Homemicro.co.uk explains the difference
HVAC systems are controlled by Building Management Systems (BMS) or Building Automation Systems (BAS) using Direct Digital Control (DDC) devices which can receive and process inputs and send command outputs.
The inputs are either Analogue Input (AI) or Digital Input (DI) reactions and the outputs are analogue Output (AO) or Digital Output (DO) actions.
Analogue (or analog): having continuously variable (modulating) characteristics representing physical measurements
An analogue signal is a continuous signal that contains time-varying quantities representing physical measurement. An analogue input is a variable signal that either measures a voltage (0-10V) or current (0-20mA) from an input device and an analogue output will supply a variable voltage or current signal. Analogue I/O is used for devices which are continuously changing or variable.
Digital: signals that have discrete (individually separate and distinct) values at each sampling point
Digital refers to the state of a single-bit, or binary (0 or 1) signal. This binary signal has two states: ON (binary 1) and OFF (binary 0). Digital I/O is used for ON/OFF (Start/Stop) control or devices that trigger an action such as limit or fault switches.
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Analogue input (AI) refers to a continuously changing voltage or current signal to indicate a changing state. An example is a sensor that monitors physical data change over time, such as temperature, flow, or pressure.
An Analogue output (AO) is a variable signal to a process control such as a motor speed controller or a valve actuator. A variable AO signal to a motor will control the speed where 0V = OFF and 1-10vdc = 10%-100%. Similarly, a control valve can have a variable position between open and closed with a variable analogue output.
Digital inputs (DI) to a controller include plant status switches and inputs from other control, monitoring and alarm systems. DI will indicate the status of a device as either ON or OFF and can therefore indicate failure or fault status. With a 24Vdc control module 0V indicates the OFF state and 24V the ON state.
A digital input registers the state of an input from such devices as a tank level switch or a door-closed switch.
Digital outputs (DO) from a BMS outstation are used to switch equipment on or off (enable or disable a device),usually via relays or contactors.
Digital output signal contains the state of an output to such devices as a motor starter or panel signal light. A digital output will change or maintain a device status by performing momentary or maintained switching to start/stop pumps, fans, etc.
DO can be used to operate two-position solenoid valves (open/closed), relays, indicating lamps, etc.
A circuit that is open with no current flow when the contacts are not actuated. That is, the switch is 'off' and there is no electric circuit or signal.
A circuit where a contact is closed in the normal state allowing electrical current flow.
A sensor translates a physical property (variable – pressure, temperature, humidity etc.) to an electrical signal. In broad terms, sensors provide variable measurement and are therefore associated with analogue points. Intelligent sensors which convert the measured value or status of the measured variable into a digitally encoded signal are available.
BMS controls analogue devices include resistance thermometers, potentiometers and voltage or current-operated devices, normally functioning in the range of 0-10V or 4-20mA.
Analogue sensors will measure the variable and convert the value into an electrical signal which is input to other devices for measurement and control purposes. Usually functioning within in a range or scale where 0V represents the lowest measured value and 10V the highest.
A switch will trigger at a specific set point (fixed – pressure, temperature, humidity, etc.). Switches infers digital control with [on/off] status indication.
Variable speed drives (VSD) are used on motors for pumping fluids or moving air to regulate flow and conserve energy. A VSD is usually controlled by an analogue input from the BMS to modulate the motor speed, however a digital input could be used to start and stop the motor automatically. In the latter case the motor speed would be fixed and set at commissioning.
An analogue output could be used to provide speed, current and torque signals to the BMS.
A digital output would normally be used to indicate a fault with the VSD.
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AI-AO vs DI-DO id: hvac-05