Every signal a controller reads or writes is either discrete or analog, and knowing the difference is fundamental to understanding any automation system. Discrete I/O deals in on/off states; analog I/O deals in continuous measured values. This guide explains both, the four categories DI, DO, AI, and AO, and how each type maps from a field device into a PLC, RTU, and SCADA.
Discrete vs Analog I/O in one line: Discrete (digital) I/O handles two-state on/off signals such as a switch being open or closed, while analog I/O handles continuously variable signals such as a 4-20 mA pressure or level measurement spanning a full range.
Discrete I/O, also called digital I/O, represents a signal that has only two states - on/off, open/closed, running/stopped, or true/false. A discrete input (DI) reads the state of something: a pump run status, a limit switch, a high-level float, an ESD pushbutton, a valve's open or closed position feedback. A discrete output (DO) commands a two-state device: energize a relay, open a solenoid valve, start a motor, turn on a beacon.
Electrically, discrete signals are simply a voltage present or absent (for example 24 VDC or dry contacts). One discrete point equals one bit of information. Discrete I/O is the backbone of interlocks, shutdowns, and sequencing, where the question is always binary: is it on or off, safe or tripped?
Analog I/O represents a value that varies continuously across a range. An analog input (AI) reads a transmitter - pressure, level, flow, temperature - most commonly as a 4-20 mA current loop, where 4 mA is the bottom of the range and 20 mA is the top. An analog output (AO) drives a modulating device to a specific position, such as sending 4-20 mA to a control valve positioner to hold it 37% open.
Inside the controller, an analog signal is converted to a number by an analog-to-digital converter and then scaled from raw counts into engineering units - so 12 mA might become 50 psi. From there, discrete and analog points alike become tags in the PLC or RTU that a SCADA host reads over a protocol like Modbus or DNP3. A well-built system labels every point clearly by type, because a shutdown interlock (discrete) and a trend of tank level (analog) are handled very differently.
Discrete (digital) I/O carries two-state on/off signals like a switch or motor status, while analog I/O carries continuously variable measurements like a 4-20 mA pressure or level reading across a full range.
Discrete Input (reads an on/off state), Discrete Output (commands an on/off device), Analog Input (reads a continuous measurement), and Analog Output (drives a modulating device such as a control valve).
It is a current loop where 4 mA represents the low end of a measurement range and 20 mA the high end. Because a healthy signal is never zero, a reading of 0 mA clearly indicates a broken wire or fault, which is why it is the standard for analog signals.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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