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Direct digital control (DDC) is the automated control of a condition or process by a digital device (computer).[1][2] DDC is considered by many to be a more modern, granular and responsive update to older HVAC control systems based upon PLC technologies. In those older PLC based systems, each zone was self-sufficient and contained all of the instrumentation and control elements needed to consider analog and digital inputs and then take actions according to rules. The complexity came from the desire to expand these 'zones' from a few dozen points and a handful of controlled elements to much broader building-wide systems. Connecting PLCs together becomes complex, and the creation of rules which would be loaded individually into each PLC impractical.
DDC on the other hand takes a more centralized network-oriented approach. All instrumentation is gathered by various analog and digital converters which use the network to transport these signals to the central controller. The centralized computer then follows all of its production rules (which may incorporate sense points anywhere in the structure) and causes actions to be sent via the same network to valves, actuators, and other HVAC components that can be adjusted.
