WHAT IS THE DIFFERENCE BETWEEN PLC AND DCS
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Answer Posted / mohammed ibrahim khaleel
1. In PLC the heart of the system is a controller,
which contains all logics.
In DCS the heart of the system is HMI.
2. In PLC Operators primary role is to handle
exceptions where as in DCS Operators interaction is
required to keep the process in target.
3. In PLC Status information on/off, Run/Stop is
critical information for operator. In DCS analog trends are
critical, alarm management is Key to safe operation &
failure of HMI could force a shut down.
4. in PLC logic scan is very fast approx. 10 ms is
required to perform function i.e.to change status of motor
where as in DCS scan time is large i.e. 100~500 ms.
5. In PLC redundancy may not be cost justified where
as in DCS system redundancy is often required.
6. In PLC you can’t change configuration online. it
requires offline means shut down where as in DCS online
parameters can be changed i.e. no shut down is required.
7. In PLC analog control is simple PID control where
as in DCS simple to advanced PID control up to advanced
process control.
8. In PLC's use Ladder logic to configure application
where as in DCS use function blocks to configure
application.
9. PLC is useful for small plant application where as
in DCS is used for large plants or number of small plants
to a single platform.
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