Direct Digital Control

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Direct Digital Control refers to the ability to control HVAC devices via programming.

[edit] Overview

Central controllers and most terminal unit controllers are programmable, meaning the direct digital control program code may be customized for the intended use. The program features include time schedules, setpoints, controllers, logic, timers, trend logs, and alarms. The unit controllers typically have analog and digital inputs, that allow measurement of the variable (temperature, humidity, or pressure) and analog and digital outputs for control of the medium (hot/cold water and/or steam). Digital inputs are typically (dry) contacts from a control device, and analog inputs are typically a voltage or current measurement from a variable (temperature, humidity, velocity, or pressure) sensing device. Digital outputs are typically relay contacts used to start and stop equipment, and analog outputs are typically voltage or current signals to control the movement of the medium (air/water/steam) control devices.Usually abbreviated as "DDC".

When DDC controllers are networked together the 'control system' may speak 'proprietary' or 'open protocol' language such as BACnet. (Building Automation Control Network) These systems may be mated with a software package that graphically allows operators to monitor, control, alarm and diagnose building equipment remotely.