PLCMentor.com
Joined: 26 Mar 2009
Posts: 23
Location: North Carolina, USA
|
 13 May 2009 16:58 plc dcs forum |
|
|
|
|
You have hit upon a discussion that has been raging in the industry for 20-30 years. Over the years, the two have come together to the point that there is not a whole lot of difference. The biggest difference is probably in their origins.
PLC's originated with discrete control. Designed to replace large amounts of relays. This also is the reason for PLC's dominant programming language being ladder logic. The target audience was electrical engineers and electricians. Ladder logic is well suited to the core training of both disciplines so EE's and electricians can understand ladder programs due to their similarity to electrical ladder diagrams. Over the years the functionality required by advanced ladder programs has somewhat negated this advantage. Many of the commands available are in no way comparable to electrical components. PLC's have also expanded to include structured text programming, function block programming and step programming. Analog I/O and capabilities have been developed to cover a full spectrum of possibilities. SCADA systems have developed to be paired with PLC's to allow full visualization and data manipulation.
DCS's originated to fill in with analog control. Developed for process industries, they were designed around process engineers - usually chemical engineers. They were designed for large systems and generally were programmed with some form of high level language or function block programming. They now have ladder and discrete functionality and even tend to look more like the PLC's. DCS's tend to integrate the visualization and data manipulation better than their PLC counterparts as their configuration is usually a single system designed to work as a whole; however, the PLC manufacturers have made strides into duplicating this capability. The costs of DCS systems are generally higher than comparable PLC systems.
I really dont see a lot of difference today between the two platforms. EE's tend to gravitate toward the PLC's still and Chem E's tend to still gravitate towards the DCS's. The PLC manufacturers are constantly trying to break the image of small system and/or discrete machine control even after all these years. Many of them have gone to calling their upper end controllers PAC's or programmable automation controllers. Change the name lose the stigma.
Hope this helps
|
|
