Glossary · PLC and control programming
PLC input
Also known as: Digital input, Analog input, I/O input
German: Eingang (SPS)
In PLC technology, an input is a connection point on an input module or on the controller through which a field signal, such as a sensor contact or an analog measurement, is fed into the controller and made available to the program.
- PLC programming
In one sentence
A PLC input connects a field signal, such as a sensor contact or analog measurement, to the controller and its program.
Example
The inductive proximity switch at the end position of a cylinder is wired to digital input %I4.2, which the program reads as CylinderExtended.
How it applies
- Engineering: Digital inputs read on/off signals; analog inputs read ranges such as 4 to 20 mA or 0 to 10 V; special inputs count pulses or capture fast events. Input type, signal level, sinking or sourcing wiring and filter times are defined in the hardware configuration and the electrical drawings.
- Commissioning: Every input is checked with the real device (loop check): the sensor switches, the correct address changes in the program, and the symbolic name fits.
- Maintenance: Status LEDs and online views help locate faulty sensors and wiring. Wire-break detection on analog inputs and plausibility logic improve Sensor data integrity.
- Documentation: The I/O list, which maps each input to device tag, address, symbol, signal type and meaning, is a core document for commissioning and service. Keep it synchronized with the program's symbol table.
PLC input vs. input variable
A PLC input is a physical signal on a module. An Input variable is a software parameter of a program unit. Name them differently to avoid confusion in documentation.