Discrete I/O (Digital Inputs and Outputs)

Discrete I/O are controller channels that handle two-state signals: inputs sense on or off conditions from devices such as push buttons, limit switches and proximity sensors, while outputs switch loads such as relays, solenoids, lamps and contactors.

A discrete input module converts a field voltage, commonly 24 V DC in modern machines or mains AC in some installations, into a logic 1 or 0 that the PLC reads each scan. Optical isolation separates field wiring from controller electronics, and input filters suppress contact bounce and electrical noise. Discrete outputs use transistors for fast DC switching, relays for mixed or higher-power loads, or triacs for AC loads, each with rated current and switching limits.

Discrete signals cover much of the I/O on typical machines: start and stop buttons, guard door switches, part-present sensors, valve solenoids, indicator lamps and motor contactors. They can be wired directly to modules in the controller rack, or to remote I/O stations and IO-Link masters near the machine to reduce cable runs.

Selection involves voltage type and level, sourcing or sinking configuration, channel density, isolation, response time and diagnostics such as wire-break or short-circuit detection. Inductive loads need suppression to protect output contacts and limit interference. Standard discrete I/O must not be used for safety functions, which require safety-rated I/O, and field wiring must follow OEM documentation and applicable electrical standards, carried out by qualified personnel.

Key points

Handle two-state signals from switches and sensors and to lamps and solenoids
24 V DC is common in modern machines; AC inputs exist in some installations
Output types include transistor, relay and triac, each with rated limits
Optical isolation and input filtering improve noise immunity
Safety functions need safety-rated I/O, not standard channels

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Related terms

Terms that refer to Discrete I/O (Digital Inputs and Outputs)

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