What Is Direct Digital Control?

Jordan Weagly

Direct digital control usually refers to a system in which a digital device such as a computer automatically controls some process based on certain conditions. There are many applications for direct digital control, many of which are related to building automation or HVAC systems. In general, these kinds of systems include multiple components that are controlled from some central location.

A Nest thermostat, which uses direct digital control.
A Nest thermostat, which uses direct digital control.

To accomplish whatever task the system has been created to maintain, it might rely on collecting and processing data before changing conditions or attempting to meet certain specifications. For instance, a direct digital control system meant to monitor and maintain a certain temperature and air pressure within a building can monitor various factors. Should the temperature drop below acceptable levels, direct digital control systems can make the proper adjustments, whether those adjustments involve turning on more radiators or encouraging air circulation by turning on an array of fans and air ducts.

Sensors and encoders make it possible to automate tasks that require exact positioning and have minimal tolerance for error.
Sensors and encoders make it possible to automate tasks that require exact positioning and have minimal tolerance for error.

Many safety systems can also benefit from the technologies offered by direct digital control. For instance, a safety instrumented system might monitor the wellhead pressure of an oil well and take action if the pressure of that well exceeds what is considered safe. This type of application shows the benefit to this type of control, because a system build to monitor safety may be able to detect malfunctions long before human intervention is possible. Much like industrial control systems, this type of control may be essential to high-risk jobs such as drilling and extracting oil.

One of the most common applications for direct digital control is in HVAC systems. These systems input the sometimes complex settings required to heat an entire building or an entire complex of buildings using one energy source based on relatively simple user input. For instance, it's possible to type the desired room temperature into a thermostat and have the direct digital control system maintain that temperature almost indefinitely. As computers and processing technology improves, the possibilities for many of these systems may also improve.

Systems of building automation, HVAC, safety instrumented systems, and many other types of direct digital control are possible largely because of data communications and microprocessor technologies. Not only can the devices maintaining control over something work faster and more efficiently, but they could also communicate with remote locations such as a home computer or central monitoring facility. Having data communicated faster between the many components of a system means faster control and faster configuration of the proper parameters.

You might also Like

Readers Also Love

Discussion Comments


@David09 - A DDC control system is not just limited to air conditioners or heaters however. For example, we have a monthly monitoring service that is not only tied to our security system but to the fire alarm as well.

I don’t know all the technical specs but I am sure that in the event of a fire, the unit sends a message to the central monitoring facility, based on temperature sensors. Fortunately we’ve never had to use this part of the service but it’s good to know that it’s there.


@allenJo - I think that’s a fair point. There is no way to make sure that the temperature never gets away from the target temperature; after all, it’s the very increases or decreases in temperature that act as the triggers for the heater or air conditioning units to kick in.

But the important point is that these units are computer controlled. With microprocessors as efficient as they are, I think they can act quickly to restore the target temperature. Whatever lag times may exist will be minimal in my opinion.


In order for the computer to maintain a certain temperature in an HVAC control system, it needs to kick in well before room temperature rises or falls from the target temperature, I would think.

For example, if you wanted the room temperature at 75 degrees and you set the thermostat accordingly, the computer would activate the heater or AC until that desired temperature was reached – and then shut off.

But what happens if the temperature begins to rise again? The computer kicks in with the AC to bring the temperature back down to 75. When does it kick in – at 76 degrees or 77 degrees? After it does kick in, how long does it take to bring the temperature back down to 75?

The point I am making is that there may be some lag time as the HVAC is trying to “maintain” room temperature.

Post your comments
Forgot password?