Glossary · Electrical engineering and control panels
Digital input
Also known as: DI, Discrete input, Binary input
German: Digitaleingang
In industrial control, a digital input is a controller input channel that detects one of two signal states, typically the presence or absence of 24 V DC, from devices such as pushbuttons, limit switches or proximity sensors.
- Control panels
In one sentence
A digital input detects one of two states, typically 24 V DC present or absent, from switches and sensors.
Example
An inductive proximity sensor on a cylinder end position is wired to a digital input; the PLC sees TRUE when the piston is extended.
How it applies
- Engineering: IEC 61131-2 defines input types (Type 1, 2 and 3) with voltage and current thresholds, which determine which sensors can be connected. Choose sinking or sourcing inputs to match the sensors, and set input filter times to suppress contact bounce without missing short signals.
- Commissioning: Check every input in the I/O check: actuate the device and confirm the right address changes state.
- Documentation: The I/O list maps each input address to its device, tag and meaning, including whether the signal is normally open or normally closed. For safety-relevant signals, document that they use safety inputs, not standard ones.
Digital input vs. safety input
A standard digital input has no fault detection. A safety input on a safety controller or Safety relay uses redundancy and diagnostics, so it can be part of a safety function.