PLC vs. DDC: Choosing the Right Controls for Your Facility
By Charles Tinsley
Most buildings’ control systems utilize Direct Digital Control (DDC) technology to manage the output and actions of HVAC systems, especially in the commercial market. However, for the industrial market and other mission-critical facilities, Programmable Logic Controllers (PLC) are often a better, faster and more reliable option that should be considered.
Although PLC and DDC controls both use sensor data to make decisions and write outputs, PLCs provide a level of sophistication specifically tailored for mission-critical environments that DDCs can’t always deliver. Check out the chart below for a side-by-side comparison.
Hardware
Software
Contractor
PLC
Higher cost
Battle-tested, extensive evaluation periods
Manufacturers have higher barriers to entry because of the lengthy testing and evaluation required
Slower to innovate and evolve because of the expensive evaluation process
Longer expected life
Fewer failures
Redundancy can be built in more easily:
Dual processors
Dual power supplies
More fault tolerant
More safety features
More universal ladder logic used, making serviceability across contractors easier
Better process monitoring available
Fast processing power
Fewer software updates and patches required
More safety features
Less likely to be proprietary and instead utilizes open-source tools
Higher cost
Highly specialized integrator experts in certain industrial processes
Typically longer mobilization lead times
More prefabrication and preprogramming involved in execution as opposed to on the jobsite
Potentially faster startup and commissioning durations
DDC
Lower cost
Failures more likely to occur but not necessarily imminent
Rapidly evolving and innovating due to lower barriers to entry in market
Redundancy can be built but is much more difficult to execute:
Single processor
Single power supplies
Redundancy is not typically built into the network frameworks
Not fault tolerant unless the design requires additional hardware and software be added
Each product has its own programming language and makes multiple vendor service more difficult
Sufficient processing power
More software updates and patches required
Fewer safety features
More frequent product end-of-life periods
Can be proprietary and have closed distribution models.
Lower cost
Contractors focus on the commercial market:
Office buildings
Hospitality
Healthcare
Institutional
Shorter mobilization lead times
Field programming and tinkering required, typically minimal prefabrication
Potentially longer startup and commissioning durations
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Most buildings’ control systems utilize Direct Digital Control (DDC) technology to manage the output and actions of HVAC systems, especially in the commercial market. However, for the industrial market and other mission-critical facilities, Programmable Logic Controllers (PLC) are often a better, faster and more reliable option that should be considered.