DDC vs. PLC: Which to Choose for Your Building Automation Project

Modern building automation, the control system is the core of energy efficient and safe operation of the building, HVAC, energy management, etc. need to be supported by reliable control technology, of which DDC (Direct Digital Control) and PLC (Programmable Logic Controllers) are the two most commonly used core program, selection directly affects the project results, cost and scalability.

Although both belong to the field of automatic control, but the original design intent, functional positioning and applicable scenarios differ significantly, no absolute advantages and disadvantages, only the existence of adaptive differences. In this paper, we will analyze the two from a multi-dimensional analysis of the building automation project selection can provide a reference to help avoid misunderstandings, realize the technology and demand for accurate matching.

What is DDC (Direct Digital Control)?

Direct digital control (DDC) is a digital microcontroller-based control technology, the core role is to automatically manage all kinds of physical processes in the building, such as temperature, pressure, humidity and other parameters of the regulation, or according to a specific logic to respond to the state of operation of the equipment.

As the most widely used control technology in the heating, ventilation and air conditioning (HVAC) industry, DDC not only realizes precise control of parameters, but also has core functions such as programmability, network interconnection, data interaction and remote management, making it the mainstream choice for HVAC automation in modern buildings.

In terms of application scenarios, DDC is tailor-made for building automation, especially for the control of air conditioning units (AHU), variable air volume (VAV) systems, chillers, boilers, and other HVAC equipment, and can accurately match the comfort needs of temperature and humidity, air quality, and energy saving goals in the building.

Main Components of DDC

The core of a complete DDC control system consists of three major components: input devices, DDC controllers, and output devices, each of which has a clear division of labor and works together to achieve precise control of building HVAC equipment.

Input Devices

Various types of sensors, including temperature sensors, humidity sensors, carbon dioxide sensors, static pressure sensors, etc., the core role is to collect the physical parameters of the building, convert them into standard electrical signals (commonly 0-5V, 0-10V, 4-20mA, etc.), and then transmitted to the DDC controller through the control line, to provide data support for the control of decision-making.

DDC Controller

As the “brain” of the system, it has built-in equipment operation program (operation sequence SOO), which can read the electrical signals transmitted by the input devices, judge and calculate according to the preset control logic, and then convert the results of the operation into control commands to be transmitted to the output devices to dominate the operation of the whole control system.

Output Devices

Including relays, actuators, drives, etc., is responsible for receiving the control instructions from the DDC controller, drive HVAC equipment to complete specific operations (such as start / stop fan, adjust the valve opening, control the compressor operation, etc.); at the same time, with the electrical isolation function, the DDC controller can protect the DDC controller from high-voltage equipment from the impact of power surges and interference to protect the system’s stable operation.

To put it simply, the working logic of DDC can be summarized as “perceive-judge-execute”: the building environment parameters are perceived through sensors, and the controller judges whether adjustment is needed according to the preset logic, and then executes the regulating action through the output devices to form a closed-loop control to ensure that the building environment is always in the preset state.

What is Programmable Logic Controller (PLC)?

A Programmable Logic Controller (PLC) is an industrial computer designed for industrial control with customizable input/output interfaces, and its core function is to control and monitor industrial equipment according to user-written programs. Unlike DDC, which focuses on building HVAC, PLC has a wider range of applications, covering manufacturing, power plants, steel mills, and other industrial scenarios, and is capable of adapting to diverse control needs.

PLC shape and size differences, small PLC compact size can be carried around, large PLC requires special heavy-duty rack mounting; at the same time with a high degree of modularity, the basic model is equipped with only the core I / O (input / output) interfaces, according to the actual needs of the expansion of the backplane and functional modules, such as analog I / O modules, communication modules, display modules, etc., to flexibly adapt to the different scales and types of industrial control scenarios. and types of industrial control scenarios.

Key Components of PLC

The stable operation of PLC system relies on five key components, each component has a clear division of labor, synergistic cooperation, constituting a complete industrial control core, specifically as follows:

Central Processing Unit (CPU)

As the “brain” of PLC, the core is responsible for the execution of the preset control program, processing input modules to transmit various types of signals, logic operations, data processing and instructions issued, is the control core of the entire PLC system, determines the system’s response speed and processing capacity.

