A digital PLC input represents a defined state, such as a switch being on or off. An analog input reads a changing signal from a compatible sensor or instrument. Check the device output, PLC module, voltage or current range, channel count, wiring and configuration for the exact equipment.
Choose a PLC input to match the sensor or switch output. A digital input represents a defined state; an analog input reads a changing signal. The module and field device specifications determine compatible voltage or current ranges and wiring.
PLC input comparison chart
| Check | Digital input | Analog input |
|---|---|---|
| Signal meaning | Defined state, such as on or off | Changing value within a supported range |
| Example device | Push button, limit switch or compatible sensor output | Compatible measurement transmitter |
| Module details | Signal voltage, channel count and input type | Voltage or current range, resolution and channel count |
| Configuration | Input addressing and state logic | Range scaling and engineering units where supported |
| Common check | Input polarity and wiring | Signal range, grounding, shielding and scaling |
Siemens lists separate digital input/output modules and analog expansion modules in its automation product information. These are examples of product families; use the exact PLC and sensor manuals for a project.
Match the field signal
- Read the sensor or switch datasheet and record its output type and range.
- Check the PLC module’s supported signal type, channel count and electrical details.
- Confirm controller, module and wiring compatibility in the manufacturer’s manuals.
- For an analog input, document the signal range and the scaling used by the program.
- Check grounding, shielding, isolation and installation requirements with the system documentation.
- Label spare channels and record the final module configuration.
Review how to choose a PLC, compare PNP and NPN sensor outputs, and browse PLCs and controllers.
Sources
Common questions
What is a digital PLC input?
It reads a defined input state from a compatible device, such as an on/off switch or sensor output.
What is an analog PLC input?
It reads a changing electrical signal within the input module's supported range.
Can an analog sensor connect to a digital input?
Not as a direct substitute. Match the sensor output to the PLC input type and module specification.
What should I check before selecting an input module?
Compare signal type and range, channel count, wiring, electrical isolation and controller compatibility in the manuals.