What is DDC or Direct Digital Controller in BMS System?

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What is DDC ?

To understand the DDC, we need to know a little bit of history about what was the things before the DDC invention and why it was invented? So that we can have a broader view of the primary purpose of DDCs.

The Programmable Logic Controller or PLC used to control and monitor the Process mainly in the industry like automobile and other manufacturing factories.

Richard Morley invented PLC in 1968 to fulfil the primary needs of control and protect the production capacity of machines and manufacturing lines in the industry, and this PLC used initially was in the area of transfer lines in automotive plants.

Due to these PLC or Programmable logic controllers were designed and invented mainly for controlling and monitoring or automating the productions in the industry.

But when it comes to buildings, these PLC cannot fulfil the exact needs in terms of tenants comfort, environmentally green or can say effective management system for buildings. And still, we can use PLC for Building automation whereas it will be an excessive investment and different performance.

So here DDC or direct digital controller invented in order the process and automated the building equipment needs almost which PLC can do with minimal investment from installation to engineering.

What is the Main Difference between PLC and DDC?

difference between DDC and PLC

Read more about PLC Vs DDC on here

What is DDC or Direct Digital Controller?

In a nutshell, DDC is a controller which use the analogue or digital signals from various devices of a field sensor and actuators and then process and control the system based on the programme written inside the controllers and has the capability to sends the information to another controller or DDC.

Basic Features of DDC

DDC or Direct digital controller usually has the followings features

  • The Analogue Inputs is to monitors the fields sensors values.
  • Digital Inputs to monitors the on/off status from switches/contactors.
  • The analogue output is to control the field actuators devices.
    Digital Output is to control relay or provide low voltages.
  • DDC must have internal ROM/RAM to store control logic and sensor values.
  • It must have networking protocols inbuilt to transfer the data between the devices.
  • Modern DDC controller should have the capability to implement BACnet protocols for communication.

Note that there are various DDC controllers available in the market from the different manufacturer and those DDCs are available with a variety of function and features based on the specific needs like controller has all inputs/outputs like Analog inputs, Digital input, analogue output and digital output and some controller has only digital/analogue inputs.

Let us see some DDC Controller from Alerton Honeywell.

DDC Controller


  • Eleven 10-bit universal inputs whereas we connect either analogue input or digital input using a jumper select, eight binary outputs, and eight analogue outputs.
  • Terminal 23,24 used to connect other DDC controller to communicate between devices through BACnet over MSTP.
  • It has non-volatile memory used to store program and work independently.
  • It has the 24vdc used to give power for field devices.

How DDC used to control the BMS System?

Consider the followings scenario which we need to control and monitor through above DDC 
Let us say in Building, we need to control Pump control and control filling sequence through DDC whereas we have 
2 Booster pumpS, one is for filling the water tank and other is to pump the water to buildings purpose to tenants like toilet etc.
This two-pump motor is controlled through the pump control panel by manually and it should work automatically based on the following sequence 
  • Pump-1 should run if the water level below the high level and stops once above the high level.
  • Pump-2 should run if the pressure on the supply line lesser than the defines let us say 2.5bar.
  • Pump-2 Should not run if water lesser than the lower level switch even pressure lesser than defined.
So based on the above sequence we will have following parameters to monitor and control
  • Booster pump-1 Run status from control panel-Binary Input
  • Booster pump-1 Run command from control panel-Binary output
  • Booster pump-2 Run status from control panel-Binary Input
  • Booster pump-2 Run command from control panel-Binary output
  • Water Low-Level status-Binary Input
  • Water High-Level status-Binary input
  • Liquid pressure on supply line-Analog Input
Let us connect the above points in DDC Controllers as follows
BP-1 Run sts- IN-1
BP-2 Run sts- IN-2
Low-Level Sts- IN-3
High-Level Sts-IN-4
Liquid Pressure-IN-5
BP-1 Run Command-BO-0
Bp-2 Run Command-BO-1
Logic will be as follows to execute the above sequence
If IN4==1        ##(means lesser than high-level status)
BP1=1             ##(On Pump)
else                 ##(means above than high-level status)    
BP1=0             ##(Off-Pump)
If (IN5<2.5 and IN4==1)    ##( if pressure lesser than 2.5bar and water above the low-level sts)
then BP2=1     ##(on Pump)
BP2=0             ##(Off-Pump)
Note that this program may change for each vendor controllers.
Not only this small sequence but also DDC can execute complex and critical sequence in BMS System for HVAC 
Share and comment if need any better explanation

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16 thoughts on “What is DDC or Direct Digital Controller in BMS System?”

  1. We offer different type of telemetry devices which are used in order to track the level of fluids or gases in a particular tank system. Whether you are going to go for monitoring any liquid, the sensor is a mandatory part which is included in all metric devices. This is actually the component which is going to be detecting the level of the liquid and providing you with the data. With this in mind, there are quite a few different types of level sensors and yet the telemetry level sensor is amongst the most popular ones. More details about Computer accessories, PLC, HMI, Inverter, Microcontroller products

  2. A BMS lighting system uses a sophisticated web of sensors and occupancy controls. Your managers can automatically shut down lighting during scheduled times of the day or night, or when areas are unoccupied.

  3. I was trying to search something for BMS integration part to my PLC System. Fortunately I read this PLC/BMS comparison, this made me ROFL. Whoever wrote it either have no or very limited knowledge about PLC system. Thanks you for article, I had a good laugh.

  4. can you explain difference in below again ..
    If IN4==1 ##(means lesser than high-level status)
    If (IN5<2.5 and IN4==1) ##( if pressure lesser than 2.5bar and water above the low-level sts)

    thanks alot

  5. I do not agree with the differences you have listed.
    PLC is much much better than DDC.
    Centralized architecture – for DCS yes, but not for single controller so you can choose to go on single or distributed control.
    Single Point Failure – you can implement redundancy.
    High installation cost – its the same,
    High engineering cost – minimal.
    Limited connectivity – Better connectivity (Ethernet, Serial) Multi protocol support
    Very limited subsystem integration capability – better, there are PLC you can choose with Bacnet connectivity other are Modbus, CAN
    Inconsistent Support – Multi level Support, almost all electrician can program in Ladder Programming.
    Limited Channel expertise – No, Multi Channel Expertise
    General Purpose – Yes, which is better, I can even put all nice features from different DDC features into a PLC, it is flexible.
    Fully Programmable – Correct, which is much better.

    What about robustness? Robust that it can withstand very high temp without ventilation or Fan designed for harsh environment.

    • I do agree your points,.

      Its not actually PLC vs DDC.

      Difference of purposes…

      Each has its own unqiue functionality.

      Industrial/factory VS buildings..


  6. Correct your statements of differences.
    DDC will soon be supersede by PLC

    PLC SCADA is now the futures.
    SCADA was for industrial before but not anymore there is now Facilities SCADA

  7. Hello .. i would just like to ask something importrant about BMS. Where should the Direct Digital Controller be placed ideally? Hope you can provide me with this answer. Thank you.

  8. It varies as per site conditions, however the basic criteria is to find a place where you get :
    1. Minimum copper cable run.
    2. Concentration of the relevant equipment.
    3. Better environmental condition. (wet area, dust, chemicals, sun, safe working space i.e. clearance, light).
    4. Within the communication cable limits.


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