Input Module

The main function is to receive the signals from on-site sensors, switches and other devices (e.g., equipment operation status, detection parameters, etc.), and convert these analog or digital signals into electrical signals that can be recognized by the PLC, and transfer them to the CPU for processing, which is the “signal input interface” between the PLC and the on-site devices.

Output Module

It receives the control instructions from CPU, converts them into electrical signals that can be recognized by the field equipment, drives the industrial equipment to complete specific operations (e.g., starting, stopping, speed regulation, etc.), and is the “signal output interface” for PLC to transmit control instructions to the field equipment.

Power Supply Unit

Provides stable power support for the entire PLC system, converts external AC power supply into DC power required by the PLC components, ensures the normal operation of the CPU, input/output modules and other core components, and is the basis for the stable operation of the PLC system.

Programming Equipment

Used for users to write, modify, debug the control logic program, common programming equipment, including special programmers, computers equipped with programming software, etc., connected to the PLC through the communication interface to achieve program uploading, downloading and debugging, is the PLC to achieve the key tool for custom control.

How Does a PLC Work?

PLC’s core work is “scanning cycle”, follow the “cycle scanning, real-time response” principle of work, the whole process is a cycle, continuous process, to ensure real-time control of industrial equipment, the specific workflow is divided into four stages The specific workflow is divided into four stages, each of which is executed sequentially and cyclically:

Working Principle

PLC through the “input – processing – output” closed-loop logic, to achieve the control of industrial equipment. Firstly, it collects the signals from the field equipment through the input module, then the CPU performs logical operation and processing on the signals, and then gives control instructions through the output module to drive the field equipment;

At the same time, it continuously detects the changes of input signals through cyclic scanning and timely adjusts the output instructions to ensure the real-time and accuracy of control. Its core advantage lies in the millisecond scanning speed and predictable working cycle, which can meet the strict requirements of timing control in industrial scenarios.

Workflow

Input Scanning Phase

PLC detects the status of all connected input devices (sensors, switches, etc.) one by one, and stores the detected signals (e.g. on/off and analog values) into the input image area of the memory, which is only responsible for signal acquisition and does not carry out any logic processing to ensure the accuracy and integrity of the input signals.

Program Execution Stage

The CPU reads the signals in the input image area from the memory according to the preset control program (written and downloaded by the programming device), carries out arithmetic, judgment and processing according to the program logic (e.g. ladder diagram, structured text, etc.), arrives at the control result, and stores the result in the output image area.

Output Update Stage

PLC will convert the control results in the output image area into electrical signals that can be recognized by the field equipment, and accurately transfer them to all kinds of industrial equipment through the output module to update the status of the output equipment in real time, so as to realize the accurate control of the equipment (e.g., starting the fan, adjusting the opening of valves, stopping the compressor operation, etc.).

Housekeeping Stage

Completing the system internal self-test (checking the operating status of each component), communication with external devices (such as monitoring host, touch screen), program storage backup and other auxiliary work, to ensure that the PLC system itself is running stable, ready for the next scanning cycle.

The entire scanning cycle takes only milliseconds, cyclic, continuous operation, so that the PLC can quickly respond to changes in the state of the field equipment to achieve real-time, reliable control of industrial equipment, which is the core reason for the PLC is widely used in industrial control scenarios.

Core Differences Between DDC and PLC Systems

Architecture and Design Philosophy

DDC adopts distributed, network-centered architecture design, usually deploying multiple small controllers in different areas of the building, all the controllers are connected to the monitoring server through BACnet/IP or BACnet MS/TP network, and each controller is responsible for controlling the peripheral HVAC equipments, realizing “distributed control and centralized monitoring”, which is more suitable for the building. It can realize “distributed control and centralized monitoring”, which is more suitable for the characteristics of dispersed building space and scattered equipment layout.

The core of its design is “HVAC-specific”, all functions and hardware are optimized around the building environment control, the shape of the compact integrated shell, without the need for complex rack mounting. In addition, the ecology of the DDC suppliers are mostly proprietary systems, different manufacturers of equipment and software compatibility is poor, difficult to cross-brand integration.

PLC architecture to centralized or distributed deployment of the main, the core of a central rack, various types of input / output modules through a dedicated line and field equipment to achieve connectivity; even with a distributed layout, the entire system will also show a clear hierarchical structure, to facilitate unified management and operation and maintenance.

