DDC - direct digital control


Jayaharan C J

Direct Digital Control in other words is PC based control where the PC acts as a controller i.e the control algorithm is executed in the PC which may be interfaced with an I/O system.

A PLC system on the other hand has a dedicated CPU and I/O where the control algorithm is downloaded and executes independently in the CPU.

Hope this helps

Jayaharan C J
[email protected]

so far that i knew about that different is DDC use PC as their controller. So the devices at field connect straight to the PC.

PLC can operate stand alone without PC, because it has a processor and memory. but if you want to monitor or control from PC, you can do that.

DDC is old technology, and PLC is more better than DDC.


Take a look at this control.com thread: http://www.control.com/1026184497/index_html#1026184605

Many DDC controls are available today that conform to an open and robust international standard, ISO 16484-5 - BACnet.

This means you can mix controllers and devices from different manufacturers on the same network. They all talk to each other "out of the box" and you're not married to any single supplier. It works!

Good luck,

Try the Dictionary of Automation at http://www.isa.org.

Basically, DDC is an archaic term referring to the way control was done with monolithic computers prior to the era of the DCS (approximately 30 years ago). PLC means "programmable logic controller" and dates from the same era, invented by Otto Struger and Dick Morley among others.


Walt Boyes ([email protected])
Editor in chief, CONTROL magazine

Blogging "Sound Off" at controlglobal.com

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This is a frequently asked doubt. The answer is simple. PLCs are standalone systems. It accepts inputs and basing on your strategy, sends outputs. Say you have three equipments of different manufacturers. These equipments are being operated by three PLCs of different vendors. Through a network system, by properly mapping the inputs and outputs of these three PLCs, one can have a single centralised control system, called DCS system (Distributed control System). If you feel otherwise, kindly correct me.

Friedrich Haase

Moin Mrs. Anita,
moin all,

the term DDC, short for "direct digital control," was first used when process computers replaced analog controllers - electrical circuits or even something that used air under pressure. Maybe in the early 1970th - I am not sure about the date.

With DDC the control actions are calculated mathematically. This also allowed to use other control methods as opposed to PID controllers. Usually designed in z-space or discrete state space. Dead-beat controllers are an example. BTW, some people do not consider a discrete PID controller as DDC.

Nowadays we use DDC in most cases, but rarely mention it. The PID-controllers in a PLC or in a PC-based control, in an old fashioned process computer (like PDP-nn, HP2100 or HP1000) or in a DCS no longer uses analog circuits but calculates the control action from set-point, current value and some past values.

You can easily program a dead beat controller or a state space controller, possibly including an observer, in a modern PLC. Even most tiny PLC often have enough processing power.

Friedrich Haase

Ing.-Büro Dr. Friedrich Haase
Consulting - Automatisierungstechnik
email [email protected]
WEB http://www.61131.com

I'm fear that your're getting some real bad input to your inquiry.

Ariez's and Jayaharan's replies are way off base!

Both PLC and DDC are programmable controllers haveing CPU, I/O, communication port, etc. Both PLC and DDC, after initial set up, do their thing without the need of a PC.

Most PLCs on the market talk Modbus which is an open but proprietary protocol.

Today, many DDCs speak BACnet which is a robust modern open protocol that conforms to an ISO standard. In addition, they generally operate in peer-to-peer mode rather than master-slave.

For an excellent DDC tutorial, take a look at http://ddc-online.org

Good luck,


Dobrowolski, Jacek

I'd advise to repleace "most" with "some". "Talking Modbus" is not general property of a PLC.


Jacek Dobrowolski

Steve Myres, PE

I think "most" is fair. I've done it with ABs and PLC Directs, I know Siemens can do it, as well as Omron, and obviously, Modicon/Tele/Schneider products.

Atish K Bagchi

Comparing DDC with PLC is like comparing a concept against a product. Before early 1970s, most of the control was done by Analog computers and Pneumatic controllers.Because of the interacting nature of both Analog OPAMP circuits and pneumatic controllers people used interactive type PI oe PID algorithms.

With the advent of process computers (like PDP 11 etc)it was possible for the first time to write PID algorithm using Difference equations and other algorithms like Dahlin etc could be tried out. The Computer did the Data Acquisition, ran the Control Engine and generated Control Outputs. Inside the computer everything was digital and due to this reason this type of control was called Direct Digital Control and control systems that did this were known as DDCS (Direct Digital Control System).

Needless to say, that PLC , DCS or even a simple single loop PID controller made now a days all employ the DDC concept and do Direct Digital Control

Raghavendra V Deshpande

Basically PLC is a readymade industrial hardware which can be programmed as per application logics (I/O-analog, digital & counter channels ) with ladder programming. With PLC you can use ready made MMI or HMI as a user interface.

Whereas DDC is not a ready made hardware solutionbut its embedded controller for any appliaction (most of the time for close loop control). DDC can be design from basic concept to product design with Hardware design, firmware etc... In

DDC you can design user interface for controller's setting like configuration/ calibration as a part of product, which will be saved in flash.