of 120 mm, and height of 125 mm. Power draw from the backplane bus is 55 mA at 5 V DC, plus up to 40 mA from the load voltage L+ (unloaded), with a typical power loss of 4 W.