The design concept is “universal industrial control”, not limited to a particular field, the shape of the modular rack type, according to the needs of a flexible combination of modules, PLC supplier ecology has a good interoperability, the modules and devices of different vendors can achieve cross-brand compatibility, to adapt to a variety of industrial control scenarios.

Programming Method

DDC is mainly based on graphic/block programming, with a large number of HVAC-specific function blocks (e.g., PID regulation, time scheduling, etc.) preset by vendors. Users do not need to program complexly, and can complete the configuration by dragging and dropping and point-and-click, which is easy to start and suitable for technicians who are familiar with HVAC but lack programming experience. Some DDCs support proprietary scripts and Niagara framework for flexible configuration, but the overall customization capability is limited, and complex non-standard logic cannot be realized.

PLC programming strictly follows the IEC 61131-3 standard, supporting ladder diagrams (LD), function block diagrams (FBD), structured text (ST), sequential function charts (SFC) and other standardized programming languages, which has a relatively steep learning curve and requires the user to have a certain programming foundation and industrial control logic reserves.

However, PLC’s strong customization capabilities, able to achieve almost all complex control algorithms, whether it is special logic control or high-speed timing control, can be accurately achieved through programming, and standard programming specifications so that it can be used across vendors, flexibility is much higher than DDC.

From the point of view of application scenarios, DDC programming is more suitable for standard HVAC sequence control, projects with tight schedules, as well as operation and maintenance teams dominated by HVAC professionals; PLC programming is more suitable for projects that require complex customized algorithms, access to non-standard equipment, or high demand for flexibility in control logic.

Communication Protocol

Communication protocol is the core of the control system to realize the interconnection of equipment and data interaction, DDC and PLC in the protocol support differences, directly affecting its integration with the building management system (BAS).

DDC system takes BACnet protocol as the native communication standard, which is a protocol specially designed for building automation, and it can perfectly adapt to the control needs of HVAC equipment, and support all kinds of building control-related objects such as temperature, pressure, alarm, scheduling, etc. DDC controllers can directly communicate with the BAS system through BACnet/IP or BACnet MS/TP protocols, without the need for additional equipment to realize seamless data sharing. equipment, realizing seamless data sharing.

PLC, on the other hand, supports a wider range of industrial communication protocols, including Modbus RTU, Modbus TCP, EtherNet/IP, PROFINET, OPC UA, etc., which can easily interface with various types of industrial equipment, but the support for BACnet protocol is weaker, and it is usually necessary to integrate with the BAS system through additional gateway devices.

This not only increases the complexity of the system, but also introduces potential points of failure, raising integration costs and operation and maintenance difficulties. However, when interfacing with non-BACnet protocol industrial equipment (e.g., industrial-grade chillers, specialized production equipment), the protocol advantages of PLC are more obvious.

Control Performance

The difference in control performance determines the adaptability of DDC and PLC in different control scenarios, and the core difference is mainly reflected in the response speed, control accuracy and complex logic processing capability.

DDC control performance through the HVAC scene optimization, cycle execution cycle is usually 1-10 seconds, although the response speed is not fast, but fully meet the control needs of the HVAC system, HVAC equipment, temperature and humidity control, valve adjustment, etc. does not require millisecond response.

And DDC’s analog control capability is outstanding, built-in perfect PID regulation and serial control function, can accurately control the temperature, pressure, humidity and other continuously changing parameters, very suitable for building environment, steady-state control.

PLC control performance is more inclined to high-speed, accurate, cycle execution cycle is only 10-100 milliseconds, with a high degree of certainty and real-time, able to cope with millisecond-level control needs, such as combustion control, variable frequency drive control, servo motor control. At the same time, PLC supports full floating-point operation, and can handle complex mathematical operations and custom algorithms, which is much more advantageous than DDC in the scenarios that require high-speed response and complex logic control.

It should be noted that, for the vast majority of building HVAC projects, DDC control performance has been fully sufficient, PLC’s high-speed response capability in the general building automation scenarios often belong to the “excess performance”, only when the building contains industrial-grade equipment, the need for high-speed control, PLC’s performance advantages to be reflected. The performance advantages of PLC can be realized only when the building contains industrial-grade equipment and requires high-speed control.

