In the construction of Building Automation (BA) systems and industrial automation control systems, DDC controllers and PLC controllers are two core control devices. Many engineering firms, project owners and technical personnel struggle with equipment selection during projects, wondering how to choose between DDC and PLC, what their differences are, and which device fits different scenarios. This article comprehensively compares DDC and PLC from the perspectives of origin and characteristics, communication protocols, software functions, scalability, security, commissioning difficulty and applicable scenarios. It also elaborates on the core signal types of air-conditioning automatic control systems to provide professional references for equipment selection in building automation projects.

I. Differences in Origin and Core Positioning between DDC and PLC
From the perspective of technological development, DDC is a dedicated building control device derived from iterations of PLC technology. They share the underlying logic but have completely different application positioning.
PLC is a general-purpose core industrial control device developed for complex industrial automation scenarios. Most industrial control systems in China adopt PLC solutions, featuring strong versatility, high control precision and robust anti-interference capability. However, the implementation effect of PLC highly depends on programmers. Technicians need to be familiar with equipment processes, possess solid professional knowledge and rich on-site commissioning experience, and the overall commissioning cycle is longer.
The DDC controller is equipment specially optimized by manufacturers for building automation scenarios. Based on PLC, it embeds solidified dedicated programs for building control to meet the control requirements of HVAC, air conditioning, fresh air, water supply and drainage and other building equipment. Nevertheless, due to the solidified programs, DDC sacrifices part of its flexibility and has weaker adaptability than PLC in scenarios with complex electromagnetic interference and non-standard sophisticated control.

II. Six Core Dimension Comparison between DDC and PLC (Key for Selection)

1. Communication Protocols and Third-party Integration Capability
DDC systems are designed for building integration adaptation, natively supporting multiple general communication protocols with abundant integration interfaces. They can quickly connect to various building electromechanical equipment with strong compatibility with third-party devices. The system openness and scalability better meet the needs of intelligent building renovation.
PLC mostly works in standalone mode with single-network structure for networking. Most adopt proprietary fieldbus protocols from manufacturers, featuring low standardization and versatility. Communication connection with third-party devices and PLCs of different brands is difficult with poor integration adaptability. Currently, mainstream PLCs and DDCs both support TCP/IP network protocols for network access.

2. Host Computer Software and Operation Threshold
Supporting host software for DDC is mostly dedicated closed-source software from manufacturers, incompatible between brands. Most original versions have English interfaces, creating certain barriers for operation and later maintenance by domestic technicians with poor versatility.
PLC systems offer higher flexibility in adaptation. They can be paired with dedicated software or mature domestic Chinese general configuration software, which delivers simple configuration operations and high visualization. General configuration software covers all professional control functions and can realize extended effects such as highly simulated interfaces, voice alarms and personalized customization, fully adapting to both industrial control and building automation scenarios.

3. System Scalability and Cost Advantages
Traditional DDC systems generally adopt a user-based licensing model with high authorization fees. Many completed building automation projects cannot add sub-control operation stations due to cost constraints, limiting system expansion.
PLC belongs to an open general industrial control system without high authorization fees. It supports convenient network access and unrestricted expansion. The cost for adding control stations and expansion renovation in later stages is lower with remarkable cost-performance advantages.

4. Professional Functions and Energy-saving Control Capability
General PLC has no built-in mature energy management and energy-saving control programs. For scenarios such as building air conditioning and equipment scheduling, technicians need to conduct separate programming and customized commissioning, relying on manual experience with heavy commissioning workload.
DDC is deeply optimized for building scenarios, with built-in standard programs, accurate PID algorithms and professional energy management modules verified by extensive field tests. It comes with standardized functions including peak load control, optimized equipment start-stop, intelligent scheduling, energy-saving cycle control, multi-mode air conditioning operation, holiday/calendar/event timing, trend recording and data reports. It requires little secondary development and fits the needs of building energy conservation and operation maintenance.

5. Operational Security and Maintenance Convenience
DDC supports independent standalone operation with stronger fault tolerance. When a single DDC device fails, it can be replaced online without affecting the communication and operation of other DDC devices in the same network. The central operation station can be shut down as required to ensure stable operation of the entire BA system with extremely high security.
PLC boasts high modular integration but low fault tolerance. Once a single module fails, the whole unit usually needs to be shut down for maintenance, reprogramming and configuration, which suspends the operation of the entire control system and exerts a great impact on continuous production and equipment operation.

