Difference between PLC and DDC?

PLC
1

Basic function of both PLC and DDC is same. We use PLC for industrial / process applications and DDC for Building automation (BAS/BMS) applications.

Both has digital input, digital output, analog input, analog output for the basic operation.

Basicaly the PLC is more powerful, faster and has redundant system. Suitable for complex and high level process applications.

The main reason for selecting the DDC for BMS Application over PLC are ,

The communication protocol,
Most of the building automation devices use BacNet/Lonwork protocol. PLC don’t support these protocols. we need to use the gateway (protocol convertors). One of my project we used allen bradley compact logix plc for Building automation application. Modbus to Bacnet convertor helped me to communicate with the VRF system. It gave some problems initially. Better to use DDC for hassle free programming.

  1. Programming

DDC programming is much easier than PLC programming.

Overall PLC is designed for industrail application and DDC is designed for BMS (Building management system) applications. Use it wisely to avoid unwanted communication issues.

2

Here’s a comparison table highlighting the key differences between Programmable Logic Controllers (PLCs) and Direct Digital Control (DDC) systems:

Features PLCs DDC Systems
Purpose Control industrial processes Control building automation systems
Scope of Control Typically used for discrete control Primarily used for HVAC and environmental control
Programming Language Ladder Logic, Function Blocks, etc. Custom programming languages or protocols
Inputs and Outputs Support for a wide range of I/O devices Primarily focused on HVAC-related inputs/outputs
Communication Often supports industrial protocols Supports protocols like BACnet, Modbus, LonWorks
System Integration Commonly integrated with industrial systems Integrated with building automation systems
Operating Environment Designed for harsh industrial environments Operate in controlled indoor building environments
Scalability Easily scalable for complex control systems Scalable for building automation in various sizes
Real-Time Processing Built-in real-time processing Real-time control with fast response requirements
Fault Tolerance Typically offers redundancy options Redundancy options available for critical systems
System Monitoring Monitoring of machine/process parameters Monitoring of environmental conditions and energy usage
Cost Generally lower cost Cost may vary depending on system complexity
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