Understanding when to use Direct Digital Controllers (DDC) versus Programmable Logic Controllers (PLC) is a fundamental decision in building automation system design.
Direct Digital Control (DDC) systems are purpose-built for HVAC and building automation. They feature:
Common DDC platforms include Johnson Controls Metasys, Tridium Niagara, Honeywell EBI, and Schneider EcoStruxure.
Programmable Logic Controllers originated in industrial manufacturing and excel at:
Common PLC platforms include Allen-Bradley, Siemens S7, and Schneider Modicon.
DDC is the right choice for most building automation applications:
| Application | Why DDC Works |
|---|---|
| AHU control | Pre-built sequences, native scheduling |
| VAV systems | BACnet integration, zone coordination |
| Chiller plants | Energy optimization, load management |
| Lighting control | Scheduling, occupancy integration |
| General HVAC | Lower engineering cost, easier maintenance |
PLC is appropriate when you need:
| Application | Why PLC is Better |
|---|---|
| Central plant safety interlocks | Fast, deterministic response required |
| Boiler/burner management | Safety integrity level (SIL) requirements |
| Conveyor/material handling | High-speed sequencing |
| Critical process control | Sub-second response times |
| Integration with manufacturing | Common platform with production systems |
Many modern facilities use both:
Example: A chiller plant might use a PLC for compressor staging safety interlocks (fast response) while a DDC system handles setpoint optimization and energy reporting.
| Factor | DDC | PLC |
|---|---|---|
| Hardware cost | Lower for typical HVAC | Higher, industrial-grade |
| Engineering time | Lower (pre-built sequences) | Higher (custom programming) |
| Commissioning | Faster (standard tools) | Longer (custom testing) |
| Maintenance | Building staff can operate | Often requires specialists |
| Spare parts | Proprietary but available | Standard industrial parts |
For 90% of HVAC applications, DDC is the right choice due to lower total cost of ownership and purpose-built features. Reserve PLCs for applications requiring fast deterministic control, safety system integration, or harsh industrial environments.
The decision often comes down to: Who will maintain it? If building operations staff need to make changes, DDC wins. If industrial maintenance electricians are available, PLC may be viable.