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Old July 12th, 2004, 09:45 AM   #1
EKC-27
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PLC vs DDC

Hello everyone,
I am new to the board.
I am an engineer in the HVAC controls industry. My typical work involves systems with DDC controllers. On a current project, the HVAC is sort of a hybrid comfort/process for a petro-chem plant. The client is asking for PLC controllers with SIL 2 rating from AB, Siemens, Modicon or GE. I am totally unfamiliar with this type, but very familiar with the HVAC variety- Siemens, Staefa, Alerton, ALC, etc..

Can anyone tell me the major differences between PLC and DDC? I assume PLC is more robust, more industrial and capable of delivering more features, but am relatively lost when it comes to them.

Thanks for any help you cna provide.

Ed
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Old July 12th, 2004, 10:09 AM   #2
Bitmore
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May I suggest the following link...

http://www.ddc-online.org/

bitmore
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Old July 12th, 2004, 10:28 AM   #3
EKC-27
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Thanks Bitmore,
I took a quick look at the site. It seems to explain some very basic details on general control concepts and DDC controls. I think it covers things with which I am already familiar- mostly DDC stuff. I am OK there, but pretty weak on the PLC related things.

Ed
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Old July 12th, 2004, 10:33 AM   #4
Tom Jenkins
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DDC (Direct Digital Control) is specifically designed for HAVC applications. The DDC programming is usually brand specific, with special functions geared to HVAC like VAV control, night setback, and so on. The communications protocols for DDC are usually proprietary, although some DDC suppliers have Modbus and similar communications abiltity as well. The operator interface for a DDC is usually either a proprietary keypad of some kind or else a general purpose SCADA interface customized for the communications and with some HVAC specific icons. DDC is usually less expensive than PLCs for hardware because HVAC contractors are primarily concerned about initial cost.

PLCs are general purpose industrial controllers, generally using ladder logic for programming. Ladder logic is a graphical programming language that looks like JIC (Joint Industries Conference?) electrical schematics for relay logic control panels. The communciations protocols are sometimes open and sometimes proprietary, but they are so widely used that finding OPC or other drivers for communicationg with most brands is not a problem, although it may cost some money.

You can do anything with a PLC that you can with DDC, but perhaps not as quickly from a programming standpoint because you may have to create the logic yourself. You can do things with a PLC that would be difficult to do with DDC, such as complex process control or intricate sequencing or process recipes.
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Old July 12th, 2004, 12:37 PM   #5
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Thanks Tom,
Methinks a closer examination of the link I provided, expands on exactly the same thing, albeit not as eloquently as you!

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