6. Commissioning Difficulty and Cycle Cost
DDC features solidified programs and standardized functions, specially adapted to building equipment. The commissioning process is simple with a short cycle and low learning threshold. Ordinary professional commissioning technicians can quickly complete configuration and commissioning.
PLC commissioning involves cumbersome procedures and heavy workload. It requires accurate matching of corresponding equipment models, calling dedicated compilers and one-to-one program reading and writing. Commissioning takes a long time with high labor costs and imposes extremely high requirements on the technical experience of commissioning personnel.

III. Applicable Scenarios of DDC and PLC (Precise Selection Guide)
Based on their performance differences, mature selection criteria have been formed in the building intelligence and industrial control industries with clear applicable scenarios:
Scenarios for preferential DDC selection: building automation projects of civil buildings such as commercial complexes, star hotels and high-end office buildings, focusing on standardized HVAC, fresh air, water supply and drainage and lighting control, with emphasis on energy-saving maintenance, convenient commissioning and multi-device integration.

Scenarios for preferential PLC selection: industrial workshops, pharmaceutical clean rooms, data centers, district energy stations and other scenarios, featuring complex working conditions, strong electromagnetic interference, high-precision control, continuous non-stop operation and customized control requirements for non-standard processes.

IV. Core Signal Types of Air-conditioning Automatic Control Systems (Basic Knowledge of BA Systems)
In automatic control systems for building air conditioning, fresh air, refrigeration stations and heat exchange stations, DDC controllers complete equipment monitoring and control mainly through four types of signals, which form the core foundation for BA system operation.

1. AI Analog Input Signal
It mainly collects continuously changing physical quantities such as temperature, humidity, pressure and flow rate. Sensors and transmitters convert them into standard electrical signals and transmit to the analog input ports of DDC. Common signal types include 0~10mA current signals and 0~5V/0~10V voltage signals. If current signals are connected, matching port resistors can convert them into voltage signals for collection. A single DDC can be equipped with multiple AI input interfaces.

2. DI Digital Input Signal
It identifies two switch states of high and low levels and converts them into 1/0 digital quantities for logical operation. It is mainly connected to switch-type sensors such as flow switches, wind speed switches and differential pressure switches. Meanwhile, it can accurately measure pulse signals, pulse frequency, level width and pulse counting to meet various switch state monitoring requirements.

3. AO Analog Output Signal
Continuously adjustable electrical signals are generated by the DDC internal D/A digital-to-analog converter. Common specifications include 0~5V, 0~10V voltage signals and 0~10mA, 4~20mA current signals. The signal magnitude is precisely adjusted by control programs. It is mainly used to control regulating actuators such as air dampers and water valves to realize precise regulation of air conditioning temperature, humidity, air volume and water flow.

4. DO Digital Output Signal
Also known as switch output, it outputs switch command signals, mostly used to control intermediate relays and AC contactors to realize start-stop control of electromechanical equipment such as fans, water pumps and compressors. To ensure equipment safety and accurately monitor operating status, auxiliary contacts of contactors are usually connected to DDC input channels to feed back the actual operating conditions of equipment in real time and avoid idling and missed fault judgment.

V. Installation and Configuration Specifications for DDC Controllers

DDC controllers are mainly deployed in areas with concentrated equipment parameters such as refrigeration stations, heat exchange stations, air conditioning machine rooms and fresh air machine rooms, normally adopting wall-mounted exposed installation. During project configuration, corresponding IO interfaces shall be matched according to the signal type and quantity of controlled equipment. An interface margin of 10%-15% must be reserved to meet the requirements of later equipment expansion and system renovation and ensure system compatibility and forward-looking performance.

VI. Conclusion
There is no absolute superiority or inferiority between DDC and PLC; they only differ in scenario adaptation. DDC is selected for civil building automation to balance energy conservation, integration and low-cost maintenance. PLC is adopted for industrial high-precision and complex working condition control to guarantee stability and customization capability. A high-quality BA building automation system requires not only accurate equipment selection, but also professional engineers to complete configuration commissioning, parameter optimization and maintenance training, so as to maximize the core value of the system in energy saving, automatic control and monitoring.