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What Are Digital and Analog I/O?

The two basic types of PLC I/O, signal levels (24V DC, 4-20 mA, 0-10 V) and their typical uses.

4 min readUpdated: 2026-07-05

Digital I/O is two-state (on/off, 0/1): buttons, sensors, relays, contactors. Analog I/O carries a continuous value: temperature, pressure, level, speed reference.

Digital I/O
Typically 24 V DC
Analog current
4–20 mA (noise-resistant)
Analog voltage
0–10 V
Over long distances 4-20 mA is preferred over 0-10 V; a current signal is less affected by cable resistance and noise.

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