What are PLC, DDC, and HVAC Controllers and Where Are They Used?

What are PLC, DDC, and HVAC Controllers and Where Are They Used?

What are PLC, DDC, and HVAC Controllers and Where Are They Used?

The operations of Heating, Ventilation and Air Conditioning equipment or HVAC are regulated through the HVAC controllers. HVAC controllers are a system of sensing devices that compare the current state of the environment to the desired state, and then determines actions to be taken in order to convert the current state into the desired state.

The Direct Digital Controller, or DDC and the Programmable Logic Controller, or PLC are the two main types of HVAC controllers. Both share a lot of similar functions for processing certain digital and analog input/outputs. But DDCs are usually taken to be a miniature of a PLC.

This article examines the two different HVAC controllers and their applications.

Direct Digital Controllers

Direct Digital Controller (DDC)

Direct Digital Controller (DDC)

A DDC usually comprises a building automation system that reads and processes sensor data digitally, and then acts on the processed data by controlling actuators. A DDC is usually integrated with a desktop computer, as well as with other software infrastructure to regulate temperature, humidity, CO2, etc. From refrigerators to fan-coil units, air ducts systems and VAV terminal units, DDCs are applicable to a wide array of HVAC equipment.

However, DDCs are usually not capable of controlling sophisticated machines. That’s because they usually comprise of relatively fewer sensor/actuator wiring. Hence, they’re relatively cheaper and easier to install and maintain and are the more preferable HAVC controllers in Building Management systems.

DDCs usually come with a number of pre-installed functions, including enthalpy calculations, optimum start/stop, command mismatch alert, etc., unlike typical PLCs, which might require tedious efforts in order to incorporate these features.

In addition to these, there are a number of other advantages that make DDCs the go-to choice for builders and homeowners when it comes to HVAC. It’s much easier to program a DDC than a PLC because DDCs usually come with pre-built logic blocks.

Also, DDCs are free from redundancies, due to their architecture which supports direct connections between extension modules and the controllers. Whereas only a few PBCs come with such architecture, hence most PBCs usually come with redundancies, albeit configurable ones. However, they’re outperformed by PLCs when it comes to a number of parameters, including faster response times and greater accuracy of control.

Programmable Logic Controller

Programmable Logic Controller

Programmable Logic Controller

PLCs are hardware devices that are usually made up of either logic gates, micro-controllers or a simple CPU. Typical PLCs come with more sophisticated programming and are more versatile in their span of control. Their CPUs usually come with high-speed scan rates.

They usually come with less pre-built functions when compared to the intuitive DDCs that have generalized functions. As a result, they require much more configurations and programming than DDCs, and this could have significant cost implications. However, these programs and configurations can be done once, and then used for numerous controllers.

Because they’re more powerful and versatile, they’re usually the better fit for industrial applications. They’re the better types of HVAC controllers’ choices in robotics, servomotors, sorters, packaging systems, and conveyor machines.

input/output/network cards that provide additional functions. Their sensors are usually RTDs, prox sensors, photo eyes (also known as Photoelectric sensors), vision systems, thermistors, and scanners.

They’re mostly based on the ladder logic programming language. Other programming languages typically used include text-based languages and GPL. PLCs also come with distribution networks such as ControlNet, DeviceNet, Modbus, EtherNet, and Bacnet.

PLCs also integrate front-end infrastructure. The Wonderware, a Tridium type software, is one of the most widely used software due to its capacity to integrate multiple software in a single front-end.

Due to their sophisticated nature, one of the downsides of PLCs is the time it takes to train users. The programming language used usually don’t make it easy to assign easily memorable names to I/O. Most of these challenges, which are usually beyond the ken of the non-tech savvy, usually surface during the development of the front end.

The ladder logic programming language, though it’s easier to use when it comes to binary functions, can prove to be very challenging when it comes to coding analog outputs. But that depends on the controller and the interface.


On a basic level, PLCs and DDCs are nearly identical systems with identical architectures. However, they’re usually applied in different settings. PLCs can serve in the same capacities as DDCs, but the vice versa is mostly not true. Unlike a DDC, a PBC can integrate a number of functionalities ranging from a handful to wads of input/output counts.

PBCs, therefore, outperform DDCs in terms of scalability. In addition, they can also provide greater security and reliability. But DDCs have the cheaper cost advantage over PLCs.

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