In this article5 sections
AI is analog input, AO is analog output, DI is discrete input, DO is discrete output. They are the four signal classes carried on an I/O list, one per row, and each one names the kind of controller channel that field signal lands on.
| Class | Stands for | Signal | Typical device | Example |
|---|---|---|---|---|
| AI | Analog input | Continuous, field to controller, usually 4-20 mA | Transmitter | PT-101, pressure transmitter |
| AO | Analog output | Continuous, controller to field, usually 4-20 mA | Control valve positioner, variable-speed drive | FCV-302, flow control valve |
| DI | Discrete input | Two-state contact, field to controller | Switch, position limit switch, motor run feedback | PSH-401, pressure switch high |
| DO | Discrete output | Two-state command, controller to field | Solenoid coil, motor starter, on and off valve | XV-301, block valve |
DI and DO are also written digital input and digital output. The two names describe the same two-state channel. The class decides which PLC card the tag lands on, so it drives the card count and the cabinet sizing long before anyone lands a wire.
Every class on the I/O list ends here: a wire on a terminal, on a specific card. Get the class wrong and the wrong card gets ordered.
The four signal classes in plain terms
AI, Analog Input. A continuously variable signal arriving at the controller from a field device. In practice this is almost always a 4-20 mA current loop from a transmitter. Pressure transmitter PT-101 sends 4 mA at zero pressure and 20 mA at full scale, and the controller converts the current to engineering units. HART instruments also fall here: the 4-20 mA carries the process value and the HART overlay carries diagnostics.
AO, Analog Output. A continuously variable signal from the controller to a field device. The most common case is the 4-20 mA setpoint to a control valve positioner, which drives FCV-302 to the position that holds the flow setpoint. Variable-speed drive speed references also land here.
DI, Discrete Input. A two-state signal from the field to the controller: a closed or open contact, 24 VDC or no voltage. Pressure switches such as PSH-401, level switches, flow switches, motor run feedback contacts and position limit switches such as ZSO-501 are all DI.
DO, Discrete Output. A two-state command from the controller to a field device, which responds by opening, closing, starting or stopping. On and off block valves such as XV-301, motor starters, solenoid coils and alarm annunciators are all DO.
These four classes map directly to physical card types. On any platform, the channel count by class determines how many cards of each type go into the rack, so a wrong class shows up as the wrong cards on the procurement list.
Instrument type is not signal class
Two things are easy to run together on an I/O list, the instrument type and the signal class. The instrument type tells you what the device measures or does. The signal class tells you what kind of card it wires to. A pressure transmitter and a pressure switch both sense pressure. The transmitter sends a continuous signal and is AI. The switch sends a contact closure and is DI. Two instruments on the same variable, two different card types.
The signal class, not the instrument type, is what the I/O list is recording. Two devices can sense the same variable and still land on different cards.
For the ISA 5.1 tag structure itself, see the ISA 5.1 identification guide. For bubble shapes and signal line types, see the P&ID instrument symbols guide. For the definitions on their own, see what AI, AO, DI and DO mean.
Count signals, not bubbles
The I/O list counts wired signals. Many instruments are one channel. Local gauges, sight glasses, hand valves such as HV-305 and unwired primary elements are none. A device with several wired connections is several rows: a motorised on and off valve with open and close commands and two position limit switches is four channels, not one.
The class is a property of the wiring, which is why an engineer confirms it against the design before the list is issued. Where the design leaves it open, the row is resolved by the engineer, not guessed.
Why the class matters downstream
A wrong class moves count from one card type to another. A modulating valve placed on a discrete output card has no analog signal at commissioning. A hand valve given a channel creates a phantom point that runs through the punch list. Neither breaks the process, and both cost more the later they are found, which is why the signal class column deserves a deliberate check before the I/O list goes to the programmer and to procurement.