Hardware and I/O Interface

DDC’s hardware design is highly adapted to HVAC scenarios, and the I/O interfaces are optimized for HVAC, mainly including universal inputs (UI), analog outputs (AO), digital inputs (BI), and digital outputs (BO):

• Universal input is compatible with temperature (thermistor/Pt1000), resistance, voltage, current and other signals, suitable for all kinds of HVAC sensors;
• Analog outputs are mainly 0-10VDC, 4-20mA, which are used to control valves, actuators and other devices;
• Digital inputs are used to detect the status of equipment (such as fan operation status, water flow switch status);
• Digital outputs are mostly relay outputs, which are used to control contactors, solenoid valves and other devices.

Each DDC controller I / O points are usually 12-96 points, and built-in BACnet, Modbus and other communication ports, some of which also supports web configuration, easy installation and commissioning.

PLC hardware is based on modular design, I / O interface is divided into various types:

• Discrete I/O module (support 24VDC, 120VAC, relay output), used to docking switches, contactors and other discrete devices;
• Analog I/O modules (supporting ±10V, 0-10V, 4-20mA, thermocouples, RTDs) for docking various types of sensors and actuators;

There are also special modules for high-speed counters, motion control, communication, etc., which can be added flexibly according to project requirements.

The number of I/O points per PLC rack can be expanded from 4 points to more than 512 points, and it supports remote I/O connection through industrial networks (PROFINET, EtherNet/IP), which is extremely scalable and can be adapted to the needs of various types of systems, from small-scale control to large-scale complex systems.

Cost Comparison

Cost is one of the core factors in project selection, the cost difference between DDC and PLC is mainly reflected in four aspects: hardware, software, commissioning and integration, and the extent of the difference is closely related to the project scale, customization requirements.

Hardware Costs

DDC controller price is usually between 500-3000 U.S. dollars, and the I / O interface has been included in the controller, no need to purchase additional; PLC base unit price of 200-2000 U.S. dollars, but the I / O modules need to be purchased separately, the price of each module in the range of 100-500 U.S. dollars, for the project of the number of I / O points is less, the PLC hardware costs may be lower, but for the standard HVAC projects, DDC and PLC hardware costs are lower. standard HVAC projects, the hardware cost of DDC is more advantageous.

Software and Commissioning Costs

DDC programming software is mostly free or low-priced version (price range of $ 500-2000), and built-in preset HVAC control sequences, debugging efficiency, speed, can significantly shorten the project duration, effectively reducing the commissioning process of labor and time costs.

PLC programming software price span, from the free version to more than 5000 U.S. dollars of professional versions, and need to be fully customized programming, not only a longer commissioning cycle, commissioning costs are relatively higher; especially for complex projects, the commissioning costs may even far exceed the DDC system.

Total Installation Cost

For standard HVAC projects, the total installed cost of DDC is usually 20%-40% lower than PLC; and for projects that require a lot of customization and access to non-standard equipment, the cost advantage of PLC may gradually appear, especially for long-term operation and maintenance costs, PLC’s modular design and interoperability can reduce the later expansion and maintenance costs.

In addition, the life cycle cost of DDC is relatively low, and the professional HVAC support provided by vendors can reduce the difficulty and cost of operation and maintenance; the life cycle cost of PLC depends on the complexity of the project, and the operation and maintenance cost of complex projects may be higher.

Integration with Building Administration Systems (BAS)

In building automation projects, the integration of the control system with the BAS directly affects the centralized monitoring and management efficiency of the building, which is one of the core differences between DDC and PLC.

DDC has natural advantages in integration with BAS: as a dedicated control technology for building automation, DDC natively supports BACnet protocol, enabling seamless integration with mainstream BAS platforms, smooth data sharing between controllers, and BAS systems can directly recognize DDC’s standard objects (e.g., analog inputs AI, analog outputs AO, time scheduling, alarms, etc.), eliminating the need for additional data mapping and configuration. There is no need for additional data mapping and configuration, low integration difficulty and high stability.

The integration of PLC and BAS has certain challenges: PLC itself does not support the native BACnet protocol, and it needs to realize protocol conversion through additional gateway devices, which not only increases the hardware cost, but also requires complex data mapping (converting the data in PLC registers to the objects recognizable by BAS), and the later operation and troubleshooting are more complicated.

