Answer / www.islamqa.com

First let's take a quick brief history about controllers:

1 - For analog control loops we have a stand alone PID controller.

2 - Then we get a networked PID controllers through a hub and HMI for configuring and monitoring.
The PID itself contain a network port.
SIEMENS introduced PID 353 & PROCIDIA software for it.

3 - Then we get DDC - DIRECT DIGITAL CONTROL, single CPU, I/O'S for analog instruments only & networked PC is an option.
SIEMENS introduced APOGEE, FISHER introduced ROC.
has disadv. of system halting/failure.

4 - The next phase is DCS - Distributed Control System, and this is first introduced by HONEYWELL & YOKOGAWA.
Ability for handling large scale of I/O'S, discrete and analog, complex control loops, PID algorithms calculations and executions.

5 - PLC is basically designed and produced for discrete on/off control, limited I/O'S.
but modern PLC with great progress in h/w technology introduced itself as a less expensive option for PID, with it's complicated calculations and algorithms.

6 - by the time with great achievements in H/W tech. and networking capabilities the single PLC can handle discrete and analog instruments with wide range.

7 - Now the man can make his own DCS with multible networked PLC'S, but it will need a great engineering and effortsto get near from DCS features and functionality.

Regards,

Ahmed Sabry
Control and Instrumentation Engineer
GPC - General Petroleum Company
ahmedsabry.online@gmail.com
CELL.: 00 2 0121367310

Answer / tauqeer hassan

plc is a programmable logic controller which can handle the
binary input and output of the logic statement which are
stored in its memory.its a digitally operating system whic
are logic stored in its memory by programming system.

DCS:-Distributed control system its control all types of
process variable at a time through this we can control the
whole plant,we can see the status of plant,and handling yhe
unit.

Answer / eid elatawy

PLC is a programmable logic controller which take the inputs from the system and execute it through the program written by Ladder or STL (statement list)language and provide the output to the system.the famous CO Siemens,Allen Bradly.

DCS is distributed control system the same idea for PLC for take the input and execute it through the program to provide the output.the famous CO Yokogawa,Honeywell


We can enumerate the difference as following:-
PLC
1- no of points less.and almost of these points digital points.
2- no of analog points very less.
3- trends and history not allowed.
4- used for machines and separate units with HMI and in this case called SCADA system.
5- cost is less and easy to make extension.
6-scanning time less.

DCS
1- you can use more than 10000 points
2- no of analog points more.
3- trends and history allowed and saved in server for along time.
4- used for a big plants like oil&gas-chemicals and petrochemicals..
5-cost very high and you have to min cost of the system + no of points separate. so the extension not easy like PLC
6- Scanning time more than PLC.
7- We can monitor the all inputs and control all outputs

Regards

Eid Elatawy
I&C Engineer
TCI sanmar chemicals CO
Port said
Egypt
contact no- 002 01000470262
mail- eng.eid72@gmail.com

Answer / ghazanfar ali

In PLC the Scan time is less.but the scan time of DCS is
high.Because it Control the whole plant. So in safty llop
we used PLC

in DCS or PLC we can not put all ai or di,there is calculation through which we can accomodate certain no of digital and analog signal.I need to know that calculation

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