May 16th, 2002, 08:13 PM
Hi! I'm trying to figure out whats the advantage & disadvantage of PLC against DCS or SCADA. Is their someone here could define me the difference between those 3?? or provide me a link where I can have some information with those 3...
Thanks in advance..
May 17th, 2002, 07:34 AM
SCADA - Supervisory Control And Data Acquisition - is essentially a graphics front end for displaying data. The data can be obtained over a communications network from simple I/O (Input/Output) blocks or from a DCS or from a PLC or from a network of PLCs. Most SCADA systems can provide archiving and trend plots of selected data. SCADA systems were originally "leg savers". That is, they didn't make any control decisions, they just collected the data and displayed it in a central location so you didn't have to walk around and collect the info. This is no longer true for most SCADA software, which has added alarming, PID, and control logic.
DCS - Distributed Control System - were originally developed in the process industries, such as petrochemical refining. They were initially a way of linking many independent single loop PID controllers together to permit displaying process and tuning data at a central computer. The communications protocols were proprietary, and the systems were expensive. They were good with analog data, but not with discrete data, sequencing, timing, etc.
PLC - Programmable Logic Controller - systems were originally developed for the manufacturing industry, particularly the automotive industry. They were developed to replace hard wired relay logic panels, and were strong on sequencing, timing, and if/then type discrete I/O based logic. Communications protocols, math functions, and analog I/O capabilities were added later as the market expanded. PLC communications protocols are generally open, or at least accessible by equipment and software from multiple suppliers.
The distinction between PLC and DCS has blurred, and newer designs look similar in terms of hardware and software capabilities. DCS are generally still more expensive on small systems, but on large ones may be more economical. Use of one over the other is often dictated by the installed base and engineering familiarity in a given industry.
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