Instrumentation
Digital Inputs
Sinking/sourcing, debounce, and contact wiring behavior.
Overview
Digital inputs carry binary field state into the controller; correctness depends on matching sensor output type, input wiring convention, and debounce handling.
Engineering purpose
Deliver clean, debounced, correctly-referenced on/off signals so logic acts on real state changes rather than electrical noise.
How it works
PNP (sourcing) sensors feed sinking inputs and vice versa; the input threshold defines on/off voltage bands; mechanical contacts bounce for milliseconds and need logic or hardware debounce; status LEDs reflect terminal state.
- Match sensor output type (PNP/NPN) to the input card's sinking/sourcing configuration.
- Debounce mechanical contacts in logic (10–50 ms) to avoid false transitions.
- Chattering inputs — rapid repeated transitions — usually indicate wiring or mechanical faults.
Common faults
PNP/NPN mismatch reading permanently off; chatter from unshielded runs beside VFD cables; bounce double-triggering counters; dry contacts wetted with the wrong common reference.
Diagnostic checks
- 1Measure voltage at the input terminal against the module specification.
- 2Compare the input status LED with the logic state to separate field from program issues.
- 3Compare the input status LED against the logic state — disagreement isolates field wiring from program faults instantly.
Safety notes
Protective stop signals must use safety-rated inputs with line monitoring, not standard DI channels, regardless of how clean the wiring looks.
Commissioning notes
Exercise every input from its field device (not jumpers at the terminal) and verify debounce times against the fastest legitimate signal.
Related concepts
Sinking/sourcing, input thresholds, debounce, signal isolation, LED-versus-logic diagnosis.
digital input · di · dq · pnp · npn · dry contact · sinking · sourcing · ورودی دیجیتال · کنتاکت خشک
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When Hermes Brain uses this article
Cited for inputs reading wrong or chattering, sensor-to-card mismatches, and noise-induced false transitions.
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