PLC inputs are the eyes of a machine — sensors, switches and analog signals telling the logic what is happening in the real world. When an input goes wrong, the machine often refuses to run, runs intermittently, or behaves oddly. This guide covers a structured approach to input faults.

Manufacturer procedures differ

PLC addressing, forcing, diagnostics and online monitoring vary by manufacturer (Siemens, Allen-Bradley, Mitsubishi, Omron and others all differ). The principles below are generic — always use your manufacturer's documentation for the specific steps on your platform.

A structure for input faults

  1. Define the symptom — is the input stuck on, stuck off, or flickering?
  2. Check the field device — is the sensor or switch physically present, aligned and powered?
  3. Check the wiring — continuity, correct voltage, correct common, no damage.
  4. Check the input status at the PLC — does the input LED agree with the field device? Does the logic status agree with the LED?
  5. Check permissives and interlocks — is this input feeding a permissive that is holding the machine stopped?
  6. Interpret the signal — compare what the input is showing with what the process should be doing.

Common failure modes

| Pattern | Likely cause | |---|---| | Input stuck off | Failed sensor, broken wire, lost supply | | Input stuck on | Shorted sensor, welded switch, wire short to common | | Intermittent / flickering | Loose connection, vibration, failing sensor, noise | | Wrong intermittently on analog | Water ingress, poor shielding, calibration drift |

Permissives and interlocks

Many input faults stop the machine not directly, but because the input feeds a permissive (e.g. "guard closed", "level OK", "motor ready"). If the machine "won't start", trace which permissives are false and which input feeds each. The fault is often a single input holding everything else up.

Key points

  • Always cross-check the field device, the input LED and the logic status — a mismatch tells you where to look.
  • A single failed input can stop an entire line through a permissive.
  • Use forcing and monitoring only as a diagnostic, and only within your site's procedures — never to run a machine with a safety input bypassed.