However, it should be noted that when the building needs to access non-BACnet protocol industrial equipment (such as industrial chiller, special production equipment), PLC’s integration advantage is more obvious, and can easily docking of various types of industrial equipment, and then through the gateway and the integration of the BAS, to achieve centralized monitoring and control of the whole system.

Advantages and limitations of DDC

Advantages

• Specially designed for HVAC system optimization, accurately meets the core requirements of building environment control, can efficiently realize the steady state regulation and control of key parameters such as temperature, humidity, pressure, etc., which is widely adapted to the HVAC application scenarios of various buildings such as office buildings, hospitals and schools.
• Convenient and efficient commissioning process, built-in preset HVAC control sequences, without the need for complex programming operations, the threshold for getting started is low, which can significantly shorten the project debugging cycle and reduce commissioning costs, especially for standard HVAC projects with tight construction schedules.
• Native support for the BACnet communication protocol, seamless integration with the building management system (BAS), smooth data transmission, low integration difficulties and stable operation, without additional configuration of the gateway and complex parameters, significantly improving integration efficiency.
• In the standard HVAC project has a significant cost advantage, the total installation cost is 20%-40% lower than PLC, and the life cycle cost is controllable, the manufacturer provides professional HVAC technical support, effectively reducing the difficulty of the later operation and maintenance.
• The operation interface is simple and intuitive, suitable for HVAC professionals as the main operation and maintenance team, without the need to master the professional programming knowledge, you can easily complete the daily operation and maintenance work, improve the efficiency of operation and maintenance.

Limitations

• Limited customization capabilities, difficult to achieve non-standard complex control logic, for the need to customize the algorithm, access to non-HVAC equipment projects, poor adaptability, unable to meet the personalized control needs.
• Vendor ecology is a proprietary closed system, poor compatibility of equipment and software from different vendors, difficult to realize cross-brand integration, significantly limiting the expansion and upgrading space of the later system.
• Functionality is relatively single, the core is only adapted to HVAC system control, it is difficult to cope with the control of industrial-grade equipment in the building, high-speed timing control and other non-HVAC control tasks.
• Expandability is not good, the number of I / O points is fixed and can not be flexibly adjusted, if the later need to increase the number of control points or expand the function, often need to replace the controller, resulting in additional costs.

Advantages and limitations of PLC

Advantages

• Strong customization ability, support for IEC 61131-3 standard multiple programming languages, can be flexibly adapted to various types of complex control needs, accurate landing custom algorithms, to solve the complex control problems that are difficult to deal with DDC.
• Rugged design, suitable for industrial-grade harsh environments, can withstand high temperatures, strong electromagnetic interference and other factors, high reliability, suitable for building scenarios containing industrial equipment, which can reduce the losses caused by equipment failure.
• Extremely fast control speed, cycle execution cycle is only 10-100 milliseconds, real-time and deterministic, can effectively deal with high-speed control, timing control and other demanding response speed scenarios, to ensure accurate control.
• Excellent expandability, using modular design, the core rack can be added flexibly to all kinds of functional modules, the number of I / O points can be expanded from a few points to hundreds of points, to adapt to the project’s whole process of expansion needs, reduce the cost of upgrading later.
• Strong cross-vendor interoperability, compatible with a variety of mainstream industrial communication protocols, can be easily interfaced with various types of industrial equipment, adapted to a single brand or multi-brand mixed network of complex control scenarios, to enhance the flexibility of integration.
• Strong data processing and complex computing capabilities, support for various types of customized algorithms, to meet the industrial chiller regulation, multi-device collaboration and other high-end control needs, to improve control accuracy and operational efficiency.
• Some of the high-end models support hot-swappable and redundant modules, which can realize non-stop maintenance and seamless fault switching, significantly improve system reliability, reduce downtime losses, and adapt to scenarios with high requirements for operational continuity.

