AI, AO, DI, DO, I/O Signal Types
AI, AO, DI, and DO are the four I/O signal types every field device maps to. AI, analog input is a continuous measurement coming in, such as 4-20 mA from a transmitter. AO, analog output is a continuous command going out, such as a valve position. DI, digital input is a discrete on or off state coming in, such as a switch. DO, digital output is a discrete command going out, such as a motor start. Classifying every tag into one of these four is what turns a P&ID into an I/O list.
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Signal type, also called signal class, is the single most important attribute on an I/O list because it drives how each tag connects to the PLC or DCS and which I/O card it consumes. The four types split on two axes. Direction, input to the controller, or output from it and form, analog, a continuous value, or digital, a discrete two-state. Analog input carries a measured value such as flow, pressure, temperature, or level, usually as a 4-20 mA current loop and increasingly with a HART digital overlay. Analog output carries a continuous command, most often a valve position or a variable speed drive reference. Digital input carries a discrete state from the field, such as a limit switch, a level switch, a pushbutton, or a running or fault contact. Digital output carries a discrete command to the field, such as a solenoid valve, a motor start, or an indicator lamp. Every tag on a P&ID that physically connects to the control system is one of these four, and the count of each type is what sizes the I/O cards, the cabinets, and the commissioning effort.
The four signal types.
AI, analog input. A continuous measured value entering the controller, typically 4-20 mA from a transmitter, FT, PT, TT, LT. AO, analog output. A continuous command leaving the controller, typically a control valve position or a drive speed reference, FV, the AO half of a control loop. DI, digital input. A discrete on or off state entering the controller from a switch or status contact, ZSH, ZSL, PSH, a motor running contact. DO, digital output. A discrete command leaving the controller to a solenoid, motor starter, or lamp, XY, a motor start signal. Input and output are always named from the controller's point of view. A signal the PLC reads is an input, a signal the PLC drives is an output.
How to classify a tag.
Read the tag and the symbol together. A transmitter, T as the second letter, like FT or PT sends a continuous measurement, so it is AI. A control valve or modulating final element takes a continuous command, so it is AO. A switch, S as a letter, like PSH, LSL, ZSH reports a discrete state, so it is DI. An on or off command to a solenoid, motor, or lamp is DO. The decision is about what physically crosses the terminal. A measured value or command that varies continuously is analog. A two-state signal is digital. A single physical instrument can produce more than one signal, which is why classification is per signal, not per device.
Why signal class drives I/O card count.
Analog and digital signals use different I/O cards, so the count of each type is the direct input to hardware sizing. Each analog input consumes one AI channel. Analog inputs drive the number and type of analog input cards. Analog outputs consume AO channels on analog output cards. Digital inputs and outputs consume DI and DO channels on discrete cards, which are usually denser and cheaper per point. Summing AI, AO, DI, and DO across the I/O list, adding a spare allowance, and dividing by channels per card gives the chassis and card count. An error in the signal class column therefore feeds straight into under-specified or over-specified hardware and into long-lead procurement.
Analog and discrete in practice.
Most regulatory control loops contribute one AI, the transmitter and one AO, the valve. Most on-off and motor control contributes DI, status and limit feedback and DO, the command. A simple motorized valve can be several signals at once. A DO to open, a DO to close, and two DI limit switches for open and closed feedback. Counting signals rather than devices is what makes the I/O list match the real channel demand, and it is the difference between a tag count and an I/O count.
Soft tags and special cases.
Not every signal is a plain 4-20 mA or dry contact. HART overlays digital data on a 4-20 mA AI loop and is usually still counted as one AI channel with HART capability noted. RTD and thermocouple inputs are analog but need a specific temperature input card, not a standard 4-20 mA AI card, so they are often tracked as their own subtype. Pulse and frequency inputs, from flow meters need a dedicated card. Soft tags or internal tags exist only in the control logic and consume no physical I/O at all. A complete I/O list flags these special cases so the card selection is right the first time.
The four I/O signal types.
Every physically connected tag on a P&ID maps to exactly one of these four. Direction is always from the control system's point of view.
| Code | Direction | Form | Typical device | Example tag |
|---|---|---|---|---|
| AI | Input | Analog, continuous measurement | Transmitter, 4-20 mA or HART | FT-101 |
| AO | Output | Analog, continuous command | Control valve, variable speed drive reference | FV-101A |
| DI | Input | Digital, discrete state | Limit switch, level switch, status contact | ZSH-201 |
| DO | Output | Digital, discrete command | Solenoid, motor starter, indicator lamp | XY-301 |
Frequently asked.
What is the difference between AI and DI.
AI, analog input is a continuous measured value entering the controller, such as a 4-20 mA flow signal that varies across a range. DI, digital input is a discrete two-state signal entering the controller, such as a switch that is either open or closed. Analog carries a value. Digital carries an on or off state.
How do you decide if a tag is AI or AO.
Direction decides it, always from the controller's point of view. A transmitter sends a measurement into the controller, so it is an analog input, AI. A control valve or drive receives a continuous command from the controller, so it is an analog output, AO. The same loop usually has one of each.
Does a control valve count as one signal or two.
It depends on the valve. A simple modulating control valve is one AO. A motorized on-off valve can be several signals. A DO to open, a DO to close, and DI limit switches for open and closed feedback. The I/O list counts signals, not devices, which is why a valve can occupy more than one channel.
How are HART, RTD, and thermocouple inputs classified.
All three are analog inputs, but they are not interchangeable on the card. A HART transmitter is one AI channel on a HART-capable card. RTD and thermocouple inputs need temperature input cards rather than standard 4-20 mA AI cards. A good I/O list notes the subtype so the right card is selected.
How does signal class affect PLC sizing.
Card selection is driven by signal class. Each AI, AO, DI, and DO consumes a channel on a card of its own type. Summing each class across the I/O list, adding spare capacity, and dividing by channels per card gives the chassis and card count. An error in the signal class column flows straight into under-specified hardware and missed long-lead procurement.