Re: [foxboro] A Non I/A Query
- From: "Johnson, Alex (Foxboro)" <ajohnson@xxxxxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Tue, 24 Aug 2004 23:53:10 -0400
General thoughts:
1) Speed of calculation - PLCs tend to be slow at floating point
(traditionally) and DCSs tend to be slower doing ladder (traditionally)
2) Speed of response - PLCs typically support event triggered actions
using interrupts. DCSs tend to support logging of events at 1ms in their
I/O, but the control cycle tends to be 100ms to 500ms.
3) Comprehensive solution - PLCs tended to lack all of the pieces for a
complete system - HMI, alarm management, etc. and that had to be purchased
from a third-party (Wonderware, Intellution, etc.). DCSs tended to be more
comprehensive including all necessary parts.
4) Scaleability - PLCs traditionally did not scale well (on a single,
integrated problem obviously you can buy lots of them) due to lack of
peer-to-peer; DCSs generally scale to large integrated systems.
5) Control algorithm suite - PLCs were generally limited to LL (digital
emulation of discrete relay control) and that is best suited for certain
types of problems; DCSs typically focussed on control blocks (digital
emulation of analog control) that best suits other problems.
A corresponding question:
Why did we move away from single-loop integrity in control?
I believe it was because networks were expensive - each Linkport port in
SPECTRUM was 2500 in 1981 and that's like 7500 today and the port only
allowed 60 loops. The cost of networking stations is basically zero now and
you are starting to see a move back to single loop integrity in FF's C-I-F.
Regards,
Alex Johnson
Invensys Systems, Inc.
10707 Haddington
Houston, TX 77043
713.722.2859 (voice)
713.722.2700 (operator)
713.932.0222 (fax)
ajohnson@xxxxxxxxxxx
For the latest information on ArchestrA, go to
http://www.invensys.com/Archestra.html.
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of Ajay.Tathgir@xxxxxxx
Sent: Monday, August 23, 2004 11:52 PM
To: foxboro@xxxxxxxxxxxxx
Subject: [foxboro] A Non I/A Query
Hi! All
I got a ludicrous question here folks. Though it has nothing to do with
I/A, but this forum has some of the best brains around to put the answer to
a query like this one, in right words.
- Today we all know that any PLC can do PID control, any DCS can do ladder,
- But still we all know that there are applications where we won't use DCS,
and there are applications where we won't use PLC.
Why ???
I will summarize...
Thanks & Regards
Ajay Tathgir
Reliance Industries Limited
Mumbai - India
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