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I am new in the field of automation. Could anybody tell me the difference between PLC and DDC?

I am new in the field of automation. Could anybody tell me the difference between PLC and DDC?

By Jim Zupich on 1 August, 2004 - 12:47 pm

I'm guessing you mean DCS.. If that's the case, here's my personal observations:

- PLCs don't generally have anything more than the control--IE, the I/O, the brains, and runs the control program.

- DCS are typically super expensive and extremely proprietary. PLCs can be pretty cheap.

- DCS are more common in process control environments whereas PCs are usually a better fit for discrete and state machine tasks.

- DCS typically both control and HMI functions in one --imaging this--system. This means one tag database for both pieces. In the PLC world when you add an HMI you have generally a separate tag database that you as the engineer have to keep in sync.

- Historically, for PLCs you'd have one autonomous PLC per machine, process line or area, etc. When you want to network between them it's generally not seamless. When you have a DCS, it can be one large DCS system for a whole plant or many areas (large amounts of I/O) but you could have several processors in the one DCS system; processors would be aware of each other and can share points. This is changing as PLC manufacturers are adding distributed processing capabilities and architecures, in part enabled by newer networking technologies.


By Michael R. Batchelor on 2 August, 2004 - 1:21 pm

I would venture a guess that he means Direct Digital Control like some of the HVAC vendors are doing. I don't know a lot about them other than that I lost a bid on an HVAC control system because the PLC was expensive in comparison.

Apparently in a DDC system the end effector devices are "smart" units, and there is something like an RS422 multi-drop running to the various chill water valves in the different zones.

Try poking around the HVAC vendors web sites. My experience was that they weren't very free with their information.

Michael R. Batchelor - Industrial Informatics, Inc.
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By Dr ir Gerrit M. van der Molen on 3 August, 2004 - 7:07 pm

Hi Piyo

See this previous thread as well:

Gerrit van der Molen