Difference in PLC and DCS
| DCS | PLC | |
| 1 | The system uses multiple processors, has a central database and the
functionality is distributed. That is the controller sub system performs the
control functions, the history node connects the data, the IMS node gives
reports, the operator station gives a good HMI, the engineering station allows engineering changes to be made. |
The system has Processor and I/O's and some functional units like basic modules, communication modules and so on. Uses a SCADA for visualization. generally the SCADA does not use a central database and so if your clocks on two PC's are not synchronized, you will find that in one PC your plant tripped at 10:00 AM and at 10:02 AM in the other. |
| 2 |
DCS had blocks, several stations spread on a LAN and IEEE802.X was sacred and
meant open system. |
PLC had ladder and Statement list (from Germany) and SFC and the relative merits
of these were discussed. Lesser beings had mnemonics and hand held programmers
while the cherished ones did it through a PC. |
| 3 | Stand alone controllers or DCS for closed loop controls | Relays and PLC's for Interlocking, shutdown and sequencing. |
| 4 | Speed of discrete operations was comparatively lower. Earlier 250ms was considered good enough for most control applications. | Speed of discrete operations were high. Micro seconds to milliseconds for individual operations, total scan time in a few milliseconds. |
| 5 | In general you would be wasting the power of a DCS processor if you tried to cycle everything at even 250 msec. | Most PLC's update their outputs in 10 to 100 msec. |
| 6 | A few examples of a full DCS, such as Honeywell or Rosemount, making it easy to connect to stand-alone systems such as analyzers, flow computers,etc.. However, when it comes to security, most DCS are pretty much bulletproof. | PLC manufacturers have always been more interested in connecting to other products because they are so often used as SCADA systems. |
| 7 | The "curse" of both PLC's and DCS is that most good ones are nearly indestructible | For many applications PLC's can provide tremendous functionality with low-priced hardware. PLC hardware is generally much easier to configure for straightforward tasks. |
| 8 | Viewing the algorithms and changing them usually requires much more time and prior knowledge of the PLC software than does the DCS. Even with a simple PID control. Screens for configuration and maintenance are generally built into a DCS system. Which also adds to their size and cost. | The PLC, so far, seems unable to match the power and integration of a dedicated DCS for large applications. PLC's can implement very complicated control schemes. |
| 9 |
The engineering is strong and you can write one database for a range of controllers in a plant. |
Making two PLC's to share data requires some special programming tools. The orientation of the engineering is still on PLC and its I/O basis rather than a central database. |
Will PLC-SCADA replace DCS, seems imminent in smaller plants but long away for
bigger plants which require more functionality. DCS need not die, but can become
open, available at affordable prices.
As time goes by the systems come closer and closer and in some time we may have
the PDPCS or Programmable Distributed Process Control System as these DCS and
PLC combines would probably be called along with the improvements that will come
with PC based controls and Distributed Data Acquisition Systems.
Once again PLC or DCS or PC based or DDAS what does one go for?
Look for:
Speed
Functional blocks available.
Price.
Reliability.
future proof systems.
Open systems where you are able to connect to other systems in the
future and protocols are in the open arena.