Limitations

• Steep learning curve, need to master the IEC 61131-3 series programming language, the programming ability of operation and maintenance personnel and industrial control professional knowledge of the requirements of the higher, the corresponding personnel training costs have increased.
• The commissioning cycle is long and costly, requiring fully customized programming configurations. Especially for complex projects, the commissioning workload is heavy and time-consuming, which may lead to an extended project duration.
• Integration with the building management system (BAS) is more difficult, and additional gateway devices need to be configured to realize the BACnet protocol conversion, which not only increases the hardware investment, but also enhances the complexity of the system, and at the same time, there are potential failure hazards.
• For the standard HVAC control project, PLC has the problem of excess functionality, and the overall cost is high, belonging to the “too big for the job”, which will cause unnecessary cost waste of the project.

In Building Automation Systems, Which One is Better, PLC or DDC?

The core of selection is “demand matching”, combined with the above differences and advantages and disadvantages, the following scenarios can clearly determine the scope of application of DDC and PLC, to help the project quickly lock the appropriate control program.

Scenarios Suitable for Choosing DDC

1. Standard HVAC projects: such as office buildings, hospitals, schools, shopping malls and other HVAC control-oriented buildings, no complex custom logic needs, the core objective is to achieve accurate temperature and humidity control and energy-saving operation.
2. projects with tight schedules: projects with high schedule requirements need to complete commissioning and deployment quickly, DDC’s pre-built sequence can significantly shorten the commissioning time to ensure that the project is delivered on time.
3. Operation and maintenance team mainly consists of HVAC professionals: the team lacks professional programming experience, DDC’s intuitive operation and low learning curve can reduce the difficulty of operation and maintenance and training costs.
4. Standard projects with limited budget: pursuing cost-effective, no need for complex functions, DDC’s low-cost advantage can meet the control needs while controlling the total project expenditure.

Suitable for Selection of PLC Scenarios

1. Projects requiring complex customized logic: building automation system contains non-standard control requirements, such as industrial chiller, special production equipment control, need to customize complex algorithms.
2. High-speed control or non-BACnet device integration needs: the project requires millisecond response control (such as variable frequency drive, safety shutdown control), or need to access a large number of non-BACnet protocol industrial equipment.
3. large scalable projects: large building scale, many control points, and later expansion needs (such as additional equipment, new control functions), PLC’s modular design and high scalability to meet long-term demand.
4. industrial operation buildings: such as data centers, factory annexes, etc., the building contains industrial-grade equipment, need to adapt to harsh environments, high requirements for system reliability and control accuracy.

Selecting Between DDC and PLC

In order to ensure the scientific and accurate selection, combined with the actual needs of the project, the following five steps can be taken step by step to avoid blind selection.

• Define the scope of the project. First determine the core control requirements of the project, whether it is pure HVAC control, or a hybrid system containing industrial equipment and customized logic; clarify the number of control points, control accuracy requirements, and whether there is a later expansion needs.
• Evaluate the team’s capability. Analyze the professional background of the operation and maintenance team, whether it has rich HVAC experience or programming and industrial control capabilities; if the team lacks programming experience, give priority to DDC; if the team has programming capabilities, PLC can be selected according to demand.
• Evaluate scalability needs. Predict whether the project needs to increase the number of control points, new control functions, or access to new equipment in the later stage; if there is a clear demand for expansion, give priority to the modular design of PLC; if the demand is fixed, DDC can meet it.
• Compare the cost difference. Combined with the project scale and customization needs, compare the hardware, software, commissioning, integration costs of DDC and PLC, as well as long-term life cycle costs; standard HVAC projects give priority to DDC, complex projects can be a comprehensive assessment of the long-term cost-effectiveness of PLC.
• Confirm BAS integration requirements. Clarify whether the project needs to be integrated with the BAS system, as well as the complexity of integration; if you need to seamlessly integrate with the BAS, prioritize DDC; if you need to access non-BACnet devices, consider PLC + gateway solution.

Conclusion

DDC and PLC as the core control technology of building automation, there is no absolute advantages and disadvantages, only different adaptation scenarios; DDC is dedicated to HVAC, the advantage of convenient, low-cost, easy to integrate with the BAS, suitable for standard HVAC projects; PLC versatility, the advantage of flexibility, high speed, scalable, suitable for complex, customized and industrial equipment containing projects.https://www.corestartech.com/products-item/programmable-controls/

The core of selection is to fit the needs, standard HVAC project priority for DDC, complex logic, high-speed control or industrial equipment needs priority for PLC, need to integrate the project scope, team capacity, cost and other factors, to achieve accurate matching of technology and demand.