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Old May 20th, 2009, 05:11 AM   #16
Ken Moore
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I have/do work with both. The biggest differences in my opinion are.
1. DCS systems do fault tolerance and redundancy much better. The PLC's are not close...yet.
2. It is much easier to do complex controls with a DCS, yes you can do the same thing with a PLC, but it takes a lot more code.
3. Only one tag database in most DCS systems. With a PLC/SCADA system, each has there own list of "tags".
4. PLC/PAC controllers are much faster, but hold less. Most DCS systems have 100msec scans or slower.
"To give real service you must add something that cannot be bought or measured with money; and that is sincerity and integrity."
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Old May 20th, 2009, 05:39 AM   #17
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In short, PLC/SCADA systems may require more engineering effort, at least that is what the DCS people will tell you. Certainly this is true for continuous processes and those with relatively little discrete control. Also DCS's tend to be more easy to configure for things like redundancy.
Keeping it Simple takes thinking time.
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Old December 14th, 2014, 11:18 PM   #18
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PLC is a programmable Logic controller which is used mainly for interlocking different equipments.
DCS is Distributed Control Systems which is used to control very big plants by using simple GUI screens.PLCs are interfaced with DCS for interlocking.
SCADA is Supervisory Control and Data Aquisition which is used to control and Monitor small number of Equipments in a field.SCADA systems are typically used to perform data collection and control at the supervisory level. Some systems are called SCADA despite only performing data acquisition and not control.The supervisory control system is a system that is placed on top of a real-time control system to control a process that is external to the SCADA system (i.e. a computer, by itself, is not a SCADA system even though it controls its own power consumption and cooling). This implies that the system is not critical to control the process in real time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time constants of the process. The process can be industrial, infrastructure or facility based as described below:
Difference between PLC and DCS
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